In Discovery Education type Forged within the Earth into the search then enter.
Read the passage
Forged Within the Earth: Metamorphic Rocks
A baker mixes flour, water, eggs, and salt. He or she then
repeatedly presses down hard on the mixture, or kneads it. The pressure changes the
mixture into dough. When the baker adds
yeast, a type of chemical reaction occurs within the dough. Then
the baker places the dough into the oven. The heat changes the
dough again; it becomes bread. In a way, the original
ingredients undergo a metamorphosis, or change, from a loose
mixture into a loaf of bread.
Metamorphic rocks are rocks, minerals in the earth’s crust, that have had their form, and occasionally
their chemical composition, changed by the forces of pressure and heat within the Earth. The word
metamorphosis comes from the Greek meaning “change of form.” Inside the Earth, extremes of
pressure and/or heat change the form of the original rock, called the protolith. Extreme pressure comes
from the collision of tectonic plates. Intense heat comes from the fiery hot magma in the Earth’s mantle.
Metamorphic rocks are an integral part of the rock cycle. The
rock cycle is the process that forms different types of rocks,
breaks them down through weathering and erosion, and then
re-creates them in a variety of forms.
Diagram of the Rock cycle
Metamorphic rock was once something else—sedimentary or igneous
rock. As these rocks are pushed below the crust by tectonic forces,
they are subjected to pressure and heat. These forces transform
them into metamorphic rock. As geologic processes move minerals
through the rock cycle, the minerals
Most metamorphism occurs where the physical environment surrounding the rock deep under the
surface undergoes dramatic changes. Conditions of in tense heat, intense pressure,
and the presence of hot mineral-laden liquid are usually
present. Separately, or more likely in combination, these
conditions lead to the formation of metamorphic rock. These
conditions occur mainly along the edges of tectonic plates,
where pressure and heat are greatest. Along plate fissures,
magma rising from the mantle intrudes into cooler protolith and
causes metamorphosis. The intense heat of the magma may also
trigger chemical changes in the rock. The heat is not sufficient to
melt the protolith, but it does make the rock denser.
Metamorphic rock is also commonly formed along continental
margins, especially where two tectonic plates are colliding or
grinding past each other. The intense pressure of a tectonic
collision also serves to compact the protolith that then becomes
metamorphic rock. Compaction leads to the formation of
entirely new types of rocks. These new metamorphic rocks have
had their arrangement of component minerals altered or have
reacted with mineralized liquids that entered the rock and
changed it. Because they form under such extreme conditions,
some types of metamorphic rock look “messy” and appear
smeared, squashed, or folded. Further, because the actual
degree of heat and pressure that creates the metamorphic rock
can and does vary widely, there is a vast diversity of
metamorphic rock. Yet most metamorphic rock retains at least
some of the essential chemical characteristics of the protolith
from which it formed.
Gneiss
One type of metamorphic rock is called foliated metamorphic rock. Foliated means striped or having
clearly visible bands. The foliation occurs when intense pres sure crushes the mineral
grains until they become elongated and squashed into alignment
to form a thin band. The mineral grains in these rocks appear to
be arranged in bands. The direction of the thin bands reveals the
direction in which the pressure was applied to the rock. Granite
gneiss (pronounced “nice”) and biotite schist are types of
foliated metamorphic rock. Granite gneiss forms when the
granite protolith is buried deeply under the earth where
pressure on the rock metamorphoses it.
Non-foliated metamorphic rock does not look like a sheet or
plate of aligned minerals. Non-foliated metamorphic rocks
appear more “messy.” Some non-foliate
metamorphic rock, such as limestone,
has mineral grains that cannot be
flattened out. So limestone is always
non-foliated. However, if limestone
becomes buried deep under the earth,
heat and pressure may eventually
compact and metamorphose it into
marble. Other non-foliated metamorphi
rocks form from contact with igneous
rock. Contact metamorphism occurs
when extremely hot igneous rock
intrudes into the protolith. The heat of
the igneous rock is so intense that the
mineral structure of the protolith is
altered. Contact metamorphism usually
occurs without any additional force or
pressure.
Regional metamorphism refers to the
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major tectonic forces and events that occur in and under the
earth. Most metamorphic rocks are regional. For example,
regional metamorphic rocks form when two tectonic plates
crash into each other. The rocks are created in the region of the
tectonic collision, as in the Himalayas where two plates are
colliding and uplifting the mountains. Most of the world’s
mountains are rich in metamorphic rock. Regional
Regional metamorphism occurs in areas of
tectonic activity, such as regions of mountain
building where tectonic plates are colliding.
Contact metamorphism takes place when
igneous rock intrudes into cooler protolith and
alters it without the addition of pressure.
metamorphic rocks are also abundant
along the margins of the Pacific, where
tectonic plates are on the move and seismic activity is frequent. Becaus e
conditions vary from tectonic region to
region, metamorphic rocks in each regio
are slightly different from similar rocks i
other regions.
By studying the composition of
metamorphic rocks formed today and
comparing them with rocks formed
millions of years ago, scientists can recreate
a picture of what types of tectonic
activity occurred so long ago. In this
way, metamorphic rocks serve as a
window into Earth’s geologic past.
Schist, at the top, is a foliated metamorphic rock
derived from sedimentary shale rock. Its bands
are visible. The marble beneath has no visible
striation. Marble is a non-foliated metamorphic
rock that is derived from limestone.