Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label volcano. Show all posts
Showing posts with label volcano. Show all posts

P'u'u O'o Crater, Hawai'i, 2005

Video Credits: Th. Böckel
Puʻu ʻŌʻō (often written Puu Oo) is a volcanic cone in the eastern rift zone of the Kīlauea volcano of the Hawaiian Islands. Puʻu ʻŌʻō has been erupting continuously since January 3, 1983, making it the longest-lived rift-zone eruption of the last two centuries.
By January 2005, 2.7 cubic kilometers (0.65 cu mi) of magma covered an area of more than 117 square kilometers (45 sq mi) and added 230 acres (0.93 km2) of land to the Southeast coast of Hawaiʻi. So far, the eruption has claimed 189 buildings and 14 kilometres (8.7 mi) of highways, as well as a church, a store, the Wahaʻula Visitor Centre, and many ancient Hawaiian sites, including the Wahaʻula heiau. The coastal highway has been closed since 1987, as it has been buried under lava up to 35 meters (115 ft) thick.

Different styles of Volcano

Different styles of Volcano

There are different styles of volcano on the face of Earth and yes the subsurface too.

Volcano Architecture 

Crater eruptions and fissure eruptions come from conduits of different shapes.
Melting in the upper mantle and lower crust produces magma, which rises into the upper crust. Typically, this magma accumulates underground in a magma chamber, a zone of open spaces and/or fractured rock that can contain a large quantity of magma. A portion of the magma may solidify in the magma chamber and transform into intrusive igneous rock, whereas the rest rises through an opening, or conduit, to the Earth’s surface and erupts from a volcano.  The conduit may have the shape of a vertical pipe, or chimney, or may be a crack called a fissure (figure above a and b). At the top of a volcanic edifice, a circular depression called a crater (shaped like a bowl, up to 500 m across and 200 m deep) may develop. Craters form either during eruption as material accumulates around the summit vent, or just after eruption as the summit collapses into the drained conduit.
The formation of volcanic calderas.
During major eruptions, the sudden draining of a magma chamber produces a caldera, a big circular depression up to thousands of meters across and up to several hundred meters deep. Typically, a caldera has steep walls and a fairly flat floor and may be partially filled with ash.
Different shapes of volcanoes.
Geologists distinguish among several different shapes of subaerial (above sea level) volcanic edifices. Shield volcanoes, broad, gentle domes, are so named because they resemble a soldier’s shield lying on the ground (a in figure above). They form when the products of eruption have low viscosity and thus are weak, so they cannot pile up around the vent but rather spread out over large areas. Scoria cones (informally called cinder cones) consist of cone-shaped piles of basaltic lapilli and blocks, generally from a single eruption (b in figure above). Strato-volcanoes, also known as composite volcanoes, are large and cone-shaped, generally with steeper slopes near the summit, and consist of interleaved layers of lava, tephra, and volcaniclastic debris (c in figure above). Their shape, exemplified by Japan’s Mt. Fuji, supplies the classic image that most people have of a volcano; the prefix strato- emphasizes that they can grow to be kilometres high.

Concept of Eruptive Style: Will It Flow, or Will It Blow? 

Kilauea, a volcano on Hawaii, produces rivers of lava that cascade down the volcano’s flanks. Mt. St. Helens, a volcano near the Washington–Oregon border, exploded catastrophically in 1980 and blanketed the surrounding countryside with tephra. Clearly, different volcanoes erupt differently and, as we've noted, successive eruptions from the same stratovolcano may differ markedly in character from one another. Geologists refer to the character of an eruption as eruptive style. Below, we describe several distinct eruptive styles and explore why the differences occur.
Contrasting eruptive styles.

Effusive eruptions 

The term effusive comes from the Latin word for pour out, and indeed that’s what happens during an effusive eruption lava pours out a summit vent or fissure, filling a lava lake around the crater and/or flowing in molten rivers for great distances (a in figure above). Effusive eruptions occur where the magma feeding the volcano is hot and mafic and, therefore, has low viscosity. Pressure, applied to the magma chamber by the weight of overlying rock, squeezes magma upward and out of the vent; in some cases, the pressure is great enough to drive the magma up into a fountain over the vent.

Explosive eruptions 

When pressure builds in a volcano, the eruption will likely yield an explosion. Smaller explosions take place during basaltic eruptions, when gas builds up and suddenly escapes, spattering lava drops and blobs upward these then solidify and fall as tephra. Occasionally, a volcano blows up in a huge explosion. Such catastrophic explosions can be triggered by many causes. For example, if a crack forms in the flank of an island volcano, water will enter the magma chamber and suddenly turn to steam, the expansion of which blasts the volcano apart. Such explosions can also happen in felsic or andesitic volcanoes, if very viscous magma plugs the vent until huge pressure builds inside. If the plug eventually cracks, or the flank of the volcano cracks, the gas inside the volcano suddenly expands, and like a giant shotgun blast, it sprays out the molten contents of the volcano and may cause the volcano itself to break apart. Such explosions, awesome in their power and catastrophic in their consequences, can eject cubic kilometres of debris outward. In some cases, the sudden draining of the magma chamber, and the ejection of debris, causes the remnants of the volcano to collapse and form a caldera.
During a large explosion, the force of the blast shoots debris skyward in a vertical column (b in figure above). But the force can only take the material so high. The huge plumes of ash that rise to stratospheric heights above large explosions do so by becoming turbulent, billowing, convective clouds. This means that the warm mixture of volcanic ash, gas, and air is less dense than the surrounding, cooler air, so the warm mixture rises buoyantly. The resulting plume resembles a mushroom cloud above a nuclear explosion. Coarser-grained ash and lapilli settle from the cloud close to the volcano, whereas finer ash gets carried farther away. Some ash enters high-elevation winds and will be carried around the globe. The denser components collapse downward once they run out of explosive energy, and gravity pulls them back down. This phenomenon, the “collapse” of the column, produces the pyroclastic flows that surge down a volcano’s flanks. What is a pyroclastic flow like? In 1902, the people of St. Pierre, a town on the Caribbean island of Martinique, sadly found out. St. Pierre was a busy port town, about 7 km south of the peak of Mt. Pelée, a volcano. When the volcano began emitting steam and lapilli, residents of the town became nervous and debated about the need to evacuate. Meanwhile, a rhyolite dome grew and obstructed the throat of the volcano. On May 8, the dome suddenly cracked, and the immense pressure that had been building beneath the obstruction was released. In the same way that champagne bursts out of a bottle when you pull out the cork, a cloud of hot ash and pumice lapilli spewed out of Mt. Pelée, and a pyroclastic flow swept 
down Pelée’s flank. Partly riding on a cushion of air, this flow reached speeds of 300 km per hour, and slammed into St. Pierre. Within moments, all the town’s buildings had been flattened and all but two of its 28,000 inhabitants were dead of incineration or asphyxiation. Similar eruptions have happened more recently on the nearby island of Montserrat, but with a much smaller death toll because of timely evacuation (c in figure above).

Relation of eruptive style to volcanic type

Note that the type of volcano (shield, cinder cone, or composite) depends on its eruptive style. Volcanoes that have only effusive eruptions become shield volcanoes, those that generate small pyroclastic eruptions due to fountaining basaltic lava yield cinder cones, and those that alternate between effusive and large pyroclastic eruptions become composite volcanoes (stratovolcanoes). Large explosions yield calderas and blanket the surrounding countryside with ash and/or ignimbrites. Why are there such contrasts in eruptive style? Eruptive style depends on the viscosity and gas contents of the magma in the volcano. These characteristics, in turn, depend on the composition and temperature of the magma and on the environment (subaerial or submarine) in which the eruption occurs. Traditionally, geologists have classified volcanoes according to their eruptive style, each style named after a well known example.

Credits: Stephen Marshak (Essentials of Geology)

The Products of Volcanic Eruptions

Products of Volcanic Eruptions

The drama of a volcanic eruption transfers materials from inside the Earth to our planet’s surface. Products of an eruption come in three forms lava flows, pyroclastic debris, and gas. Note that we use the name flow for both a molten, moving layer of lava and for the solid layer of rock that forms when the lava freezes.

Lava Flows 

The character of a lava flow depends on its viscosity.
Sometimes it races down the side of a volcano like a fast moving, incandescent stream, sometimes it builds into a rubble-covered mound at a volcano’s summit, and sometimes it oozes like a sticky but scalding paste. Clearly, not all lava behaves in the same way when it rises out of a volcano. Therefore, not all lava flows look the same. Why? The character of a lava primarily reflects its viscosity (resistance to flow), and not all lavas have the same viscosity. Differences in viscosity depend, in turn, on chemical composition, temperature, gas content, and crystal content. Silica content plays a particularly key role in controlling viscosity. Silica poor (basaltic) lava is less viscous, and thus flows farther than does silica-rich (rhyolitic) lava (figure above). To illustrate the different ways in which lava behaves, we now examine flows of different compositions.

Basaltic lava flows 

Features of basaltic lava flows. They have low viscosity thus can flow for long distances. Their surface and interior can be complex.
Basaltic (mafic) lava has very low viscosity when it first emerges from a volcano because it contains relatively little silica and is very hot. Thus, on the steep slopes near the summit of a volcano, it can flow very quickly, sometimes at speeds of over 30 km per hour (figure above a). The lava slows down to less-than-walking pace after it starts to cool (figure above b). Most flows measure less than a few km long, but some flows reach as far as 600 km from the source. How can lava travel such distances? Although all the lava in a flow moves when it first emerges, rapid cooling causes the surface of the flow to crust over after the flow has moved a short distance from the source. The solid crust serves as insulation, allowing the hot interior of the flow to remain liquid and continue to move. As time progresses, part of the flow’s interior solidifies, so eventually, molten lava moves only through a tunnel-like passageway, or lava tube, within the flow the largest of these may be tens of meters in diameter. In some cases, lava tubes drain and eventually become empty tunnels.
The surface texture of a basaltic lava flow when it finally freezes reflects the timing of freezing relative to its movement. Basalt flows with warm, pasty surfaces wrinkle into smooth, glassy, rope-like ridges; geologists have adopted the Hawaiian word pahoehoe (pronounced “pa-hoy-hoy”) for such flows (figure above c). If the surface layer of the lava freezes and then breaks up due to the continued movement of lava underneath, it becomes a jumble of sharp, angular fragments, creating a rubbly flow also called by its Hawaiian name, a’a’ (pronounced “ah-ah”) (figure above d). Footpaths made by people living in basaltic volcanic regions follow the smooth surface of pahoehoe rather than the foot-slashing surface of a’a’. 
During the final stages of cooling, lava flows contract, because rock shrinks as it loses heat, and may fracture into polygonal columns. This type of fracturing is called columnar jointing (figure above e). 
Basaltic flows that erupt underwater look different from those that erupt on land because the lava cools so much more quickly in water. Because of rapid cooling, submarine basaltic lava can travel only a short distance before its surface freezes, producing a glass-encrusted blob, or “pillow” (figure above f). The rind of a pillow momentarily stops the flow’s advance, but within minutes the pressure of the lava squeezing into the pillow breaks the rind, and a new blob of lava squirts out, freezes, and produces another pillow. In some cases, successive pillows add to the end of previous ones, forming worm-like chains.

Andesitic and rhyolitic lava flows

This rhyolite dome formed about 650 years ago, in Panum Crater, California. Tephra (cinders) accumulated around the vent.
Because of its higher silica content and thus its greater viscosity, andesitic lava cannot flow as easily as basaltic lava. When erupted, andesitic lava first forms a large mound above the vent. This mound then advances slowly down the volcano’s flank at only about 1 to 5 m a day, in a lumpy flow with a bulbous snout. Typically, andesitic flows are less than a few km long. Because the lava moves so slowly, the outside of the flow has time to solidify; so as it moves, the surface breaks up into angular blocks, and the whole flow looks like a jumble of rubble called blocky lava.
Rhyolitic lava is the most viscous of all lavas because it is the most silicic and the coolest. Therefore, it tends to accumulate either above the vent in a lava dome (figure above), or in short and bulbous flows rarely more than 1 to 2 km long. Sometimes rhyolitic lava freezes while still in the vent and then pushes upward as a column-like spire up to 100 m above the vent. Rhyolitic flows, where they do form, have broken and blocky surfaces.

Volcaniclastic Deposits 

On a mild day in February 1943, as Dionisio Pulido prepared to sow the fertile soil of his field 330 km (200 miles) west of Mexico City, an earthquake jolted the ground, as it had dozens of times in the previous days. But this time, to Dionisio’s amazement, the surface of his field visibly bulged upward by a few meters and then cracked. Ash and sulfurous fumes filled the air, and Dionisio fled. When he returned the following morning, his field lay buried beneath a 40-m-high mound of gray cinders Dionisio had witnessed the birth of Paricutín, a new volcano. During the next several months, Paricutín erupted continuously, at times blasting clots of lava into the sky like fireworks. By the following year, it had become a steep-sided cone 330 m high. Nine years later, when the volcano ceased erupting, its lava and debris covered 25 square km. 
This description of Paricutín’s eruption, and that of  Vesuvius at the beginning of this chapter, emphasizes that volcanoes can erupt large quantities of fragmental igneous material. Geologists use the general term volcaniclastic deposits for accumulations of this material. Volcaniclastic deposits include pyroclastic debris (from the Greek pyro, meaning fire), which forms from lava that flies into the air and freezes. They also include the debris formed when an eruption blasts apart pre-existing volcanic rock that surrounds the volcano’s vent, the debris that accumulates after tumbling down the volcano in landslides or after being transported in water-rich slurries, and the debris formed as lava flows break up or shatter. 

Pyroclastic debris from basaltic eruptions

Pyroclastic debris from basaltic eruptions.
Basaltic magma rising in a volcano may contain dissolved volatiles (such as water). As such magma approaches the surface, the volatiles form bubbles. When the bubbles reach the surface, they burst and eject clots and drops of molten magma upward to form dramatic fountains (figure above a). To picture this process, think of the droplets that spray from a newly opened bottle of soda. Solidification of the pea-sized fragments of glassy lava and scoria produces a type of lapilli (from the Latin word for little stones). Pieces of this type of lapilli are informally known as cinders. Rarely, flying droplets may trail thin strands of lava, which freeze into filaments of glass known as Pelé’s hair, after the Hawaiian goddess of volcanoes, and the droplets themselves freeze into tiny streamlined glassy beads known as Pelé’s tears. Apple- to refrigerator-sized fragments called blocks (figure above b) may consist of already-solid volcanic rock, broken up during the eruption such blocks tend to be angular and chunky. In some cases, however, blocks form when soft lava squirts out of the vent and then solidifies such blocks, also known as bombs, have streaked, polished surfaces.

Pyroclastic debris from andesitic or rhyolitic eruptions

The components of an explosive eruption.
Andesitic or rhyolitic lava is more viscous than basalt, and may be more gas-rich. The lava flows tend to be blocky to start with, and blocks of flows may tumble down the volcano. Eruptions of these lavas also tend to be explosive. Debris ejected from explosive eruptions includes fragments of pumice and ash. Ash consists of particles less than 2 mm in diameter, made from both glass shards formed when frothy lava explosively breaks up during an eruption, and from pulverized pre-existing volcanic rock (figure above a). Two types of lapilli are produced by explosive eruptions: pumice lapilli consists of angular pumice fragments formed from frothy lava (figure above b); accretionary lapilli consists of snowball-like lumps of ash formed when ash mixes with water in the air and then sticks together (figure above c). 
Much of the pyroclastic debris erupted from an exploding volcano billows upward in a turbulent cloud that can reach stratospheric heights (figure above d). Some, however, rushes down the flank of the volcano in an avalanche-like current known as a pyroclastic flow (figure above e). Pyroclastic flows were once known as nuées ardentes (French for glowing cloud), because the debris they contain can be quite hot 200C to 450C. 
Unconsolidated deposits of pyroclastic grains, regardless of size, constitute tephra. Ash, or ash mixed with lapilli, becomes tuff when buried and transformed into coherent rock. Tuff that formed from ash and/or pumice lapilli that fell like snow from the sky is called air-fall tuff, whereas a sheet of tuff that formed from a pyroclastic flow is an ignimbrite. Ash and pumice lapilli in an ignimbrite is sometimes so hot that it welds together to form a hard mass.

Other volcaniclastic deposits

In cases where volcanoes are covered with snow and ice, or are drenched with rain, water mixes with debris to form a volcanic debris flow that moves downslope like wet concrete. Very wet, ash-rich debris flows become a slurry called a lahar, which can reach speeds of 50 km per hour and may travel for tens of kilometers. When debris flows and lahars stop moving, they yield a layer consisting of volcanic debris suspended in ashy mud.

Volcanic Gas 

The gas component of volcanic eruptions.
Most magma contains dissolved gases, including water, carbon dioxide, sulphur dioxide, and hydrogen sulphide (H2O, CO2, SO2, and H2S). In fact, up to 9% of a magma may consist of gaseous components, and generally, lavas with more silica contain a greater proportion of gas. Volcanic gases come out of solution when the magma approaches the Earth’s surface and pressure decreases, just as bubbles come out of solution in a soda when you pop the bottle top off. 
In low-viscosity magma, gas bubbles can rise faster than the magma moves, and thus most reach the surface of the magma and enter the atmosphere before the lava does. Thus some volcanoes may, for a while, produce large quantities of steam, without much lava (figure above a). The last bubbles to form, however, freeze into the lava and become holes called vesicles (figure above b). In high-viscosity magmas, the gas has trouble escaping because bubbles can’t push through the sticky lava. When this happens, explosive pressures build inside or beneath the volcano.

Credits: Stephen Marshak (Essentials of Geology)

Plate tectonics activity

Plate-Tectonic Context of Igneous Activity 

Melting occurs only in special locations where conditions lead to decompression, addition of volatiles, and/or heat transfer. The conditions that lead to melting and, therefore, to igneous activity, can develop in four geologic settings: (figure below) (1) along volcanic arcs bordering oceanic trenches; (2) at hot spots; (3) within continental rifts; (4) along mid-ocean ridges. Let’s look more carefully at melting and igneous rock production at these  settings, in the context of plate-tectonics theory, with a focus on the types of igneous rocks that may form in each setting.

The tectonic setting of igneous rocks

Products of Subduction 

A chain of volcanoes, called a volcanic arc (or just an arc), forms on the overriding plate, adjacent to the deep-ocean trenches that mark convergent plate boundaries. The word “arc” emphasizes that many of these chains define a curve on a map. Continental arcs, such as the Andean arc of South America and the Cascade arc in the northwestern United States, grow along the edge of a continent, where oceanic lithosphere subducts beneath continental lithosphere. Island arcs, such as the Aleutian arc of Alaska and the Mariana arc of the western Pacific, protrude from the ocean at localities where one oceanic plate subducts beneath another. Beneath volcanic arcs, a variety of intrusions plutons, dikes, and sills develop, to be exposed only later, when erosion has removed the volcanic overburden. In some localities, arc-related igneous activity produces huge batholiths. How does subduction trigger melting? Some minerals in oceanic crust rocks contain volatile compounds (mostly water). At shallow depths, volatiles are chemically bonded to the minerals. But when subduction carries crust down into the hot asthenosphere, “wet” crustal rocks warm up. At a depth of about 150 km, crust becomes so hot that volatiles separate from crustal minerals and diffuse up into the overlying asthenosphere. Addition of volatiles causes the hot ultramafic rock in the asthenosphere to undergo partial melting, a process that yields mafic magma. This magma either rises directly, to erupt as basaltic lava, or undergoes fractional crystallization before erupting and evolves into intermediate or felsic lava. In continental volcanic arcs, not all the mantle-derived basaltic magma rises directly to the surface; some gets trapped at the base of the continental crust, and some in magma chambers deep in the crust. When this happens, heat transfers into the continental crust and causes partial melting of this crust. Because much of the continental crust is mafic to intermediate in composition to start with, the resulting magmas are intermediate to felsic in composition. This magma rises, leaving the basalt behind, and either cools higher in the crust to form plutons or rises to the surface and erupts. For this reason, granitic plutons and andesite lavas form at continental arcs.

Products of Hot Spots 

Most researchers think that hotspot volcanoes form above plumes of hot mantle rock from deep in the mantle, though some studies suggest that some hot spots may originate due to other processes happening at shallower depths. According to the plume hypothesis, a column, or “plume” of very hot rock rises like soft plastic up through the overlying mantle beneath a hot spot. (Note that a plume does not consist of magma; it is solid, though relatively soft and able to flow.) When the hot rock of a plume reaches the base of the lithosphere, decompression causes it to undergo partial melting, a process that generates mafic magma. The mafic magma then rises through the lithosphere, pools in a magma chamber in the crust, and eventually erupts at the surface, forming a volcano. In the case of oceanic hot spots, mostly mafic magma erupts. In the case of continental hot spots, some of the mafic magma erupts to form basalt; but some transfers heat to the continental crust, which then partially melts itself, producing felsic magmas that erupt to form rhyolite. 

Large Igneous Provinces (LIPs) 

A map showing the distribution of large igneous provinces (LIPs) on Earth. The red areas are or once were underlain by immense volumes of basalt; not all of this basalt is exposed.
In many places on Earth, particularly voluminous quantities of mafic magma have erupted and/or intruded (figure above). Some of these regions occur along the margins of continents, some in the interior of oceanic plates, and some in the interior of continents. The largest of these, the Ontong Java Oceanic Plateau of the western Pacific, covers an area of about 5,000,000 km2 of the sea floor and has a volume of about 50,000,000 km3. Such provinces also occur on land. It’s no surprise that these huge volumes of igneous rock are called large igneous provinces (LIPs). More recently, this term LIP has been applied to huge eruptions of felsic ash too.

Flood basalts form when vast quantities of low-viscosity mafic lava "floods" over the landscape and freezes into a thin sheet. Accumulation of successive flows builds a flat-topped plateau.
Mafic LIPs may form when the bulbous head of a mantle plume first reaches the base of the lithosphere. More partial melting can occur in a plume head than in normal asthenosphere, because temperatures are higher in a plume head. Thus, an unusually large quantity of unusually hot basaltic magma forms in the plume head; when the magma reaches the surface, huge quantities of basaltic lava spew out of the ground. If the plume head lies beneath a rift, added decompression can lead to even more melting (figure above a). The particularly hot basaltic lava that erupts at such localities has such low viscosity that it can flow tens to hundreds of kilometres across the landscape. Geoscientists refer to such flows as flood basalts. Flood basalts make up the bedrock of the Columbia River Plateau in Oregon and Washington (figure above b and c), the Paraná Plateau in southeastern Brazil, the Karoo region of southern Africa, and the Deccan region of southwestern India. 

Igneous Rocks at Rifts 

Successful rifting splits a continent in two and gives birth to a new mid-ocean ridge. As the continental lithosphere thins during rifting, the weight of rock overlying the asthenosphere decreases, so pressure in the asthenosphere decreases and decompression melting produces basaltic magma, which rises into the crust. Some of this magma makes it to the surface and erupts as basalt. However, some of the magma gets trapped in the crust and transfers heat to the crust. The resulting partial melting of the crust yields felsic (silicic) magmas that erupt as rhyolite. Thus, a sequence of volcanic rocks in a rift generally includes basaltic flows and sheets of rhyolitic lava or ash. Locally, the felsic and mafic magmas mix to form intermediate magma.

Forming Igneous Rocks at Mid-Ocean Ridges 

Most igneous rocks at the Earth’s surface form at mid-ocean ridges, that is, along divergent plate boundaries. Think about it the entire oceanic crust, a 7- to 10-km-thick layer of basalt and gabbro that covers 70% of the Earth’s surface, forms at mid-ocean ridges. And this entire volume gets subducted and replaced by new crust, over a period of about 200 million years. Igneous magmas form at mid-ocean ridges for much the same reason they do at hot spots and rifts. As sea-floor spreading occurs and oceanic lithosphere plates drift away from the ridge, hot asthenosphere rises to keep the resulting space filled. As this asthenosphere rises, it undergoes decompression, which leads to partial melting and the generation of basaltic magma. This magma rises into the crust and pools in a shallow magma chamber. Some cools slowly along the margins of the magma chamber to form massive gabbro, while some intrudes upward to fill vertical cracks that appear as newly formed crust splits apart. Magma that cools in the cracks forms basalt dikes, and magma that makes it to the sea floor and extrudes as lava forms pillow basalt flows.
Credits: Stephen Marshak (Essentials of Geology)

Why does magma form?

Why Does Magma Form, and What Is It Made of? 

It’s Hot Inside the Earth 

Where does the heat that can cause the production of magma come from? Some of the Earth’s internal heat is a relict of the planet’s formation. In fact, during the first 700 million years or so of its existence, the Earth was very hot, and at times may even have been largely molten. But our planet has had a long time to cool since then, and probably would have become too cool to melt at all were it not for the presence of radioactive elements. Every time a radioactive element decays, it generates new heat. The Earth produces enough radioactive heat to keep its inside quite hot.

Causes of Melting 

Even though the Earth is very hot inside, the popular image that the crust floats on a sea of molten rock is wrong. The crust and the mantle of this planet are almost entirely solid. Magma forms only in special places where preexisting solid rock undergoes melting. Below, we describe conditions that lead to melting. We’ll briefly note the settings, in the context of plate tectonics, in which melting conditions develop, but will wait until the end of this chapter to characterize specific types of igneous rocks that form at these settings.

Melting due to a decrease in pressure (decompression) 

The concept of decompression melting.
Beneath typical oceanic crust, temperatures comparable to those of lava occur in the upper mantle (figure above a). But even though the upper mantle is very hot, its rock stays solid because it is also under great pressure from the weight of overlying rock, and pressure prevents atoms from breaking free of solid mineral crystals. Because pressure prevents melting, a decrease in pressure can permit melting. Thus, if the pressure affecting hot mantle rock decreases while the temperature remains unchanged, magma forms. This kind of melting, called decompression melting, occurs where hot mantle rock rises to shallower depths in the Earth. Such movement occurs in mantle plumes, beneath rifts, and beneath midocean ridges (figure above b).

Melting as a result of the addition of volatiles 

Flux melting and heat-transfer melting.
Magma also forms at locations where chemicals called volatiles mix with hot mantle rock. Volatiles, are substances such as water (H2O) and carbon dioxide (CO2) that evaporate easily and can exist in gaseous forms at the Earth’s surface. When volatiles mix with hot, dry rock, they help break chemical bonds so that the rock begins to melt (figure above a). In effect, adding volatiles decreases a rock’s melting temperature. Melting due to addition of volatiles is sometimes called flux melting.

Melting as a result of heat transfer from rising magma

When magma from the mantle rises up into the crust, it brings heat with it. This heat raises the temperature of the surrounding crustal rock and, in some cases, the rise in temperature may be sufficient for the crustal rock to begin melting. To picture the process, imagine injecting hot fudge into ice cream; the fudge transfers heat to the ice cream, raises its temperature, and causes it to melt (figure above b). We call such melting heat transfer melting, because it results from the transfer of heat from a hotter material to a cooler one. 

The Major Types of Magma 

All magmas contain silica, a compound of silicon and oxygen. But magmas also contain varying proportions of other elements such as aluminium (Al), calcium (Ca), sodium (Na), potassium (K), iron (Fe), and magnesium (Mg); each of these ions also bonds to oxygen to form a metal-oxide compound. Because magma is a liquid, its molecules do not lie in an orderly crystalline lattice but are grouped instead in clusters or short chains, relatively free to move with respect to one another.
Geologists distinguish between “dry” magmas, which contain no volatiles, and “wet” magmas, which do. In fact, wet magmas include up to 15% dissolved volatiles such as water, carbon dioxide, nitrogen (N2), hydrogen (H2), and sulphur dioxide (SO2). These volatiles come out of the Earth at volcanoes in the form of gas. Usually, water constitutes about half of the gas erupting at a volcano. Thus, magma contains not only the molecules that constitute solid minerals in rocks but also the molecules that become water or air.

The Four Categories of Magma
Magmas differ from one another in terms of the proportions of chemicals that they contain. Geologists distinguish four major compositional types depending, overall, on the proportion of silica (SiO2) relative to other metal oxides (table above). Mafic magma contains a relatively high proportion of iron oxide (FeO or Fe2O3) and magnesium oxide (MgO) relative to silica. The “ma-” in the word stands for magnesium, and the “-fic” comes from the Latin word for iron. Ultramafic magma has an even higher proportion of magnesium and iron oxides, relative to silica. Felsic magmas have a relatively high proportion of silica, relative to magnesium and iron oxides. (Occasionally, geologists use the term “silicic” interchangeably with felsic.) Intermediate magma gets its name because its composition is partway between mafic and felsic. Why are there so many kinds of magma? Several factors control magma composition, including those described below.

Phenomena that can affect the composition of magma.
  • Source rock composition: The composition of a melt reflects the composition of the solid from which it was derived. Not all magmas form from the same source rock, so not all magmas have the same composition. 
  • Partial melting: Under the temperature and pressure conditions that occur in the Earth, only about 2% to 30% of an original rock can melt to produce magma at a given location the temperature at sites of magma production simply never gets high enough to melt the entire original rock before the magma has a chance to migrate away from its source. Partial melting refers to the process by which only part of an original rock melts to produce magma (figure above a). Magmas formed by partial melting are more felsic than the original rock from which they were derived. For example, partial melting of an ultramafic rock produces a mafic magma. 
  • Assimilation: As magma sits in a magma chamber before completely solidifying, it may incorporate chemicals dissolved from the wall rocks of the chamber or from blocks that detached from the wall and sank into the magma (figure above b). This process is called contamination or assimilation. 
  • Magma mixing: Different magmas formed in different locations from different sources may enter a magma chamber. In some cases, the originally distinct magmas mix to create a new, different magma. Thoroughly mixing a felsic magma with a mafic magma in equal proportions produces an intermediate magma.
Credits: Stephen Marshak (Essentials of Geology)

    Volcanism and Igneous Rocks

    Magma and Igneous Rocks

    Igneous Rocks are  formed by crystallization from a liquid, or magma. They include two types
    • Volcanic or extrusive  igneous rocks form when the magma cools and crystallizes on the surface of the Earth
    • Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.
    Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and  properties of a liquid.
    Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called aplutonic or intrusive igneous rock.
    Types of Magma
    Chemical composition of magma is controlled by the abundance of elements in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is so abundant, chemical analyses are usually given in terms of oxides. SiO2 is the most abundant oxide.
    1. Mafic or Basaltic--  SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 
    2. Intermediate or Andesitic--  SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 
    3. Felsic or Rhyolitic--  SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na.
    Gases - At depth in the Earth nearly all magmas contain gas.  Gas gives magmas their explosive character, because the gas expands as pressure is reduced.
    • Mostly H2O with some CO2 
    • Minor amounts of Sulfur, Cl , and F 
    • Felsic magmas usually have higher gas contents than mafic magmas.
    Temperature of Magmas
    • Mafic/Basaltic - 1000-1200o
    • Intermediate/Andesitic -  800-1000o
    • Felsic/Rhyolitic -  650-800oC.
    Viscosity of Magmas

    Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content.  
    • Higher SiO2 content magmas have higher viscosity than lower SiO2 content magmas 
    • Lower Temperature magmas have higher viscosity than higher temperature magmas.

    Summary Table
    Magma TypeSolidified Volcanic RockSolidified Plutonic RockChemical CompositionTemperatureViscosityGas Content
    Mafic or BasalticBasaltGabbro45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na1000 - 1200 oCLowLow
    or Andesitic
    AndesiteDiorite55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K800 - 1000 oCIntermediateIntermediate
    Felsic or RhyoliticRhyoliteGranite65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na650 - 800 oCHighHigh

    Origin of Magma
    As we have seen the only part of the earth that is liquid is the outer core.  But the core is not likely to be the source of magmas because it does not have the right chemical composition.  The outer core is mostly Iron, but magmas are silicate liquids.  Thus magmas DO NOT COME FROM THE MOLTEN OUTER CORE OF THE EARTH.  Thus, since the rest of the earth is solid, in order for magmas to form, some part of the earth must get hot enough to melt the rocks present. We know that temperature increases with depth in the earth along thegeothermal gradient.  The earth is hot inside due to heat left over from the original accretion process, due to heat released by sinking of materials to form the core, and due to heat released by the decay of radioactive elements in the earth.  Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid.  Thus, magmas form only under special circumstances.  To understand this we must first look at how rocks and mineral melt.
    As pressure increases in the Earth, the melting temperature changes as well.  For pure minerals, there are two general cases.

    • For a pure dry (no H2O or CO2present) mineral, the melting temperate increases with increasing pressure.
    • For a mineral with H2O or CO2present, the  melting temperature first decreases with increasing pressure

    Since rocks mixtures of minerals, they behave somewhat differently.  Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures.  Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma.  The two general cases are:
    • Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.  The degree of partial melting can range from 0 to 100%
    • Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals, melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt.
    WetRockMelt.GIF (9309 bytes)

    Three ways to Generate MagmasFrom the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way.
    The geothermal gradient can be raised by upwelling of hot material from below either by uprise solid material (decompression melting) or by intrusion of magma (heat transfer). Lowering the melting temperature can be achieved by adding water or Carbon Dioxide (flux melting).
    Decompression Melting  - Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle.  Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient.  Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. 
    If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form.  Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids.  Basaltic magmas appear to originate in this way.
    Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and beneath continental rift valleys.  Thus, generation of magma in these three environments is likely caused by decompression melting.
    Transfer of Heat-  When magmas that were generated by some other mechanism intrude into cold crust, they bring with them heat.  Upon solidification they lose this heat and transfer it to the surrounding crust.   Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas.
    Transfer of heat by this mechanism may be responsible for generating some magmas in continental rift valleys, hot spots, and subduction related environments.
    Flux Melting - As we saw above, if water or carbon dioxide are added to rock, the melting temperature is lowered.   If the addition of water or carbon dioxide takes place deep in the earth where the temperature is already high, the lowering of melting temperature could cause the rock to partially melt to generate magma.  One place where water could be introduced is at subduction zones. Here, water present in the pore spaces of the subducting sea floor or water present in minerals like hornblende, biotite, or clay minerals would be released by the rising temperature and then move in to the overlying mantle.   Introduction of this water in the mantle would then lower the melting temperature of the mantle to generate partial melts, which could then separate from the solid mantle and rise toward the surface.

    Chemical Variability of Magmas
    The chemical composition of magma can vary depending on the rock that initially melts (the source rock), and process that occur during partial melting and transport.
    Initial Composition of Magma
    The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting.   In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas.  Melting of crustal sources yields more siliceous magmas.
    In general more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions can be generated. So, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma.
    Magmatic Differentiation
    But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.   These processes are referred to asmagmatic differentiation and include assimilation, mixing, and fractional crystallization.

    Assimilation - As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous.

    Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks.
    Fractional Crystallization - When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. The processes is called magmatic differentiation by Fractional Crystallization.
    Because mafic minerals like olivine and pyroxene crystallize first, the process results in removing Mg, Fe, and Ca, and enriching the liquid in silica. Thus crystal fractionation can change a mafic magma into a felsic magma.

    Crystals can be removed by a variety of processes. If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case.
    Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO2. Initially the composition of this magma is expressed as 50% SiO2 and 50% MgO. i.e.

    Now let's imagine I remove 1 MgO molecule by putting it into a crystal and removing the crystal from the magma. Now what are the percentages of each molecule in the liquid?
    If we continue the process one more time by removing one more MgO molecule

    Thus, composition of liquid can be changed.

    Bowen's Reaction Series

    Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature.  As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first.  Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase.  But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO2 rich.  If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature

    Igneous Environments and Igneous Rocks
    The environment in which magma completely solidifies to form a rock determines:
    1. The type of rock
    2. The appearance of the rock as seen in its texture
    3. The type of rock body.
    In general there are two environments to consider:
    The intrusive or plutonic environment is below the surface of the earth. This environment is characterized by higher temperatures which result in slow cooling of the magma.  Intrusive or plutonic igneous rocks form here.
    Where magma erupts on the surface of the earth, temperatures are lower and cooling of the magma takes place much more rapidly.  This is the extrusive or volcanic environment and results in extrusive or volcanic igneous rocks.
    Extrusive Environments
    When magmas reach the surface of the Earth they erupt from a vent called a volcano.  They may erupt explosively or non-explosively.
    • Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas and sometimes rhyolitic magma).
      • Usually begin with fire fountains due to release of dissolved gases
      • Produce lava flows on surface
      • Produce Pillow lavas if erupted beneath water

    • Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).
      • Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure
      • High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface.
      • Bursting of bubbles fragments the magma into pyroclasts and tephra (ash).
      • Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

    Tephra that falls from the eruption column produces a tephra fall deposit.EruptColumn.GIF (17691 bytes)
    If eruption column collapses a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed.  This is the most dangerous type of volcanic eruption.  The deposits that are produced are called ignimbrites.PyroclasFlow.GIF (12927 bytes)
    Intrusive Environments
    Magma that cools at depth form bodies of rocks called intrusive bodies or plutonic bodies called plutons, from Greek god of the underworld - Pluto. When magma intrudes it usually affects the surrounding rock and is also affected by the surrounding rock.  It may metamorphose the surrounding rocks or cause hydrothermal alteration. The magma itself may also cool rapidly near the contact with the surrounding rock and thus show a chilled margin next to the contact.  
    It may also incorporate pieces of the surrounding rocks without melting them.  These incorporated pieces are called xenoliths (foreign rocks).

    Magma intrudes by injection into fractures in the rock and expanding the fractures.   The may also move by a process called stoping, wherein bocks are loosened by magma at the top of the magma body with these blocks then sinking through the magma to accumulate on the floor of the magma body. 
    In relatively shallow environments intrusions are usually tabular bodies like dikes and sills or domed roof bodies called laccoliths.
    • Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude.  Discordant means that they cut across preexisting structures.  They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth.
    dike.gif (5977 bytes)
    • Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude.  Sills usually are fed by dikes, but these may not be exposed in the field. 
    sill.gif (4277 bytes)
    • Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion.  They are also concordant types of intrusions.
    laccolith.gif (9944 bytes)

    Deeper in the earth intrusion of magma can form bulbous bodies called plutons and the coalescence of many plutons can form much larger bodies called batholiths.
    • Plutons are large intrusive bodies, of any shape that intrude in replace rocks in an irregular fashion. 
    • Stocks are smaller bodies that are likely fed from deeper level batholiths.  Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed.

    • If multiple intrusive events occur in the same part of the crust, the body that forms is called abatholith.  Several large batholiths occur in the western U.S. - The Sierra Nevada Batholith, the Coast Range Batholith, and the Idaho Batholith, for example (See figure 6.10d in your text).
    batholith.gif (8597 bytes)

    During a magmatic event there is usually a close relationship between intrusive activity and extrusive activity, but one cannot directly observe the intrusive activity.   Only after erosion of the extrusive rocks and other rock above the intrusions has exposed the intrusions do they become visible at the earth's surface (see figure 6.10a in your text).
    The rate of cooling of magma depends largely on the environment in which the magma cools.   Rapid cooling takes place on the Earth's surface where there is a large temperature contrast between the atmosphere/ground surface and the magma.  Cooling time for material erupted into air and water can be as short as several seconds.   For lava flows cooling times are on the order of days to weeks.   Shallow intrusions cool in months to years and large deep intrusions may take millions of years to cool.

    Because cooling of the magma takes place at a different rate, the crystals that form and their interrelationship (texture) exhibit different properties.
    • Fast cooling on the surface results in many small crystals or quenching to a glass. Gives rise to aphanitic texture (crystals cannot be distinguished with the naked eye), or obsidian (volcanic glass).
    • Slow cooling at depth in the earth results in fewer much larger crystals, gives rise to phaneritic texture.
    • Porphyritic texture develops when slow cooling is followed by rapid cooling. Phenocrysts = larger crystals, matrix orgroundmass = smaller crystals.
    Classification of Igneous Rocks 

    Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization.   You will explore the classification of igneous rocks in the laboratory portion of this course.

    Extrusive/Volcanic Rocks
    Basalts, Andesites, and Rhyolites are all types of volcanic rock distinguished on the basis of their mineral assemblage and chemical compostion (see figure 6.13 in your text).  These rocks tend to be fine grained to glassy or porphyritic.  Depending on conditions present during eruption and cooling, any of these rock types may form one of the following types of volcanic rocks.
    • Obsidian - dark colored volcanic glass showing concoidal fracture and few to no crystals. Usually rhyolitic .
    • Pumice - light colored and light weight rock consisting of mostly holes (vesicles) that were once occupied by gas, Usually rhyolitic or andesitic.
    • Vesicular rock - rock filled with holes (like Swiss cheese) or vesicles that were once occupied by gas. Usually basaltic and andesitic.
    • If vesicles in a vesicular basalt are later filled by precipitation of calcite or quartz, the fillings are termed amygdules and the basalt is termed an amygdularl basalt.
    • Pyroclasts = hot, broken fragments. Result from explosively ripping apart of magma. Loose assemblages of pyroclasts called tephra. Depending on size, tephra can be classified as bombs. lapilli, or ash.
    • Rock formed by accumulation and cementation of tephra called a pyroclastic rock or tuff. Welding, compactioncause tephra (loose material) to be converted in pyroclastic rock.

    Intrusive/Plutonic Igneous Rocks
    Shallow intrusions like dikes and sills are usually fine grained and sometimes porphritic because cooling rates are similar to those of extrusive rocks.   Classification is similar to the classification for volcanic/extrusive rocks.  Coarse grained rocks, formed at deeper levels in the earth include gabbros, diorites, and granites.  Note that these are chemically equivalent to basalts, andesites, and rhyolites, but may have different minerals or different proportions of mineral because their crystallization history is not interrupted as it might be for extrusive rocks (see figure 6.13 in your text).
    Pegmatites are very coarse grained igneous rocks consisting mostly of quartz and feldspar as well as some more exotic minerals like tourmaline, lepidolite, muscovite.  These usually form dikes related to granitic plutons.
    Distribution of Igneous Activity
    Igneous activity is currently taking place as it has in the past in various tectonic settings.   These include diverging and converging plate boundaries, hot spots, and rift valleys.

    Divergent Plate Boundaries
    At oceanic ridges, igneous activity involves eruption of basaltic lava flows that form pillow lavas at the oceanic ridges and intrusion of dikes and plutons beneath the ridges.   The lava flows and dikes are basaltic and the plutons mainly gabbros.   These processes form the bulk of the oceanic crust as a result of sea floor spreading.  Magmas are generated by decompression melting as hot solid asthenosphere rises and partially melts.
    Convergent Plate Boundaries
    Subduction at convergent plate boundaries introduces water into the mantle above the subduction and causes flux melting of the mantle to produce basaltic magmas.  These rise toward the surface differentiating by assimilation and crystal fractionation to produce andesitic and rhyolitic magmas.  The magmas that reach the surface build island arcs and continental margin volcanic arcs built of basalt, andesite, and rhyolite lava flows and pyroclastic material.  The magmas that intrude beneath these arcs can cause crustal melting and form plutons and batholiths of diorite and granite

    Hot Spots
    As discussed previously, hot spots are places are places where hot mantle ascends toward the surface as plumes of hot rock.  Decompression melting in these rising plumes results in the production of magmas which erupt to form a volcano on the surface or sea floor, eventually building a volcanic island.  As the overriding plate moves over the hot spot, the volcano moves off of the hot spot and a new volcano forms over the hot spot.  This produces a hot spot track consisting of lines of extinct volcanoes leading to the active volcano at the hot spot.  A hot spot located beneath a continent can result in heat transfer melting of the continental crust to produce large rhyolitic volcanic centers and plutonic granitic plutons below.   A good example of a continental hot spot is at Yellowstone in the western U.S.  Occasionally a hot spot is coincident with an oceanic ridge.  In such a case, the hot spot produces larger volumes of magma than normally occur at ridge and thus build a volcanic island on the ridge.  Such is the case for Iceland which sits atop the Mid-Atlantic Ridge.
    Rift Valleys
    Rising mantle beneath a continent can result in extensional fractures in the continental crust to form a rift valley.  As the mantle rises it undergoes partial melting by decompression, resulting in the production of basaltic magmas which may erupt as flood basalts on the surface.   Melts that get trapped in the crust can release heat resulting in melting of the crust to form rhyolitic magmas that can also erupt at the surface in the rift valley.  An excellent example of a continental rift valley is the East African Rift. 
    Large Igneous Provinces
    In the past, large volumes of mostly basaltic magma have erupted on the sea floor to form large volcanic plateaus, such as the Ontong Java Plateau in the eastern Pacific.   Such large volume eruptions can have affects on the oceans because they change the shape of ocean floor and cause a rise in sea level, that sometimes floods the continents.   The plateaus form obstructions which can drastically change ocean currents. These changes in the ocean along with massive amounts of gas released by the magmas can alter climate and have drastic effects on life on the planet. 

    330 million years old volcano discovered near Mullingar, Co Westmeath

    How Mullingar might look with a volcano nearby
    Three hundred million year old volcanoes have been found under the midlands.

    A low flying air ship utilizing the most recent mapping innovation has revealed the 330 million-year-old rock developments on the Westmeath/Offaly outskirt south of Mullingar.

    It additionally indicated groups of volcanic rocks a few kilometers under the ground close Strokestown, Co Roscommon – some piece of a noteworthy deficiency line that can be followed through Ireland to Scotland.

    The revelations were made in the most recent piece of the Government's Tellus program.

    Flying at 90metres, air ship use innovation to see through Ireland's profound frigid stores and broad peat spread.

    Specialists said the destinations will be of enthusiasm to organizations watchful for mineral stores.
    Beam Scanlon, important geologist at the Geological Survey of Ireland, said: "Tellus keeps on uncovering unprecedented new detail in Ireland's geographical scene covered underneath our feet, expanding after existing information holes and creating regular asset opportunities.

    "A comprehension of Ireland's topography is indispensable for natural, wellbeing and financial reasons and the information will be invited by a wide scope of partners for agrarian, radon aversion, groundwater security and mineral investigation purposes."

    Tellus plans to finish a topographical jigsaw of the island of Ireland and wants to have a large portion of the nation secured before the end of 2017.

    The Midlands discoveries are huge as they make up another bit of the jigsaw which started in 2007.

    The fourth period of Tellus is in progress crosswise over eastern Ireland where the overview over Offaly, Kildare , Meath , Dublin and northern parts of Wicklow and Laois is just about 60 for every penny complete.

    Right now the undertaking is centered around country district Dublin .

    The uniquely prepared flying machine conveys three instruments on load up measuring attraction, conductivity and regular radiation of the stones and soils underneath.

    Tellus is a piece of Ireland's earth science office, the Geological Survey of Ireland, established in 1845.

    The overview is in charge of gathering land data and giving guidance and data including maps, reports and databases.

    It is a Department's piece of Communications, Energy and Natural Resources and has around 50 staff.

    Geology: Definitely NOT a Boring Science!!

    I recently shifted my major in college to geoscience so that I could finally pursue a long-time, childhood interest of mine: paleontology. I love paleontology. I absolutely adore the idea of studying ancient creatures that are long-extinct, and yet are the precursors of life on Earth today. I think our knowledge of evolution is truthfully amazing, and the thought that there is a field of science that studies evolution in a broad context is awesome to me. Paleontology has strong (and obvious) ties to geology (because you have to dig the damned fossils out of the dirt to get to them), and so there is quite a lot to learn about the rocks you’ll be digging in before you can get to the “goods”, so to speak. What’s actually the most surprising is that there is a rather staggering amount one can learn from studying geology; there’s more to it than just rocks!

    With that thought in mind, it always interested me that most people think of geology as being one of the more “boring” sciences. I really have no way of relating to the idea that any field of science is boring, and so this line of thought really intrigues me from a sort of interpersonal academic kind of direction. Whenever I hear the words “geology is boring” I am always the first to jump up and ask: “How could volcanoes, continental drift, catastrophic disasters and mountain building be boring? It’s awesome!” Usually when I say that sort of thing, the response of “well yeah, those things are cool, but geology itself is really boring” is usually what I get in return. I’m always a little dumbfounded by that response. In essence, all of those things are what make geology REALLY awesome and fun to study, making it worthy of serious research as one of the hard sciences, but people usually don’t think of those things when they think of geology, and there are a few reasons for that (that I will get to later). For this blog posting, I want to begin by covering what geology is and why it is an important science (also illustrating along the way why it is that geology is awesome), and then I want to wrap this up with my thoughts on why many people don’t share my enthusiasm for it.
     Do you know what this is? Do you know why it looks that way?
    Understanding geological processes is interesting for many reasons. The academic interest of just wanting to know more about the world is one of the big attractions for many people to get into the field, as studying geology can give a researcher a lot of insight into the processes that have built the geography of the planet we live on in a very interesting deep time perspective. Geology gives a certain amount of deep context and meaning to things that we might just normally look at as, let’s say, interesting formations on a streambed, or something. Studying the rocks of the Earth might seem like a pretty basic and almost boring science, but it gives deep insight into things such as volcanism or erosion, forces that shape the world around us even today.
    On fire: Kawika Singson was shooting in the volcanoes of Hawaii, which was so hot his tripod and shoes caught alight
    The science of geology also allows scientists to study more outwardly exciting topics such as volcanism, making those oh-so-awesome lava flows we see on National Geographic every now and again that much more exciting for a very simple reason: we can not only know, but inform people as to why that happens. It changed your perspective on your home planet when you realize that there are oceans of molten rock beneath your feet, heated by immense pressure, blasting through the surface from time to time. In understanding volcanism, one also begins to understand plate tectonics. Think about the idea of plate tectonics for a minute; the surface of the Earth being broken up into multiple gigantic plates of rock is still a new idea (comparatively speaking), and continental drift is fascinating in its own right. Just knowing a little about the theory of plate tectonics really takes your mind into a sort of psychological time-warp, because you suddenly realize that your world is dynamic, and has been changing for hundreds of millions, even BILLIONS of years. That kind of perspective on time is brought to you courtesy of your friendly neighborhood geologist.

    Figuring out the ages of rocks also falls under the blanket of geological science. This is one of the places where geology crosses over into two other fields, these being physics and chemistry. To know the age of a rock, one needs to take a sample of the rock and vaporize it in a mass spectrometer so that one can read the spectral lines to determine its chemical content. The kind of understanding required to comprehend those lines of color (or lack thereof in terms of absorption lines) requires an understanding of physics with direct reference to atomic structure and electron orbitals; this also requires knowledge of chemistry to really grasp exactly what that sample was made of. The dating of rocks usually falls into one of two columns: absolute dating and relative dating. Absolute dating relies on the ratio of original to what are usually called “daughter” isotopes within a given sample of rock as unstable isotopes of many elements decay over time. The understanding of why those atoms decay the way they do requires another venture into physics to understand quantum tunneling and the Weak Nuclear Force, fields of research that are fascinating in their own right. Erosion, which is another phenomenon that geologists study, is often influenced not only by mechanical factors such as landslides, rushing rivers, etc. (which have gravity involved in their processes, which requires another aside into physics) but also by chemical factors, which requires a geologist to more fully delve into chemistry. In this way we can see geology as being a science that is fundamentally intertwined with other major sciences, creating a blend of knowledge that many might not imagine was originally there.

    But physics and chemistry are not the only sciences contributing to greater geological knowledge; astronomy has something to say, as well. Some geological processes, like the erosion of a coastal cliff-face due to the pounding of the waves, have astronomical machinery at work. The tides are driven by the gravitational attraction of the Moon on the Earth’s oceans, causing large bodies of water to oscillate in tides that help shape and/or destroy different features on coastlines everywhere. The noticeable gravitational tug on our tiny world from both the Sun and Moon may also play a role in how tectonic plates move and help further shape the planet we live on. Many geologists also study climate, both ancient and modern, which requires at least some understanding of things like the solar wind, atmospheric chemical composition, etc. Though astronomy may seem far away from everyday geologic study, it sometimes stands at the forefront.

    The study of geology also yields clues as to how and why certain organisms evolve the way they did. We have to keep in mind here that biological evolution is not only driven by things like predation and sex, but also by climate, weather, erosion, uplift, deposition, ocean currents (which can be changed due to the movement of tectonic plates) and many other factors that have their roots in geology. While the slow uplift of a mountain range might not seem like it would effect such a plastic and adaptable thing such as life, that mountain range may one day splinter a single population of organisms into two, three, or four different groups, giving evolution lots to work with and do its magic on. Paleontology, with these thoughts in mind, is sort of where geology and biology collide and create a science that is both and neither at the same time. In my own opinion, it is the interrelatedness of geology with all the other sciences that makes it so interesting and magnificent as its own study.

    Geology also has a ton of subfields, such as:

    So why don’t more people get really excited about geology shows on TV, geology lectures, or anything else related to such a fascinating science? I think the author of the “For Dummies” edition on Geology said it best. The author mentioned that rocks are everyday things, and so we don’t really think about them as being important, because they are essentially everywhere. A geologist might tell you something truly amazing, but because of that association with the mundane it might not be paid attention to, whereas an astronomer can say something relatively mundane about their own field and be thought of as sharing truthfully groundbreaking information simply because stars are far outside our everyday experience (other than twinkling in the night sky, that is). We associate geology with the mundane, and so, to us, it is immensely boring. I think we have also built a cultural picture of what a geologist is, as well. We picture geologists as boring, verbose little men that say a lot of big words, and to us that is unappealing. In a way, we’ve done that with all the sciences by constantly depicting scientists as often short-sighted and socially inept people wearing labcoats and stinking of Firefly fandom. In a way, interest in geology suffers from social conceptions of what geology is and what geologists are, but it also suffers because rocks are freakin’ everywhere, and looking at a rock is almost never fun.
     Sure isn’t boring to me.

    Just a thought.