In this lesson, we will discuss the three main types of rocks
and how they are formed. The lesson also gives an introduction into how
matter locked in rocks can be cycled through the earth.
Rock Cycle
The rock cycle can take thousands of years to complete
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When the earth was first created, the materials that
it made were so hot from the contractions that formed the planet and
from the heat generated by the core that the outer layers were mostly
liquid. Over time, it slowly cooled, and a hard crust formed on the
outer surface. This was much like the thin layer of ice that first forms
over a pond when the temperature dips below freezing. And just as there
is liquid under the layer of ice, there is still liquid magma flowing
under the crust. As this happened, a cycle of events began to be put
into place that causes changes in the rocks that exist on the earth.
This cycle is known as the
rock cycle. The rock cycle is a model used to describe the creation, alteration, and destruction of the rocks that form from magma.
It
is important to note that new matter is never created on the earth. All
the matter that the earth contains was present when the earth formed
and still is here today. It may be in different forms, but it is still
present. The rock cycle is the earth's way of recycling the matter used
to make up rocks from formation to destruction and back to formation.
Igneous Rock
Granite is a common igneous rock
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That's right, Crystal. To begin the process, magma
cools and hardens either under the ground or on the earth's surface. As
the rock cools, the minerals that make up the rock take shape. All magma
that solidifies becomes one of two types of igneous rock: either
extrusive rocks, rocks that form on the earth's surface, or
intrusive rocks,
rocks that form under the earth's surface. Hawaii is made up mostly of
extrusive rock called basalt that came from the volcanic activity common
to the area.
The most common type of intrusive rock is
granite,
recognized by its pink or grey color flecked with light- and
dark-colored mineral specks. Igneous rocks are constantly being formed
on the earth whenever magma leaks to the surface through cracks or
erupts in volcanoes.
Sedimentary Rock
Limestone is a kind of sedimentary rock
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Yes, Crystal, rocks that are exposed to the weathering
nature of the elements can be broken apart. Small grains of exposed
rock can be broken off by the action of wind and water or broken apart
as water freezes in cracks. This can affect any type of rock, not just
igneous. As time passes, these grains of rock accumulate in the slow
currents in rivers or river mouths, low areas, or other quiets spots.
As sediment piles up, the first
sediments
get buried deeper and are subjected to increased heat and pressure.
Sometimes water will flow through the sediment, dissolving and
redepositing glue-like substances to bind the grains together into
layers. Rock made from eroded material broken off other rocks and
cemented together is called
sedimentary rock. One of the best examples of sedimentary rock is the layers that are seen in the Grand Canyon.