Rock
(mineral), naturally occurring solid material consisting of one or
more minerals. Minerals are solid, naturally occurring chemical elements or
compounds that are homogenous, meaning they have a definite chemical composition
and a very regular arrangement of atoms. Rocks are everywhere, in the ground,
forming mountains, and at the bottom of the oceans. Earth’s outer layer, or
crust, is made mostly of rock. Some common rocks include granite and basalt.
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TYPES
OF ROCKS
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Rocks are divided into
three main types, based on the ways in which they form. Igneous rocks are made
of old rocks that have melted within the earth to form molten material called
magma. Magma cools and solidifies to become igneous rocks. Sedimentary rocks
form as layers of material settle onto each other, press together, and harden.
Metamorphic rocks are created when existing rocks are exposed to high
temperatures and pressures, and the rock material is changed, or metamorphosed,
while solid.
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Igneous
Rock
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New
Lava
Magma that reaches the earth’s surface is called lava. In
this picture, red-hot lava flows from a volcano on Réunion, an island off the
coast of Africa in the Indian Ocean. The lava wrinkles because the exterior and
interior of the flow cool at different rates. The surface of the flow cools
relatively quickly, forming a skin that becomes deformed as the hotter lava
moves underneath.
Igneous rocks are rocks
formed from a molten or partly molten material called magma. Magma forms deep
underground when rock that was once solid melts. Overlying rock presses down on
the magma, and the less dense magma rises through cracks in the rock. As magma
moves upward, it cools and solidifies. Magma that solidifies underground
usually cools slowly, allowing large crystals to form. Magma that reaches
Earth’s surface is called lava. Lava loses heat to the atmosphere or ocean very
quickly and therefore solidifies very rapidly, forming very small crystals or
glass. When lava erupts at the surface again and again, it can form mountains
called volcanoes.
Igneous
Rock: Pegmatite
Pegmatite is a variety of igneous rock with extremely large
crystals; individual crystals can be as large as a bathtub. Pegmatites are the
last rocks to crystallize from a solidifying body of magma. The slow rate of
cooling and the presence of large amounts of water dissolved in the magma
account for the large size of the crystals.
Igneous rocks commonly
contain the minerals feldspar, quartz, mica, pyroxene, amphibole, and olivine.
Igneous rocks are named according to which minerals they contain. Rocks rich in
feldspar and quartz are called felsic; rocks rich in pyroxene, amphibole, and
olivine, which all contain magnesium and iron, are called mafic. Common and
important igneous rocks are granite, rhyolite, gabbro, and basalt. Granite and
rhyolite are felsic; gabbro and basalt are mafic. Granite has large crystals of
quartz and feldspar. Rhyolite is the small-grained equivalent of granite.
Gabbro has large crystals of pyroxene and olivine. Basalt is the most common
volcanic rock.
B
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Sedimentary
Rock
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Sedimentary
Rock
Sedimentary rock, such as this sandstone, forms when layers
of sediment are compacted and cemented together. Nearly 5% of the top 15 km (10
mi) of the earth’s crust is made of sedimentary rock. However, sedimentary rock
constitutes nearly 75% of the exposed rock on the earth’s surface.
Sedimentary rock forms when
loose sediment, or rock fragments, hardens. Geologists place sedimentary rocks
into three broad categories: (1) clastic rocks, which form from clasts,
or broken fragments, of pre-existing rocks and minerals; (2) chemical rocks,
which form when minerals precipitate, or solidify, from a solution, usually
seawater or lake water; and (3) organic rocks, which form from accumulations of
animal and plant remains. It is common for sedimentary rocks to contain all
three types of sediment. Most fossils are found in sedimentary rocks because
the processes that form igneous and metamorphic rocks prevent fossilization or
would likely destroy fossils.
The most common types of
clastic rocks are sandstone and shale (also known as mudrock). Sandstone is
made from sand, and shale is made from mud. Sand particles have diameters in
the range 2.00 to 0.06 mm (0.08 to 0.002 in), while mud particles are smaller
than 0.06 mm (0.002 in). Sand and mud form when physical or chemical processes
break down and destroy existing rocks. The sand and mud are carried by wind,
rivers, ocean currents, and glaciers, which deposit the sediment when the wind
or water slows down or where the glacier ends. Sand usually forms dunes in
deserts, or sandbars, riverbeds, beaches, and near-shore marine deposits. Mud
particles are smaller than sand particles, so they tend to stay in the wind or
water longer and are deposited only in very still environments, such as lake
beds and the ocean floor.
Sedimentary rock forms when
layers of sand and mud accumulate. As the sediment accumulates, the weight of
the layers of sediment presses down and compacts the layers underneath. The
sediments become cemented together into a hard rock when minerals (most
commonly quartz or calcite) precipitate, or harden, from water in the spaces
between grains of sediment, binding the grains together. Sediment is usually
deposited in layers, and compaction and cementation preserve these layers,
called beds, in the resulting sedimentary rock.
The most common types of
chemical rocks are called evaporites because they form by evaporation of
seawater or lake water. The elements dissolved in the water crystallize to form
minerals such as gypsum and halite. Gypsum is used to manufacture plaster and
wallboard; halite is used as table salt.
The most common organic
rock is limestone. Many marine animals, such as corals and shellfish, have
skeletons or shells made of calcium carbonate (CaCO3). When these
animals die, their skeletons sink to the seafloor and accumulate to form large
beds of calcium carbonate. As more and more layers form, their weight
compresses and cements the layers at the bottom, forming limestone. Details of
the skeletons and shells are often preserved in the limestone as fossils.
Coal is another common
organic rock. Coal comes from the carbon compounds of plants growing in swampy
environments. Plant material falling into the muck at the bottom of the swamp
is protected from decay. Burial and compaction of the accumulating plant
material can produce coal, an important fuel in many parts of the world. Coal
deposits frequently contain plant fossils.
C
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Metamorphic
Rock
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Metamorphic
Rock
Metamorphic rock forms when extreme temperatures and
pressures deep within the earth alter the mineralogical or structural aspects
of existing rock and change it into metamorphic rock. This temperature and
pressure alteration within rock causes its set of minerals, or mineral
assemblages, to change into other minerals. The change from one mineral
assemblage to another is called metamorphism.
Metamorphic rock forms when
pre-existing rock undergoes mineralogical and structural changes resulting from
high temperatures and pressures. These changes occur in the rock while it
remains solid (without melting).
The changes can occur
while the rock is still solid because each mineral is stable only over a
specific range of temperature and pressure. If a mineral is heated or
compressed beyond its stability range, it breaks down and forms another
mineral. For example, quartz is stable at room temperature and at pressures up
to 1.9 gigapascals (corresponding to the pressure found about 65 km [about 40 mi]
underground). At pressures above 1.9 gigapascals, quartz breaks down and forms
the mineral coesite, in which the silicon and oxygen atoms are packed more
closely together.
In the same way, combinations
of minerals are stable over specific ranges of temperature and pressure. At
temperatures and pressures outside the specific ranges, the minerals react to
form different combinations of minerals. Such combinations of minerals are
called mineral assemblages.
In a metamorphic rock,
one mineral assemblage changes to another when its atoms move about in the
solid state and recombine to form new minerals. This change from one mineral
assemblage to another is called metamorphism. As temperature and pressure
increase, the rock gains energy, which fuels the chemical reactions that cause
metamorphism. As temperature and pressure decrease, the rock cools; often, it
does not have enough energy to change back to a low-temperature and
low-pressure mineral assemblage. In a sense, the rock is stuck in a state that
is characteristic of its earlier high-temperature and high-pressure
environment. Thus, metamorphic rocks carry with them information about the
history of temperatures and pressures to which they were subjected.
The size, shape, and distribution
of mineral grains in a rock are called the texture of the rock. Many
metamorphic rocks are named for their main texture. Textures give important
clues as to how the rock formed. As the pressure and temperature that form a
metamorphic rock increase, the size of the mineral grains usually increases.
When the pressure is equal in all directions, mineral grains form in random
orientations and point in all directions. When the pressure is stronger in one
direction than another, minerals tend to align themselves in particular directions.
In particular, thin plate-shaped minerals, such as mica, align perpendicular to
the direction of maximum pressure, giving rise to a layering in the rock that
is known as foliation. Compositional layering, or bands of different minerals,
can also occur and cause foliation. At low pressure, foliation forms fine, thin
layers, as in the rock slate. At medium pressure, foliation becomes coarser,
forming schist. At high pressure, foliation is very coarse, forming gneiss.
Commonly, the layering is folded in complex, wavy patterns from the pressure.
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THE
ROCK CYCLE
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The rock cycle describes
how rocks change, or evolve, from one type to another. For example, any type of
rock (igneous, sedimentary, or metamorphic) can become a new sedimentary rock
if its eroded sediments are deposited, compacted, and cemented. Similarly, any
type of rock can become metamorphic if it is buried moderately deep. If the
temperature and pressure become sufficiently high, the rock can melt to form
magma and a new igneous rock.
