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 glacier. Show all posts
Showing posts with label glacier. Show all posts

Guest Blog: How Speleothems Are Used To Determine Past Climates?

About author: Alex Graham is an undergraduate student at University of Newcastle, Australia. He is interested in Geology as a whole but his major interests include fluvial processes, karst systems and ocean science. During his visit to New Zealand, he has obeserved the glow worms in Waitomo Caves and spelunking in Nikau Caves.

Speleothems, more commonly known as stalactites or stalagmites, consist of calcium carbonate (calcite or aragonite) crystals of various dimensions, ranging from just a few micrometers to several centimetres in length, which generally have their growth axis perpendicular to the growth surface. Speleothems are formed through the deposition of calcium carbonate minerals in karst systems, providing archives of information on past climates, vegetation types and hydrology, particularly groundwater and precipitation. However, they can also provide information on anthropogenic impacts, landscape evolution, volcanism and tectonic evolution in mineral deposits formed in cave systems.

Stalagmite Formation
Rainfall containing carbonic acid weathers the rock unit (generally either limestone or dolomite) and seeps into the cracks, forming caverns and karst systems. The groundwater, percolating through such cracks and caverns, also contains dissolved calcium bicarbonate. The dripping action of these groundwater droplets is the driving force behind the deposition of speleothems in caves.
Core drilling of an active stalagmite in Hang Chuot cave.
Speleothems are mainly studied as paleoclimate indicators, providing clues to past precipitation, temperature and vegetation changes over the past »500,000 years. Radioisotopic dating of speleothems is the primary method used by researchers to find annual variations in temperature. Carbon isotopes (d^13C) reflect C3/C4 plant compositions and plant productivity, where increased plant productivity may indicate greater amounts of rainfall and carbon dioxide absorption. Thus, a larger carbon absorption can be reflective of a greater atmospheric concentration of greenhouse gases. On the other hand, oxygen isotopes (d^8O) provide researchers with past rainfall temperatures and quantified levels of precipitation, both of which are used to determine the nature of past climates.

Stalactite and stalagmite growth rates also indicate the climatic variations in rainfall over time, with this variation directly influencing the growth of ring formations on speleothems. Closed ring formations are indicative of little rainfall or even drought, where-as wider spaced ring formations indicate periods of heavy rainfall or flooding. These ring formations thus enable researchers to potentially predict and model the occurrence of future climatic patterns, based off the atmospheric signals extrapolated from speleothems. Researchers also use Uranium –Thorium radioisotopic dating, to determine the age of speleothems in karst formations. Once the layers have been accurately dated, researchers record the level of variance in groundwater levels over the lifetime of the karst formation. Hydrogeologists specialise in such areas of quantitative research. As a result, speleothems are widely regarded as a crucial geological feature that provide useful information for researchers studying past climates, vegetation types and hydrology.

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Explore Fascinating Geology of Lofoten Islands, Norway

It is probably going to be boring what you are going to read, but if you are a geologist, please continue reading.
 What started as a simple fun trip with some friends to Lofoten Islands in northern Norway, just became a unique geological experience. This, because I think that, as a geologist, it is completely impossible to separate fun from my profession while traveling. It's just amazing to mix your profession with your favorite hobby. 
Trying to understand the rocks, the configuration of the landscapes and their phenomena, is simply priceless.

Reinebringen Mountain, Norway.
View to the town of Reine and Fjords.
Photo Credits: J. Sebastian Guiral
This time I got completely impressed with the beauty of the Fjords in Lofoten (help: what is a fjord? well basically, a fjord is a narrow and deep channel that allows the sea to enter to the land. They can be several kilometers long, so they are often confused with rivers or lakes, and can reach great depths, exceeding 1000 m. These geomorphological units are the product of sea flooding of valleys created by glacial activity).
Reinebringen mountain, Reine, Norway.
View to the town of Reine and Kirkefjord. U-shaped valleys and geomorphological features associated with intense tectonic activity. Glacial lake
Photo Credits: J. Sebastian Guiral
Hiking through the perfectly carved U-shaped valleys left me speechless (above mentioned glacial valleys). In each valley, it was possible to appreciate the sediments associated with the activity of the glacier, that is, the Moraines (frontal and lateral), till and reworked proglacial sediments.

Skelfjord, Lofoten, Norway.
Photo Credits: J. Sebastian Guiral

In addition, the typical vegetation of Tundra is impressive (help: what is Tundra? In simple words, it is a biome characterized by the lack of trees, the soils are mainly covered with mosses and lichens, characteristic of circumpolar latitudes. The subsoil is almost permanently frozen). This vegetation covered the base of the mountain chains and snowy hills, contrasting in a perfectly artistic way and offering a breathtaking view. 

Å, Moskenes, Norway.
Mosses on Precambrian gneisses and migmatites.
Photo Credits: J. Sebastian Guiral
Reine, Lofoten Norway.
View to Reinefjorden and snowy peaks
Photo Credits: J. Sebastian Guiral
Hamnøy, Lofoten Norway. Snowy Peaks.
Photo Credits: J. Sebastian Guiral
Haukland beach, Leknes, Lofoten, Norway

Snowy Peaks at Hamnøy, Lofoten Norway.
Photo Credits: J. Sebastian Guiral
What about lithologies? Well, broadly all those landscapes are conformed by a Precambrian basement represented by an Archean and Paleoproterozoic metamorphic complexes of ortho- and paragneisses, intruded by anorthosites and suites of charnokite-granites. This basement is in tectonic contact with amphibolites and paragneisses, which were intruded by tonalitic magmas at 470 Ma. Subsequently, at the top of the sequence, in a rather complex structural context, volcano-sedimentary sequences are found, ranging from the Permian to the Paleogene. These volcano-sedimentary sequences are part of the sea floor between Greenland and Norway. All these units are in well-marked tectonic contacts.

Utakleiv Beach, Leknes, Lofoten, Norway.
Paleoproterozoic amphibolites and gneisses.
Photo Credits: J. Sebastian Guiral
Utakleiv Beach, Leknes, Lofoten, Norway.
Paleoproterozoic amphibolites and gneisses
Photo Credits: J. Sebastian Guiral 

Paleoproterozoic amphibolites and gneisses at Haukland beach, Leknes, Lofoten, Norway
Photo Credits: J. Sebastian Guiral 

Finally, in addition to the geological stuff, the sunsets, perfect beaches, rainbows, snowstorms, the strong rain and a whole bunch of climatic phenomena associated with these high latitudes, make the Lofoten Islands one of the places. I have enjoyed a lot being a geologist. 

Reine, Lofoten Norway.
View of Reinefjorden and snowy peaks
Photo Credits: J. Sebastian Guiral 

 This is what I like about this profession, trying to understand a bit about such a complex, beautiful and huge planet.

If you are a geologist and feel the same as me while traveling, let me congratulate you.

You have a beautiful profession!

Sebas enjoying rain in Å, Moskenes, Norway.
Photo Credits: J. Sebastian Guiral 
Sebas exploring Paleoproterozoic amphibolites and gneisses at Utakleiv Beach, Leknes, Lofoten, Norway.
Photo Credits: J. Sebastian Guiral 
About authorJ. Sebastian Guiral is a Geological Engineer from the National University of Colombia. He is currently pursuing his master's program in Georesources Engineering at the Luleå University of Technology in Sweden. He also has  studied at the University of Liege in Belgium and at University of Lorraine in France. As a geologist, he has worked in important engineering and research projects in his country, which include geomechanics of underground excavations, geodynamics and geomorphology. Currently, his interests are focused on economic geology, exploration, mining and mineral processing techniques. 
You can contact with J. Sebastian Gujral at [email protected] or at Instagram: @sebasguiralv

We are grateful to J. Sebastian Gujral for sharing his knowledge and adventures with us. You can also contribute share your geological adventures with us. See details here.

Consequences of Continental Glaciation

Consequences of Continental Glaciation

Ice Loading and Glacial Rebound 

The concept of subsidence and rebound, due to continental glaciation and deglaciation. (Not to scale.)
When a large ice sheet (more than 50 km in diameter) grows on a continent, its weight causes the surface of the lithosphere to sink. In other words, ice loading causes glacial subsidence. Lithosphere, the relatively rigid outer shell of the Earth, can sink because the underlying asthenosphere is soft enough to flow slowly out of the way (figure above). Because of ice loading, much of Antarctica and Greenland now lie below sea level, so if their ice were instantly to melt away, these continents would be flooded by a shallow sea.
What happens when continental ice sheets do melt away? Gradually, the surface of the underlying continent rises back up, by a process called glacial rebound, and the asthenosphere flows back underneath to fill the space. This process doesn't take place instantly, the asthenosphere flows so slowly (at rates of a few millimetres per year) that it takes thousands of years for ice-depressed continents to rebound. Thus, glacial rebound is still taking place in some regions that were covered by ice during the Pleistocene Ice Age.

Deposition Associated with Glaciation

Deposition Associated with Glaciation 

The Glacial Conveyor 

The glacial conveyor and the formation of lateral and medial moraines on glaciers.
Glaciers can carry sediment of any size and, like a conveyor belt, transport it in the direction of flow (that is, toward the toe;  figure above a). The sediment load either falls onto the surface of the glacier from bordering cliffs or gets plucked and lifted from the substrate and incorporated into the moving ice. Geologists refer to a pile of debris carried by or left by glaciers as a moraine. Sediment dropped on the glacier’s surface moves with the ice and becomes a stripe of debris. Stripes formed along the side edges of the glacier are lateral moraines. When a glacier melts, lateral moraines lie stranded along the side of the glacially carved valley, like bathtub rings. Where two valley glaciers merge, the debris constituting two lateral moraines merges to become a medial moraine, running as a stripe down the interior of the composite glacier (figure above b). Trunk glaciers created by the merging of many tributary glaciers contain several medial moraines. Sediment transported to a glacier’s toe by the glacial conveyor accumulates in a pile at the toe and builds up to form an end moraine.

Carving and Carrying by Ice

Carving and Carrying by Ice

Glacial Erosion and Its Products 

Products of glacial erosion. Ice is a very aggressive agent of erosion.
During the last ice age, valley glaciers cut deep, steep-sided valleys into the Sierra Nevada mountains of California. In the process, some granite domes were cut in half, leaving a rounded surface on one side and a steep cliff on the other. Half Dome, in Yosemite National Park, formed in this way (figure above a); its steep cliff has challenged many rock climbers. Such glacial erosion also produces the knife-edge ridges and pointed spires of high mountains (figure above b) and broad expanses where rock outcrops have been stripped of overlying sediment and polished smooth (figure above c). In many localities, the rock surface visible today is the same rock surface once in contact with ice. In some places, subsequent rockfalls and river erosion have substantially modified the surface.
As glaciers flow, clasts embedded in the ice act like the teeth of a giant rasp and grind away the substrate. This process, glacial abrasion, produces long gouges, grooves, or scratches called glacial striations (figure above d). Striations range from 1 cm to 1 m across and may be tens of centimeters to tens of meters long. As you might expect, striations run parallel to the flow direction of the ice. Rasping by embedded sand yields shiny glacially polished surfaces. 
Glaciers pick up fragments of their substrate in several ways. During glacial incorporation, ice surrounds debris so the debris starts to move with the ice. During glacial plucking (or glacial quarrying), a glacier breaks off fragments of bedrock. Plucking occurs when ice freezes around rock that has just started to separate from its substrate, so that movement of the ice can lift off pieces of the rock. At the toe of a glacier, ice may actually bulldoze sediment and trees slightly before flowing over them. 

Landscape features formed by the glacial erosion of a mountains landscape.
Let’s now look more closely at the erosional features associated with a mountain glacier (figure above a). Freezing and thawing during the fall and spring help fracture the rock bordering the head of the glacier (the ice edge high in the mountains). This rock falls on the ice or gets picked up at the base of the ice, and moves downslope with the glacier. As a consequence, a bowl-shaped depression, or cirque, develops on the side of the mountain. If the ice later melts, a lake called a tarn may form at the base of the cirque. The shape of a cirque may be maintained or even amplified by rockfalls after the glacier is gone. An arête (French for ridge), a residual knife-edge ridge of rock, separates two adjacent cirques. A pointed mountain peak surrounded by at least three cirques is called a horn. The Matterhorn, a peak in Switzerland, is a particularly beautiful example of a horn; each of its four faces is a cirque (figure above b).
Glacial erosion severely modifies the shape of a valley. To see how, compare a river-eroded valley with a glacially eroded valley. If you look along the length of a river in unglaciated mountains, you’ll see that it typically flows down a V-shaped valley, with the river channel forming the point of the V. The V develops because river erosion occurs only in the channel, and mass wasting causes the valley slopes to approach the angle of repose. But if you look down the length of a glacially eroded valley, you’ll see that it resembles a U, with steep walls. A U-shaped valley (figure above c) forms because the combined processes of glacial abrasion and plucking not only lower the floor of the valley but also bevel its sides.
Glacial erosion in mountains also modifies the intersections between tributaries and the trunk valley. In a river system, the trunk stream serves as the local base level for tributaries, so the mouths of the tributary valleys lie at the same elevation as the trunk valley. The ridges (spurs) between valleys taper to a point when they join the trunk valley floor. During glaciation, tributary glaciers flow down side valleys into a trunk glacier. But the trunk glacier cuts the floor of its valley down to a depth that far exceeds the depth cut by the tributary glaciers. Thus, when the glaciers melt away, the mouths of the tributary valleys perch at a higher elevation than the floor of the trunk valley. Such side valleys are called hanging valleys. The water in a post-glacial stream that flows down a hanging valley  cascades over a spectacular waterfall to reach the post-glacial trunk stream (figure above d). As they erode, trunk glaciers also chop off the ends of spurs (ridges) between valleys, to produce truncated spurs.

A roche moutonnée is an asymmetric bedrock hill shaped by the flow of glacial ice.
Now let’s look at the erosional features produced by continental ice sheets. To a large extent, these depend on the nature of the pre-glacial landscape. Where an ice sheet spreads over a region of low relief, such as the Canadian Shield, glacial erosion creates a vast region of polished, flat, striated surfaces. Where an ice sheet  spreads over a hilly area, it deepens valleys and smooths hills. Glacially eroded hills may end up being elongate in the direction of flow and may be asymmetric, for glacial rasping smoothes and bevels the upstream part of the hill, creating a gentle slope, whereas glacial plucking eats away at the downstream part, making a steep slope. Ultimately, the hill’s profile may resemble that of a sheep lying in a meadow such a hill is called a roche moutonnée, from the French for sheep rock (figure above a, b).

Fjords: Submerged Glacial Valleys 

One of the many spectacular fjords of Norway. The water is an arm of the sea that fills a glacially carved valley. Tourists are standing on Pulpit Rock (Prekestolen).
As noted earlier, where a valley glacier meets the sea, the glacier’s base remains in contact with the ground until the water depth exceeds about four-fifths of the glacier’s thickness, at which point the glacier floats. Thus, glaciers can carve U-shaped valleys even below sea level. In addition, during an ice age, water extracted from the sea becomes locked in the ice sheets on land, so sea level drops significantly. Therefore, the floors of valleys cut by coastal glaciers during the Pleistocene Ice Age were cut much deeper than present sea level. Today, the sea has flooded these deep valleys, producing fjords. In the spectacular fjord-land regions along the coasts of Norway, New Zealand, Chile, and Alaska, the walls of submerged U-shaped valleys rise straight from the sea as vertical cliffs up to 1,000 m high (figure above). Fjords also develop where an inland glacial valley fills to become a lake.
Credits: Stephen Marshak (Essentials of Geology)

Ice and the Nature of Glaciers

Ice and the Nature of Glaciers 

What Is Ice? 

The nature of ice and the formation of glaciers. Snow falls like sediment and metamorphoses to ice when buried.
Ice consists of solid water, formed when liquid water cools below its freezing point. We can apply concepts introduced in our earlier discussions of rocks and minerals to distinguish among various occurrences of ice. For example, we can think of a single ice crystal as a mineral specimen, for it is a naturally occurring, inorganic solid, with a definite chemical composition (H2O) and a regular crystal structure. Ice crystals have a hexagonal form, so snowflakes grow into six-pointed stars (figure above a). We can picture a layer of fresh snow as a layer of sediment, and a layer of snow that has been compacted so that the grains stick together as a layer of sedimentary rock (figure above b). We can also think of the ice that appears on the surface of a pond as an igneous rock, for it forms when molten ice (liquid water) solidifies. Glacial ice, in effect, is a metamorphic rock. It develops when pre-existing ice recrystallizes in the solid state, meaning that the molecules in solid water rearrange to form new crystals (figure above c).

Alpine Glacier Basics