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 metamorphism. Show all posts
Showing posts with label metamorphism. Show all posts

Where Does Metamorphism Occur?

Where Does Metamorphism Occur? 

So far, we've discussed the nature of changes that occur during metamorphism, the agents of metamorphism (heat, pressure, compression and shear, and hydrothermal fluids), the rock types that form as a result of metamorphism, and the concepts of metamorphic grade and metamorphic facies. With this background, let’s now examine the geologic settings on Earth where metamorphism takes place, as viewed from the perspective of plate tectonics theory.
Because of the wide range of possible metamorphic environments, metamorphism occurs at a wide range of conditions in the Earth. You will see that the conditions under which metamorphism occurs are not the same in all geologic settings. That’s because the geothermal gradient (the relation between temperature and depth), the extent to which rocks endure compression and shear during metamorphism, and the extent to which rocks interact with hydrothermal fluids all depend on the geologic environment.

Types of Metamorphic Rocks

Types of Metamorphic Rocks 

Coming up with a way to classify and name the great variety of metamorphic rocks on Earth hasn't been easy. After decades of debate, geologists have found it most convenient to divide metamorphic rocks into two fundamental classes: foliated rocks and non-foliated rocks. Each class contains several rock types. We distinguish foliated rocks from each other partly by their component minerals and partly by the nature of their foliation, whereas we distinguish non-foliated rocks from each other primarily by their component minerals. 

Foliated Metamorphic Rocks 

To understand this class of rocks, we first need to discuss the nature of foliation in more detail. The word comes from the Latin folium, for leaf. Geologists use foliation to refer to the parallel surfaces and/or layers that can occur in a metamorphic rock. Foliation can give metamorphic rocks a striped or streaked appearance in an outcrop, and/or can give them the ability to split into thin sheets. A foliated metamorphic rock has foliation either because it contains inequant mineral crystals that are aligned parallel to one another, defining preferred mineral orientation, and/or because the rock has alternating dark-coloured and light-coloured layers.

Consequences and Causes of Metamorphism

What Is a Metamorphic Rock? 


If someone were to put a rock on a table in front of you, how would you know that it is metamorphic? First, metamorphic rocks can possess metamorphic minerals, new minerals that grow in place within the solid rock only under metamorphic temperatures and pressures. In fact, metamorphism can produce a group of minerals that together make up what geologists call a “metamorphic mineral assemblage.” And second, metamorphic rocks can have metamorphic texture defined by distinctive arrangements of mineral grains not found in other rock types. Commonly, the texture results in metamorphic foliation, due to the parallel alignment of platy minerals (such as mica) and/ or the presence of alternating light-coloured and dark-coloured layers. When metamorphic minerals and/or textures develop, a metamorphic rock becomes as different from its protolith as a butterfly is from a caterpillar. For example, metamorphism of red shale can yield a metamorphic rock consisting of aligned mica flakes and brilliant garnet crystals (a in figure above), and metamorphism of a limestone composed of cemented-together fossil fragments can yield a metamorphic rock consisting of large interlocking crystals of calcite (b in figure above). The process of forming metamorphic minerals and textures takes place very slowly it may take thousands to millions of years and it involves several processes, which sometimes occur alone and sometimes together. The most common processes are: 
  • Recrystallization, which changes the shape and size of grains without changing the identity of the mineral making up the grains (a in figure above). 
  • Phase change, which transforms one mineral into another mineral with the same composition but a different crystal structure. On an atomic scale, phase change involves the rearrangement of atoms. 
  • Metamorphic reaction, or neocrystallization (from the Greek neos, for new), which results in growth of new mineral crystals that differ from those of the protolith (b in figure above). During neocrystallization, chemical reactions digest minerals of the protolith to produce new minerals of the metamorphic rock. 
  • Pressure solution, which happens when a wet rock is squeezed more strongly in one direction than in others. Mineral grains dissolve where their surfaces are pressed against other grains, producing ions that migrate through the water to precipitate elsewhere (c in figure above). 
  • Plastic deformation, which happens when a rock is squeezed or sheared at elevated temperatures and pressures. Under these conditions, grains behave like soft plastic and change shape without breaking (d in figure above). 

Caterpillars undergo metamorphosis because of hormonal changes in their bodies. Rocks undergo metamorphism when they are subjected to heat, pressure, compression and shear, and/or very hot water. Let’s now consider the details of how these agents of metamorphism operate.

Metamorphism Due to Heating 

When you heat cake batter, the batter transforms into a new material cake. Similarly, when you heat a rock, its ingredients transform into a new material metamorphic rock. Why? Think about what happens to atoms in a mineral grain as the grain warms. Heat causes the atoms to vibrate rapidly, stretching and bending chemical bonds that lock atoms to their neighbours. If bonds stretch too far and break, atoms detach from their original neighbours, move slightly, and form new bonds with other atoms. Repetition of this process leads to rearrangement of atoms within grains, or to migration of atoms into and out of grains, a process called solid-state diffusion. As a consequence, recrystallization and/or neo-crystallization take place, enabling a metamorphic mineral  assemblage to grow in solid rock. Metamorphism takes place at temperatures between those at which diagenesis occurs and those that cause melting. Roughly speaking, this means that most metamorphic rocks you find in outcrops on continents formed at temperatures of between 250C and 850C.

Metamorphism Due to Pressure 

As you swim underwater in a swimming pool, water squeezes against you equally from all sides in other words, your body feels pressure. Pressure can cause a material to collapse inward. For example, if you pull an air-filled balloon down to a depth of 10 m in a lake, the balloon becomes significantly smaller. Pressure can have the same effect on minerals. Near the Earth’s surface, minerals with relatively open crystal structures can be stable. However, if you subject these minerals to extreme pressure, the atoms pack more closely together and denser minerals tend to form. Such transformations involve phase changes and/or neo-crystallization.

Changing Both Pressure and Temperature 

So far, we've considered changes in pressure and temperature as separate phenomena. But in the Earth, pressure and temperature change together with increasing depth. For example, at a depth of 8 km, temperature in the crust reaches about 200C and pressure reaches about 2.3 kbar. If a rock slowly becomes buried to a depth of 20 km, as can happen during mountain building, temperature in the rock increases to more than 500C, and pressure to 5.5 kbar. Experiments and calculations show that the “stability” of certain minerals (the ability of a mineral to form and survive) depends on both pressure and temperature. When pressure and temperature increase, the original mineral assemblage in a rock becomes unstable, and a new assemblage forms out of minerals that are stable. Thus, a metamorphic rock formed at 8 km does not contain the same minerals as one formed at 20 km.

Compression, Shear, and Development  of Preferred Orientation 


Imagine that you have just built a house of cards and, being in a destructive mood, you step on it. The structure collapses because the downward push you apply with your foot exceeds the push provided by air in other directions. We can say that we have subjected the cards to compression (a in figure above). Compression flattens a material (b in figure above). Shear, in contrast, moves one part of a material sideways, relative to another. If, for example, you place a deck of cards on a table, then set your hand on top of the deck and move your hand parallel to the table, you shear the deck (c in figure above). When rocks are subjected to compression and shear at elevated temperatures and pressures, they can change shape without breaking. As it changes shape, the internal texture of a rock also changes. For example, platy (pancake-shaped) grains become parallel to one another, and elongate (cigar shaped) grains align in the same direction. Both platy and elongate grains are inequant grains, meaning that the dimension of a grain is not the same in all directions; in contrast, equant grains have roughly the same dimensions in all directions (d in figure above). The alignment of inequant minerals in a rock results in a preferred orientation (e in figure above).

The Role of Hydrothermal Fluids 

Metamorphic reactions commonly take place in the presence of hydrothermal fluids (very hot-water solutions). Where does the water in hydrothermal fluids come from? Some of it was originally bonded to minerals in the protolith, for metamorphic reactions can release such water into its surroundings. Some of it may seep up into the protolith from a nearby igneous intrusion, or down from overlying groundwater reservoirs. Notably, under extremely high pressures and temperatures, the water of hydrothermal fluids is in neither gas nor liquid state, but rather is in a “supercritical” state, meaning that it has characteristics of both gas and liquid. Such hydrothermal fluids chemically react with rock; they accelerate metamorphic reactions, because atoms involved in the reactions can migrate faster through a fluid than they can through a solid, and hydrothermal fluids provide water that can be absorbed by minerals during metamorphic reactions. Finally, fluids passing through a rock may pick up some dissolved ions and drop off others, as a bus picks up and drops off passengers, and thus can change the overall chemical composition of a rock during metamorphism. The process of changing a rock’s chemical composition by reactions with hydrothermal fluids is called metasomatism.

Metamorphic rocks

Metamorphic rocks are formed by the heating of pre-existing rocks. The heat provide to a rock changes it mineralogical and physical changes which are called metamorphic rocks.

These rocks forms mostly where magma chamber is available to heat enough for mineralogical changes occurrence. These rocks have multiple features in distinguishing like schistosic, gneissic and slaty texures.


Texture is the physical character or a pattern.

Slaty texture

Slate are formed by the metamorphism of shale. Sheets are formed in the slates where it can broke  into sheets. This help in determining the slates.

Schistose

Schistose is formed after metamorphism of slate where it rearrange in forming irregular sheets like character which is schistosic texture.

Gneissic

Gneiss is the high grade metamorphic rock of shale which is distinguished by regular interval of dark and light bands present in it. These are called gneissic bands which is the recognizable character.

Metamorphism and its changes are following

Shale- Slate- Schist - Gneiss
Shale is sedimentary rock when metamorphose produces slate and slate to schist and schist to gneiss with increasing metamorphic grades. In the same way marble is produced by the metamorphism of limestone.