Definition of Rock
Rock can be defined as the solid mineral material which the Earth is made of.
Types/Classes of Rocks
Rocks are categorised into three different types. The categorisation is based on how the rocks are formed.
- Igneous rock
- Sedimentary rock
- Metamorphic rock
Igneous rocks are formed from molten magma (molten materials in the earth's crust). Igneous basically means fire or heat.
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Types of Igneous Rocks
- Intrusive or Plutonic Igneous Rock
- Extrusive or Volcanic Igneous Rock
Intrusive, or plutonic igneous rock forms when magma is trapped deep inside the Earth. Examples of intrusive igneous rocks include:
Extrusive, or volcanic igneous rock is produced when magma exits and cools above (or very near) the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. Examples of extrusive igneous rocks include:
All igneous rocks have one thing in common: they occur from the cooling and solidification of molten materials.
Sedimentary rocks are formed from particles such as sand, shells, pebbles, etc, which are later cemented on the earth’s surface and within bodies of water. Generally, sedimentary rocks is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock. Sedimentary rocks are the only type of rocks that contain fossils.
Types of Sedimentary Rocks
Sedimentary rocks are differentiated based on the fragments that come together to form them. There are three types of sedimentary rock:
- Clastic Sedimentary Rock
- Chemical Sedimentary Rock
- Organic Sedimentary Rock
Clastic Sedimentary Rocks are composed of fragments of rocks. Examples of clastic sedimentary rocks include: breccia, conglomerate, sandstone, siltstone, and shale
Chemical Sedimentary Rocks are formed when standing water evaporates and results in mineral constituents which later become supersaturated and inorganically precipitate. Examples of chemical sedimentary rocks include: halite (rock salt), sylvite, barite and gypsum.
Organic Sedimentary Rocks are formed from the accumulation of sedimentary debris caused by organic processes. For instance, the hard parts of animals, such as bones and shells, can become cemented together over time to make rock. Examples of organic sedimentary rocks include: fossiliferous limestone and coal
A metamorphic rock is a result of a transformation of a pre-existing rock which has been subjected to very high heat and pressure. Examples of metamorphic rocks include: gneiss, slate, marble, schist, and quartzite.
Types of Metamorphic Rocks
Metamorphic rocks can be classified into two types based on their physical quality and level of exposure to heat. The two types of metamorphic rocks are:
- Foliated Metamorphic Rocks - such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure
2. Non-foliated Metamorphic Rocks - such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance.