Types of Rocks

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Types of Rocks

Down to a depth of 16 km from the surface of land, 95 per cent of the earth materials found in the earth’s crust consist of rocks. Rocks are made up of individual substances, called minerals, found mostly in solid state. Rocks are usually classified into the following three major types.

1. Igneous Rocks

These rocks are formed directly from the molten material, when it gets solidified. Example – Mica, Granite, etc. These are rocks that solidified directly from molten silicates, which geologists call magma. Examples : Granite, Basalt, Pumice and Flint ( which is a form of quartz ).

Morphology and Setting

Intrusive :

Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. Surrounded by pre – existing rock ( called country rock ), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes.

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Typical Intrusive Formations

  • Batholiths
  • Stocks
  • Laccoliths
  • Sills
  • Dikes

Extrusive :

Extrusive igneous rock is made from lava released by volcanoes Basalt ( an extrusive igneous rock in this case ); light coloured tracks show the direction of lava flow.

The melted rock, with or without suspended crystals and gas bubbles, is called magma. It rises because it is less dense than the rock from which it was created. When magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine. Black smokers and mid – ocean ridge basalt are examples of submarine volcanic activity.

2. Sedimentary Rocks

Sedimentary rocks are formed from pre – existing rocks or pieces of once – living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.

Clastic Sedimentary Rock

Clastic sedimentary rocks are the group of rocks most people think of when they think of sedimentary rocks. Clastic sedimentary rocks are made up of pieces ( clasts ) of pre – existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.

Biologic Sedimentary Rock

Biologic sedimentary rocks form when large numbers of living things die, pile up, and are compressed and cemented to form rock. Accumulated carbon – rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert.

Chemical Sedimentary Rock

Chemical sedimentary rocks are formed by chemical precipitation. The stalactites and stalagmites you see in caves form this way, so does the rock salt that table salt comes from. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals can be redeposited, or precipitated, when the water evaporates away or when the water becomes over –  saturated with minerals.

3. Metamorphic Rocks

Metamorphic rocks are rocks that have undergone a change from their original form due to changes in temperature, pressure or chemical alteration. The classification of metamorphic rocks is based on the minerals that are present and the temperature and pressure at which these minerals form. Determination of this information is not easily accomplished in this lab. Therefore, a simplified system is used based on texture and composition.

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Texture : 

Texture is divided into two groups. Foliated textures show a distinct planar character. This means that the minerals in the rock are all aligned with each other. This planar character can be flat like a piece of slate or folded. Non – foliated textures have minerals that are not aligned. Essentially, the minerals are randomly oriented.

Foliation :

Foliated textures show four types of foliation. Slaty cleavage is composed of platy minerals that are too small to see. Typically, these rocks split along parallel, planar surfaces. Phyllitic foliation is composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye.

The larger size gives the foliation a slighly shiny appearance. Schistose foliation is composed of larger minerals which are visible to the unaided eye. Platy minerals tend to dominate. Gneissic banding is the easiest of the foliations to recognize. It is composed of alternating bands of dark and light minerals.

Non – Foliation :

Non – foliated textures are identified by their lack of planar character. Further identification of non – foliated rocks is dependent on the composition of the minerals or components in the rock. Anthracite coal is similar to bituminous coal. Both are black in color , and is composed of carbon.

Anthracite coal is generally shiny in appearance and breaks with a conchoidal fracture ( broken glass also shows this type of fracture ). Metaconglomerate is composed of pebbles and gravel that have been flattened due to directed pressure. Quartzite is composed of quartz sand grains. Quartz has a hardness of 7, which makes it difficult to scratch. Marble is composed of calcite and will readily react to a small drop of HCl.

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