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Types of rock

From a geological point of view, a rock is a solid material, found below the surface of the Earth. Rocks can consist of fragments (called clasts), or minerals. Geological deposits that are not yet solidified, are also counted among rocks. From this point of view, a loose layer of sand or clay is also a rock. Even a large amount of land ice can be regarded as a rock.

Rocks can be classified into sedimentary rocks, metamorphic rocks, and igneous rocks, depending on how they formed. The building components of rocks are minerals. These minerals can occur in many varieties.

Igneous rocks for example, are formed during the solidification (or cooling) of magma or lava. Sedimentary rocks are formed by sedimentation or deposition of materials, after erosion. Metamorphic rocks are formed by exposing rocks to high temperatures and/or pressures (deep inside the earth's crust).

Igneous rocks

As we already know, igneous rocks are formed by the solidification of magma or lava. If the rock occurs deep inside the earth's crust, forming major intrusions or plutons, we call these rocks plutonic rocks. For small intrusions we use the term “gangues”. When lava solidifies on the surface we call these type of rocks  “extrusive”.

Magma however, is subjected to various processes before solidification. These processes can change the composition of the magma, and ultimately the composition and type of igneous rock. Examples of plutonic rocks are: granite and gabbro. Typical gangues are: dolerite or granofite. Examples of typical extrusive rocks are: basalt, andesite and obsidian.

Sedimentary rocks

It takes a continuous cycle of erosion and sedimentation, to weather rocks into their singular elements and loosened particles, which are then transported by rivers, wind and ice and deposited upon the earth's surface. This deposition process is referred to as sedimentation, and the material, which is often located within layers, are called sediments. When these sediments harden into rock, during the course of time, we use the term sedimentary rock. Sandstone for example is nothing more than the solidified sedimentary deposit of sand grains. Shale on the other hand, is formed by the solidification of mudstone, or other very fine-grained sediments. And last but not least, a conglomerate is the solidified product of gravel deposits ...

Sedimentary rocks can also occur in alternative forms. For example, plant material (peat) hardens over time, and under, pressure into lignite or coal. Limestone usually forms as a result of the accumulation of carbonate based (exo)skeletons of marine life forms, on the bottom of a sea or ocean. Under special circumstances, some sedimentary rocks can also be formed as a result of  the precipitation of dissolved minerals in groundwater. Example: gypsum and rock salt(s).

An important aspect during the sedimentation process, lies in the fact that organisms, or parts of organisms, can be buried along in between the different layers. Under the right circumstances (pressure, quick covering, temperature, and anoxic environments) these organisms can fossilize.

Sedimentary rocks, often display various amounts of sedimentary structures. These structures can be used to reconstruct the original environment in relation to the stratigraphy and the characteristics of the rocks in a given geological layer.

The following table describes the most common sediments. But keep in mind that, the term “grain size” refers to the average grain size.

Slate
Is in fact ' Petrified ' clay consisting of particles smaller than 0.004 millimeters. Slate usually breaks into fairly thin slices. The rock is quite soft and erodes quickly. The rock is an erosion product of igneous rocks.
 
Siltstone
Siltstone contains pellets between 0.004 to 0.0062 millimeter. It is classified according to grain size, between sandstone and shale. This rock is also an erosion product of igneous rocks.  
Sandstone
Sandstone contains grains between 0.062 to 2 millimeter. This rock is also the erosion product of igneous rocks.  
Conglomerate

A conglomerate is a composite stone with rounded gravel or pebbles incrusted in a finer matrix. The gravel can be between 2 millimeters to 4 feet! Also this rock is the decomposition product of igneous rocks.

 
Breccia   
Breccia is basically the same as a conglomerate, but the incrusted components are more angular rather than rounded.  Most of the time breccia indicates a geological event such as rockslide, or collapse.  
Coal 
Coal is formed as a result of compressed plant remains. Coal is usually black and greasy.  
Limestone
Limestone consists of calcium carbonate. This rock contains lots of fossils, and usually occurs when carbonate (micro) skeletons of sea creatures sink to the bottom and accumulate.  
Marl    
Marl is a rock consisting of both clay and limestone.  
Halite / Gypsum   
This rock is formed by the “precipitation” of seawater with high salinity.  The seawater evaporates and the salt concentration becomes too high forming evaporates.  

Metamorphic rocks

Sediments buried deep inside the Earth's crust , are exposed to a high temperatures and pressure.  Mineral changes occur inside these rocks. This process is  called metamorphosis. Also igneous rocks such as granite can be transformed, the product of this metamorphism is called gneiss. Rocks that then can be formed are:

 


 


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