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 tectonics. Show all posts
Showing posts with label tectonics. Show all posts

3D Geological Model of Pakistan

We are GLAD to inform you that one of our admin, Muhammad Qasim Mehmood with his team have prepared a geological model which was presented at All Pakistan Science Fest hosted by UET Science Society at 20/04/17. 

Here is the brief introduction of model:

It is a 3D geological model of Pakistan that shows mainly tectonic division of mountain ranges of Pakistan. The model demonstrates the major/famous deposits of Pakistan like Petroleum, Minerals/Gemstones, Uranium, Coal including other geographical features like dams and rivers.

It is a non-working model (size: 5
6 ft approximately) supported mainly by wooden boards and other cementing material. It is using thermocol sheets, maps, large paper sheets, graphs, paints and mechanical and scientific goods as per the requirement of a particular model.

This unique model cover the topics of Plate Tectonics, Structural Geology, Economic Geology and others. Also the students have added the future aspects of Geo-economics like Kalabagh Dam, CPEC route, oil and gas wells in Baluchistan and offshore wells in Arabian Sea near Gwadar.

The model is showing the following geological aspects of Pakistan:

1. Major Geological Basins of Pakistan i.e. Indus Basin and Balochistan Basin
2. Major Thrusts in Northern Pakistan
     Main Karakoram Thrust (MKT)
     Main Mantle Thrust (MMT)
     Main Central Thrust (MCT)
     Main Boundary Thrust (MBT)
     Salt Range Thrust / Himalayan Frontal Thrust
3.  Mountain Ranges of Pakistan
     Some mountain ranges of Pakistan is shown on the model located in North-West to              South-West of Pakistan which has important geological significance in distinguishing            Indus Basin from Balochistan Basin
4.  Famous Peaks of Pakistan
     Mount Godwin-Austen (k2) - World's 2nd highest peak
     Nanga Parbat ( The Killer Mountain) - World's 9th highest peak
     Tirich Mir - highest peak in Hindukush Range

5. Major Fuel of Pakistan
     Oil wells in Potwar Plateau and in Balochistan
     Gas wells in Sui, Balochistan - biggest gas reserve in Pakistan
     Coal reserves in Thar - World's 16th largest coal reserve in Pakistan
     Uranium reserves in Siwalik Hills west of Dera Ghazi Khan
6. Famous Gem Stone of Pakistan
    Emerald from Mingora, Swat 
    Aquamarine from Hunza Valley,Gilgit-Baltistan    Tourmaline from Skardu District, Gilgit Baltistan

And two future prospects for the improvement of Geo-economics of Pakistan:
1. Kalabagh Dam (to be made)
2. China Pakistan Economic Corridor -CPEC (construction under process) western route.

Following are some of the maps (obtained from internet) which we consider during the preparing of our model

Map showing Geological Basins of Pakistan
Source: GSP

Tectonic Map of Pakistan
Source: GSP

Political Map of Pakistan
Source: Unknown
Tectonic Map of Pakistan & India showing major regional thrusts
Source: Unknown
CPEC map
Source: CPEC website
And some photos captured during the preparation of model

Cutting of thermocol sheet

coloring thermocol sheet with finishing paint

Hasan creating "finishing of paint" with paint spatula scraper

final look of Stage 1
Umer Amin sketching map on model and fixing sticks for projections and heights

sketches of mountain ranges and river tributaries and sticks for average height of each range and peak
all things are made perfect due to plotting of each point according to longitude and latitude

 a great Atlas Book

maps and maps

a rough look of model showing mountain ranges made with Plaster of Paris
Completion of Stage 2

team work!!!

after using distemper paint

And finally after painting and drawing river tributaries, fixing sign boards of cities and much more, the model is:

 3D Geological Model of Pakistan

 3D Geological Model of Pakistan

Legend for the model

Geological tools, Gemstones, Rocks and Fuel (Coal and Crude Oil)
Featuring Qasim Mehmood (Co founder of Learning Geology), on left
Rana Faizan Saleem, my class mate and Founder of Geology for Beginners
Students of Institute of Geology, University of the Punjab

Geologic Contacts

A geologic contact is where one rock type touches another. There are three types of geologic contact:1. Depositional contacts are those where a sedimentary rock (or a lava flow) was deposited on an older rock
2. Intrusive contacts are those where one rock has intruded another
3. Fault contacts are those where rocks come into contact across fault zones.
Learn in detail about fault here

Following are the some pictures showing each type of geologic contact

Depositional Contacts

1. Angular Unconformity, Siccar Point, Scotland

This place is known as Siccar Point which is the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.
gently sloping strata of 370-million-year-old Famennian Late Devonian Old Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.

2. Cretaceous Sandstone overlying Conglomerate    Kootenai Formation, SW Montana

Photo Courtesy:

3. Dun Briste Sea Stack, IrelandDun Briste is a truly incredible site to see but must be visited to appreciate its splendour. It was once joined to the mainland. The sea stack stands 45 metres (150 feet) tall.

Dun Briste and the surrounding cliffs were formed around 350 million years ago (during the 'Lower Carboniferous Period'), when sea temperatures were much higher and the coastline at a greater distance away.  There are many legends describing how the Sea Stack was formed but it is widely accepted that an arch leading to the rock collapsed during very rough sea conditions in 1393. This is remarkably recent in geological terms

Photo Courtesy: 

Fault Contacts

1. Normal Faulting in the Cutler Formation near Arches National Park

Photo Courtesy:

2. Normal Fault in Titus Canyon, Death Valley, California 

Photo Courtesy:

Horst and Graben Structure in Zanjan, Iran

Photo Courtesy: Amazhda

Intrusive Contacts 

Pegmatite and aplite dikes and veins in granitic rocks on Kehoe Beach, Point Reyes National Seashore, California.

2. Spectacular mafic dyke from Isla de Socorro from Pep Cabré. The Isla de Socorro is a volcanic island off the west coast of Mexico and it is the only felsic volcano in the Pacific Ocean

Photo Courtesy:

3. The margins of this Granite dyke cooled relatively quickly in contact with this much older Gabbro.
Photo near Ai-Ais Namibia

Photo Courtesy: travelinggeologist

Continental Accretion and Plate Tectonics Model

Continental Accretion

Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features, or blocks or pieces of continental crust split from other continental plates. Over "geologic time" (measured in millions of years), volcanic arcs form and may be crushed onto (or between) colliding continents with plate boundaries. Pieces of continental land masses may be ripped away and carried to other locations. For instance, Baja California and parts of southern California west of the San Andreas Fault are being ripped away from the North American continent and are slowly being carried northward. These rocks may eventually pass what-is-now San Francisco, and perhaps 70 to 100 million years from now will be crushed and accreted into the landmass currently known as Alaska!
Plate tectonics model: 
Subduction introduces oceanic crustal rocks (including sediments) back into the Asthenosphere. Water and gas helps low-temperature minerals to melt and rise as, forming new continental crust (less dense than oceanic crust). Floating on the Asthenosphere, the continental crustal materials accumulate, forming continents.

Plate Tectonic Model
Photo Courtesy: Phil Stoffer

The processes associated with subduction lead to the accretion (growth) of continents over time. As ocean crust is recycled back into the upper mantle, the lighter material "accumulates" along continental margins. Pieces of lithosphere are sometimes scraped off one plate and crushed onto and added to another plate.  

Photo Courtesy: Phil Stoffer

Relation of Volcanism to Plate Tectonics

Relation of Volcanism to Plate Tectonics 

A map showing the distribution of volcanoes around the world and the basic geologic settings in which volcanoes form, in the contact of plate tectonics theory.
Different styles of volcanism occur at different locations on Earth. Most eruptions occur along plate boundaries, but major eruptions also occur at hot spots (figure above). We’ll now look at the settings in which eruptions occur, in the context of plate tectonics theory and see why different kinds of volcanoes form in different settings.

What Drives Plate Motion, and How Fast Do Plates Move?

What Drives Plate Motion, and How Fast Do Plates Move? 

Forces Acting on Plates 

We've now discussed the many facets of plate tectonics theory but to complete the story, we need to address a major question: “What drives plate motion?” When geoscientists first proposed plate tectonics, they thought the process occurred simply because convective flow in the asthenosphere actively dragged plates along, as if the plates were simply rafts on a flowing river. Thus, early images depicting plate motion showed simple convection cells elliptical  flow paths in the asthenosphere. At first glance, this hypothesis looked pretty good. But, on closer examination it became clear that a model of simple convection cells carrying plates on their backs can’t explain the complex geometry of plate boundaries and the great variety of plate motions that we observe on the Earth. Researchers now prefer a model in which convection, ridge push, and slab pull all contribute to driving plates. Let’s look at each of these phenomena in turn.

How Do Plate Boundaries Form and Die?

How Do Plate Boundaries Form and Die? 

The configuration of plates and plate boundaries visible on our planet today has not existed for all of geologic history, and will not exist indefinitely into the future. Because of plate motion, oceanic plates form and are later consumed, while continents merge and later split apart. How does a new  divergent boundary come into existence, and how does an existing convergent boundary eventually cease to exist? Most new divergent boundaries form when a continent splits and separates into two continents. We call this process rifting. A convergent boundary ceases to exist when a piece of buoyant lithosphere, such as a continent or an island arc, moves into the subduction zone and, in effect, jams up the system. We call this process collision.

Special Locations in the Plate Mosaic

Special Locations in the Plate Mosaic 

Triple Junctions 

Examples of triple junction. The triple junction are marked by dots.
Geologists refer to a place where three plate boundaries intersect as a triple junction, and name them after the types of boundaries that intersect. For example, the triple junction formed where the Southwest Indian Ocean Ridge intersects two arms of the Mid–Indian Ocean Ridge (this is the triple junction of the African, Antarctic, and Australian Plates) is a ridge-ridge-ridge triple junction (figure above a). The triple junction north of San  Francisco is a trench-transform-transform triple junction (figure above b).

Hot Spots 

 The dots represent the locations of selected hot-spot volcanoes. The red lines represent hot-spot tracks. The most recent volcano (dot) is at one end of this track. Some of these volcanoes are extinct, indicating that the mantle plume no longer exists. Some hot spots are fairly recent and do not have tracks. Dashed tracks were broken by sea-floor spreading.
Most subaerial (above sea level) volcanoes are situated in the volcanic arcs that border trenches. Volcanoes also lie along mid-ocean ridges, but ocean water hides most of them. The volcanoes of volcanic arcs and mid-ocean ridges are plate boundary volcanoes, in that they formed as a consequence of movement along the boundary. Not all volcanoes on Earth are plate-boundary volcanoes, however. Worldwide, geoscientists have identified about 100 volcanoes that exist as isolated points and are not a consequence of movement at a plate boundary. These are called hotspot volcanoes, or simply hot spots (figure above). Most hot spots are located in the interiors of plates, away from the boundaries, but a few lie along mid-ocean ridges. What causes hot-spot volcanoes? In the early 1960s, J. Tuzo Wilson noted that active hot-spot volcanoes (examples that are erupting or may erupt in the future) occur at the end of a chain of dead volcanic islands and seamounts (formerly active volcanoes that will never erupt again). This configuration is different from that of volcanic arcs along convergent plate boundaries at volcanic arcs, all of the volcanoes are active. With this image in mind, Wilson suggested that the position of the heat source causing a hotspot volcano is fixed, relative to the moving plate. In Wilson’s model, the active volcano represents the present-day location of the heat source, whereas the chain of dead volcanic islands represents locations on the plate that were once over the heat source but progressively moved off.

The deep mantle plume hypothesis for the formation of hot-spot tracks.
A few years later, researchers suggested that the heat source for hot spots is a mantle plume, a column of very hot rock rising up through the mantle to the base of the lithosphere (figure above a–d). In this model, plumes originate deep in the mantle. Rock in the plume, though solid, is soft enough to flow, and rises buoyantly because it is less dense  than surrounding cooler rock. When the hot rock of the plume reaches the base of the lithosphere, it partially melts and produces magma that seeps up through the lithosphere to the Earth’s surface. The chain of extinct volcanoes, or hot-spot track, forms when the overlying plate moves over a fixed plume. This movement slowly carries the volcano off the top of the plume, so that it becomes extinct. A new, younger volcano grows over the plume. 
The Hawaiian chain provides an example of the volcanism associated with a hot-spot track. Volcanic  eruptions occur today only on the big island of Hawaii. Other islands to the northwest are remnants of dead volcanoes, the oldest of which is Kauai. To the northwest of Kauai, still older volcanic remnants are found. About 1,750 km northwest of Midway Island, the track bends in a more northerly direction, and the volcanic remnants no longer poke above sea level; we refer to this northerly trending segment as the Emperor seamount chain. Geologists suggest that the bend is due to a change in the direction of Pacific Plate motion at about 40 Ma. 
Some hot spots lie within continents. For example, several have been active in the interior of Africa, and one now underlies Yellowstone National Park. The famous geysers (natural steam and hot-water fountains) of Yellowstone exist because hot magma, formed above the Yellowstone hot spot, lies not far below the surface of the park. While most hot spots, such as Hawaii and Yellowstone, occur in the interior of plates, away from plate boundaries, a few are positioned at points on mid-ocean ridges. The additional magma production associated with such hot spots causes a portion of the ridge to grow into a mound that can rise significantly above normal ridgeaxis depths and protrude above the sea surface. Iceland, for example, is the product of hot-spot volcanism on the axis of the Mid-Atlantic Ridge.
Credits: Stephen Marshak (Essentials of Geology)

Transform Plate Boundaries

Transform Plate Boundaries

The concept of transform faulting.
When researchers began to explore the bathymetry of midocean ridges in detail, they discovered that mid-ocean ridges are not long, uninterrupted lines, but rather consist of short segments that appear to be offset laterally from each other (figure above a) by narrow belts of broken and irregular sea floor. These belts, or fracture zones, lie roughly at right angles to the ridge segments, intersect the ends of the segments, and extend beyond the ends of the segments. Originally, researchers incorrectly assumed that the entire length of each fracture zone was a fault, and that slip on a fracture zone had displaced segments of the mid-ocean ridge sideways, relative to each other. In other words, they imagined that a mid-ocean ridge initiated as a continuous, fence-like line that only later was broken up by faulting. But when information about the distribution of earthquakes along mid-ocean ridges became available, it was clear that this model could not be correct. Earthquakes, and therefore active fault slip, occur only on the segment of a fracture zone that lies between two ridge segments. The portions of fracture zones that extend beyond the edges of ridge segments, out into the abyssal plain, are not seismically active.

Convergent Plate Boundaries and Subduction

Convergent Plate Boundaries and Subduction 

At convergent plate boundaries, two plates, at least one of which is oceanic, move toward one another. But rather than butting each other like angry rams, one oceanic plate bends and sinks down into the asthenosphere beneath the other plate. Geologists refer to the sinking process as subduction, so convergent boundaries are also known as subduction zones. Because subduction at a convergent boundary consumes old ocean lithosphere and thus ‘‘consumes’’ oceanic basins, geologists also refer to convergent boundaries as consuming boundaries, and because they are delineated by deep-ocean trenches, they are sometimes simply called trenches. The amount of oceanic plate consumption worldwide, averaged over time, equals the amount of sea-floor spreading worldwide, so the surface area of the Earth remains constant through time. 

During the process of subduction, oceanic lithosphere sinks back into the deeper mantle.

Divergent Plate Boundaries and Sea-Floor Spreading

Divergent Plate Boundaries and Sea-Floor Spreading 

The process of sea-floor spreading.
At a divergent boundary, or spreading boundary, two oceanic plates move apart by the process of sea-floor spreading. Note that an open space does not develop between diverging plates. Rather, as the plates move apart, new oceanic lithosphere forms continually along the divergent boundary (figure above a). This process takes place at a submarine mountain range called a mid-ocean ridge that rises 2 km above the adjacent abyssal plains of the ocean. Thus, geologists commonly refer to a divergent boundary as a mid-ocean ridge, or simply a ridge. Water depth above ridges averages about 2.5  km. 
To characterize a divergent boundary more completely, let’s look at one mid-ocean ridge in more detail (figure above b). The Mid-Atlantic Ridge extends from the waters between northern  Greenland and northern Scandinavia southward across the equator to the latitude of the southern tip of South America. Geologists have found that the formation of new sea floor takes place only along the axis (centerline) of the ridge, which is marked by an elongate valley. The sea floor slopes away, reaching the depth of the abyssal plain (4 to 5 km) at a distance of about 500 to 800 km from the ridge axis. Roughly speaking, the Mid-Atlantic Ridge is symmetrical its eastern half looks like a mirror image of its western half. The ridge consists, along its length, of short segments (tens to hundreds of km long) that step over at breaks that, as we noted earlier, are called fracture zones.

What Do We Mean by Plate Tectonics?

What Do We Mean by Plate Tectonics?

The paleomagnetic proof of continental drift (plate tectonics) and the discovery of sea-floor spreading set off a scientific revolution in geology in the 1960s and 1970s. Geologists realised that many of their existing interpretations of global geology, based on the premise that the positions of continents and oceans remain fixed in position through time, were simply wrong! Researchers dropped what they were doing and turned their attention to studying the broader implications of continental drift and sea-floor spreading. It became clear that these phenomena required that the outer shell of the Earth was divided into rigid plates that moved relative to each other. New studies clarified the meaning of a plate, defined the types of plate boundaries, constrained plate motions, related plate motions to earthquakes and volcanoes, showed how plate interactions can explain mountain belts and seamount chains, and outlined the history of past plate motions. From these, the modern theory of plate tectonics evolved. Below, we first describe lithosphere plates and their boundaries, and then outline the basic principles of plate tectonics theory.

The Concept of a  Lithosphere Plate 

 Nature of the lithosphere and its behaviour.
We learned earlier that geoscientists divide the outer part of the Earth into two layers. The lithosphere consists of the crust plus the top (cooler) part of the upper mantle. It behaves relatively rigidly, meaning that when a force pushes or pulls on  it, it does not flow but rather bends or breaks (figure above a). The lithosphere floats on a relatively soft, or “plastic,” layer called the asthenosphere, composed of warmer ( 1280°C) mantle that can flow slowly when acted on by a force. As a result, the asthenosphere convects, like water in a pot, though much more slowly.
Continental lithosphere and oceanic lithosphere differ markedly in their thicknesses. On average, continental lithosphere has a thickness of 150 km, whereas old oceanic lithosphere has a thickness of about 100 km (figure above b). (For reasons discussed later in this chapter, new oceanic lithosphere at a mid-ocean ridge is much thinner.) Recall that the crustal part of continental lithosphere ranges from 25 to 70 km thick and consists largely of low-density felsic and intermediate rock. In contrast, the crustal part of oceanic lithosphere is only 7 to 10 km thick and consists largely of relatively high-density mafic rock (basalt and gabbro). The mantle part of both continental and oceanic lithosphere consists of very high-density ultramafic rock (peridotite). Because of these  differences, the continental lithosphere “floats” at a higher level than does the oceanic lithosphere. 

The location of plate boundaries and the distribution of earthquakes.
The lithosphere forms the Earth’s relatively rigid shell. But unlike the shell of a hen’s egg, the lithospheric shell contains a number of major breaks, which separate it into distinct pieces. As noted earlier, we call the pieces lithosphere plates, or simply plates. The breaks between plates are known as plate boundaries (figure above a). Geoscientists distinguish twelve major plates and several microplates. 

The Basic Principles of Plate Tectonics 

With the background provided above, we can restate plate tectonics theory concisely as follows. The Earth’s lithosphere is divided into plates that move relative to each other. As a plate moves, its internal area remains mostly, but not perfectly, rigid and intact. But rock along plate boundaries undergoes intense deformation (cracking, sliding, bending, stretching, and squashing) as the plate grinds or scrapes against its neighbours or pulls away from its neighbours. As plates move, so do the continents that form part of the plates. Because of plate tectonics, the map of Earth’s surface constantly changes.

Identifying Plate Boundaries 

How do we recognize the location of a plate boundary? The answer becomes clear from looking at a map showing the locations of earthquakes (figure above b). Recall from Chapter 1 that earthquakes are vibrations caused by shock waves that are generated where rock breaks and suddenly slips along a fault. The epicentre marks the point on the Earth’s surface directly above the earthquake. Earthquake epicentres do not speckle the globe randomly, like buckshot on a target. Rather, the majority occur in relatively narrow, distinct belts. These earthquake belts define the position of plate boundaries because the fracturing and slipping that occurs along plate boundaries generates earthquakes. Plate interiors, regions away from the plate boundaries, remain relatively earthquake-free because they do not accommodate as much movement. While earthquakes serve as the most definitive indicator of a plate boundary, other prominent geologic features also develop along plate boundaries.
Note that some plates consist entirely of oceanic lithosphere, whereas some plates consist of both oceanic and continental lithosphere. Also, note that not all plates are the same size (figure above c). Some plate boundaries follow continental margins, the boundary between a continent and an ocean, but others do not. For this reason, we distinguish between active margins, which are plate boundaries, and passive margins, which are not plate boundaries. Earthquakes are common at active margins, but not at passive margins. Along passive margins, continental crust is thinner than in  continental interiors. Thick (10 to 15 km) accumulations of sediment cover this thinned crust. The surface of this sediment layer is a broad, shallow (less than 500 m deep) region called the continental shelf, home to the major fisheries of the world. 

The three types of plate boundaries differ based on the nature of relative movement.
Geologists define three types of plate boundaries, based simply on the relative motions of the plates on either side of the boundary (figure above a–c). A boundary at which two plates move apart from each other is a divergent boundary. A boundary at which two plates move toward each other so that one plate sinks beneath the other is a convergent boundary. And a boundary at which two plates slide sideways past each other is a transform boundary.
Credits: Stephen Marshak (Essentials of Geology)

Evidence for Sea-Floor Spreading

Evidence for Sea-Floor Spreading 

For a hypothesis to become a theory, researchers must demonstrate that the idea really works. During the 1960s, geologists found that the sea-floor spreading hypothesis successfully explains several previously baffling observations. Here we discuss two: (1) the existence of orderly variations in the strength of the measured magnetic field over the sea floor, producing a pattern of stripes called marine magnetic anomalies; and (2) the variation in sediment thickness on the ocean crust, as measured by drilling.

Marine Magnetic Anomalies

Recognizing anomalies 

Geologists can measure the strength of Earth’s magnetic field with an instrument called a magnetometer. At any given location on the surface of the Earth, the magnetic field that you measure includes two parts: one produced by the main dipole of the Earth generated by circulation of molten iron in the outer core, and another produced by the magnetism of near-surface rock. A magnetic anomaly is the difference between the expected strength of the Earth’s main dipole field at a certain location and the actual measured strength of the magnetic field at that location. Places where the field strength is stronger than expected are positive anomalies, and places where the field strength is weaker than expected are negative anomalies.

The Discovery of Sea-Floor Spreading

The Discovery of Sea-Floor Spreading

New Images of Sea-Floor Bathymetry 

Bathymetry of mid-ocean ridges and abyssal plains.
Military needs during World War II gave a boost to sea-floor exploration, for as submarine fleets grew, navies required detailed information about bathymetry, or depth variations. The invention of echo sounding (sonar) permitted such information to be gathered quickly. Echo sounding works on the same principle that a bat uses to navigate and find insects. A sound pulse emitted from a ship travels down through the water, bounces off the sea floor, and returns up as an echo through the water to a receiver on the ship. Since sound waves travel at a known velocity, the time between the sound emission and the echo detection indicates the distance between the ship and the sea floor. (Recall that  velocity distance/time, so distance velocity s time.) As the ship travels, observers can obtain a continuous record of the depth of the sea floor. The resulting cross section showing depth plotted against location is called a bathymetric profile (figure above a, b). By cruising back and forth across the ocean many times, investigators obtained a series of bathymetric profiles and from these constructed maps of the sea floor. (Geologists can now produce such maps much more rapidly using satellite data.) Bathymetric maps reveal several important features.

Paleomagnetism and the Proof of Continental Drift

Paleomagnetism and the Proof of Continental Drift

More than 1,500 years ago, Chinese sailors discovered that a piece of lodestone, when suspended from a thread, points in a northerly direction and can help guide a voyage. Lodestone exhibits this behaviour because it consists of magnetite, an iron rich mineral that, like a compass needle, aligns with Earth’s magnetic field lines. While not as magnetic as lodestone, several other rock types contain tiny crystals of magnetite, or other magnetic minerals, and thus behave overall like weak magnets. In this section, we explain how the study of such magnetic behaviour led to the realization that rocks preserve paleomagnetism, a record of Earth’s magnetic field in the past. An understanding of paleomagnetism provided proof of continental drift and, contributed to the development of plate tectonics theory. As a foundation for introducing paleomagnetism, we first provide additional detail about the basic nature of the Earth’s magnetic field.

Earth’s Magnetic Field 

Features of Earth’s magnetic field.
Circulation of liquid iron alloy in the outer core of the Earth generates a magnetic field. (A similar phenomenon happens in an electrical dynamo at a power plant.) Earth’s magnetic field resembles the field produced by a bar magnet, in that it has two ends of opposite polarity. Thus, we can represent Earth’s field by a magnetic dipole, an imaginary arrow (figure above a). Earth’s dipole intersects the surface of the planet at two points, known as the magnetic poles. By convention, the north magnetic pole is at the end of the Earth nearest the north geographic pole (the point where the northern end of the spin axis intersects the surface). The north-seeking (red) end of a compass needle points to the north magnetic pole.

Wegener’s Evidence for Continental Drift

Wegener’s Evidence for Continental Drift 

Wegener suggested that a vast supercontinent, Pangaea, existed until near the end of the Mesozoic Era (the interval of geologic time that lasted from 251 to 65 million years ago). He suggested that Pangaea then broke apart, and the landmasses moved away from each other to form the continents we see today. Let’s look at some of Wegener’s arguments and see what led him to formulate this hypothesis of continental drift.

The Fit of the Continents 

Almost as soon as maps of the Atlantic coastlines became available in the 1500s, scholars noticed the fit of the continents. The northwestern coast of Africa could tuck in against the eastern coast of North America, and the bulge of eastern South America could nestle cozily into the indentation of southwestern Africa. Australia, Antarctica, and India could all connect to the southeast of Africa, while Greenland, Europe, and Asia could pack against the northeastern margin of North America. In fact, all the continents could be joined, with remarkably few overlaps or gaps, to create Pangaea. Wegener concluded that the fit was too good to be coincidence and thus that the continents once did fit together.

What Are Earth Layers Made Of?

What Are Earth Layers Made Of? 

A modern view of Earth‘s interior layers.
As a result of studies during the past century, geologists have a pretty clear sense of what the layers inside the Earth are made of. Let’s now look at the properties of individual layers in more detail (figure above a, b).

The Crust 

When you stand on the surface of the Earth, you are standing on top of its outermost layer, the crust. The crust is our home and the source of all our resources. How thick is this all important layer? Or, in other words, what is the depth to the crust-mantle boundary? An answer came from the studies of Andrija Mohorovicˇic´, a researcher working in Zagreb, Croatia. In 1909, he discovered that the velocity of earthquake waves suddenly increased at a depth of tens of kilometres beneath the Earth’s surface, and he suggested that this increase was caused by an abrupt change in the properties of rock. Later studies showed that this change can be found most everywhere around our planet, though it occurs at different depths in different locations. Specifically, it’s deeper beneath continents than beneath oceans. Geologists now consider the change to define the base of the crust, and they refer to it as the Moho in Mohorovicˇic´’s honour. The relatively shallow depth of the Moho (7 to 70 km, depending on location) as compared to the radius of the Earth (6,371 km) emphasizes that the crust is very thin indeed. In fact, the crust is only about 0.1% to 1.0% of the Earth’s radius, so if the Earth were the size of a balloon, the crust would be about the thickness of the balloon’s skin.

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)