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Rocks are everywhere! They are large and small, heavy or light, porous or dense, but rocks
in some shape or form can be found all over the planet. Different types of rock are formed
in different ways. There are three main types of rock: Igneous, Sedimentary, and Metamorphic.
Igneous is a word that means 'fiery.' Igneous rock is formed when magma or lava cools. Sometimes
magma cools slowly underneath the earth's surface. This forms intrusive igneous rock,
like granite. Other times lava cools when it comes to the surface in a volcanic eruption.
This forms extrusive igneous rock. Some examples of extrusive igneous rock are tuff, obsidian,
and pumice. Igneous rocks make up about 95% of the earth's crust.
The next type of rock is Sedimentary rock. Sediment is small particles of sand, mud,
and organic material that settle to the bottom of water or land areas, often lakes or oceans.
Sedimentary rocks are formed when sediment accumulates over time in deposits that form
layers. These layers become squeezed and compressed over time until they consolidate into a rock.
Sedimentary rocks are the types of rocks in which fossils may be found, since the process
of forming sedimentary rocks can preserve plant and animal remains that are deposited
into the sediment layers. Some examples of sedimentary rock are limestone, shale, and
sandstone.
The third type of rock is Metamorphic rock. The word 'metamorph' means to change form,
and metamorphic rock is rock that has been changed by extreme heat and pressure. Sedimentary
rock, igneous rock, or even other metamorphic rocks can be changed by heat and pressure
into new kinds of rock. Metamorphic rocks can be formed by being deep under the earth,
where pressure and temperatures are high, or when rock near the surface is heated up
by the movement of tectonic plates or magma. Different types of rocks become different
types of metamorphic rock when exposed to heat and pressure. For example, shale becomes
slate, sandstone becomes quartzite, and limestone becomes marble.
Rocks are slowly but constantly changing in something known as the 'rock cycle.' The rock
cycle begins with magma, or hot melted rock, deep beneath the earth's surface. This magma
becomes crystallized, becoming igneous rock. These rocks begin to erode, or break down
into small pieces because of wind, water, or other forces. The small fragments of rock
are carried away as sediment when water passes over them and are deposited in layers which
eventually become sedimentary rocks. Then, some sedimentary rocks are pushed below the
surface due to tectonic activity, where they are exposed to heat and pressure, transforming
them into metamorphic rocks. If the rocks are buried even deeper, they may melt and
form magma, starting the cycle all over again. Of course, sedimentary and metamorphic rocks
can be eroded into sediment, and igneous rock can become metamorphic rock or lava, but one
way or another, rocks all over the world keep changing from one form to the next.