Rocks are an aggregate of minerals. A rock can be composed of a single mineral or more commonly composed of an aggregate of two or more minerals

• Rate of cooling  is one of the most important factors that control crystal size

• Solification can occur along the surface or beneath the surface of the Earth.

  IGNEOUS ROCKS

-   are formed from the solidification of molten rock material. There are two basic types.

Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite.

Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.

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METAMORPHIC ROCKS

Metamorphic rocks have been modified by heat, pressure, and chemical processes, usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture, and chemical composition of the rocks.

There are two basic types of metamorphic rocks.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.

Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance.

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SEDIMENTARY ROCKS

Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks.

Clastic sedimentary rocks such as breccia, conglomerate, sandstone, siltstone, and shale are formed from mechanical weathering debris.

Chemical sedimentary rocks, such as rock salt, iron ore, chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution.

Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris

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