There are three main types of rocks
Each of these rocks are formed by physical changes—such as melting, cooling, eroding, compacting, or deforming—that are part of the rock cycle.
Sedimentary Rocks
Sedimentary rocks are formed by the accumulation,
compactation and cementation of sediments. There are three basic types
of sedimentary rocks.
Clastic sedimentary rocks form from the
accumulation and lithification of mechanical weathering debris. Examples
include: breccia, conglomerate, sandstone, siltstone, and
shale.
Chemical sedimentary rocks form when dissolved
materials preciptate from solution. Examples include:
chert, some dolomites, flint, iron ore, limestones, and
rock salt.
Organic sedimentary rocks form from the
accumulation of plant or animal debris. Examples include: chalk, coal,
diatomite, some dolomites, and some limestones.
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Sedimentary rocks can also be eroded back into
sediments and re-cemented into new rocks or undergo heat and pressure to
be converted into metamorphic rocks.
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 have a layered or
banded appearance that is produced by exposure to heat and directed
pressure. Examples of foliated rocks include:
gneiss, phyllite, schist, and slate.
Non-foliated metamorphic rocks do not have a
layered or banded appearance. Examples of nonfoliated rocks include:
hornfels, marble, novaculite, quartzite, and skarn.
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Metamorphic rocks can be eroded into sediments or
undergo additional heat and pressure where the crystals liquify.
Igneous Rocks
Igneous rocks are formed from the solidification of
molten material. Some form below Earth's surface. Some form on or above
Earth's surface. There are two basic types of Igneous rocks.
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:
diabase, 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 can be eroded into sediments or
undergo additional heat and pressure where the crystals liquify.