There are many specific kinds of rocks, but they all fall into three main categories. Their category is based on their system of formation. The three types of rocks include igneous, sedimentary and metamorphic. Early in the Earth’s history, every rock was igneous, having been made from the cooling of materials on the surface. They originated from the core of the earth hence they inform us about the details of the Earth’s interior properties. Now, billions of years into the Earth’s history, we have these 3 main types of rocks.
An igneous rock is one that has solidified from lava or magma after gradual cooling. They can be intrusive (for instance, solidified from magma internally) or extrusive (solidified from lava near or at the surface of the Earth). The bulk of the Earth’s crust is created from igneous rock. Examples of igneous rock include rhyolite, diorite, gabbro, granite, obsidian, basalt and pumice.
A sedimentary rock is formed as a result of accumulation of various sized sediments coming from all types of materials. They can be made from organic matter and pass through cementation, compaction or evaporation, from precipitation, or a concentrated mineral solution. Sedimentary rocks are classified as clastic (made from any size particle of pre existing rock), non-clastic (also known as chemical sedimentary) or organic (formed from flora and fauna). Sedimentary rock results from evaporation of solution that is concentrated with mineral composites. Examples of organic sedimentary rocks are limestone and coal, while clastic includes shale and conglomerate. Lastly, instances of chemical, or non-clastic, are rock salt and gypsum.
This is an igneous, sedimentary or even metamorphic rock that has either been pressed by massive force and is vulnerable to extremely high temperatures modifying its structure, chemical composition or mineral arrangement. Metamorphic rocks are classified as contact (from proximity to a magmatic intrusion), or regional (a name for rocks that were deformed over a very large area). In addition, they can be either folic or non-folic. Folation occurs when a rock is being compacted in one particular direction while forming. This causes leafing which causes the rocks to look like it is made from multiple thin sheets of rock. Metamorphic rocks can likewise be termed by the grade of metamorphism which ranges from low to high. Rocks in this group comprise schist, gneiss, marble and phylite.
There are many hundreds of different types of rocks, that are classified by their composition of specific method of formation, but they all fall into one of the above main categories.