Rocks - Fort Thomas Independent Schools

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First found May 22, 2018

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Rocks
• I. Introduction
• Millions of years to complete the
cycle
• Multiple paths a rock can take
during the cycle
• Three broad categories based on
the way the rocks are formed:
• Igneous
• Metamorphic
• Sedimentary
II. Igneous Rocks
• Make up over 70% of
continental crust and
90% of oceanic crust
• Formed when molten
rock cools and solidifies
• Two types of igneous
rocks:
• (1) Intrusive (plutonic)
• Magma cools within Earth
as opposed to on surface
of Earth
• Cooling rate is slower
resulting in coarser
grained rocks
• Minerals are visible to
naked eye
• Examples: granite, gabbro,
peridotite
• (2) Extrusive
• magma cools on Earth’s
surface, usually from volcanic
eruptions
• Cooling rate is faster resulting
in finer grained rocks
• Minerals are too fine to be seen
with naked eye – petrographic
microscope
• Examples: rhyolite, basalt, and
pumice
• Some igneous rocks have both intrusive
and extrusive features
• Result of two step process – some
cooling within Earth; some on surface
• Porphyritic texture – combination of
coarse and fine crystals
• Identifying igneous rocks
• Texture is important, but not the only
consideration
• Mineral composition, especially silica
content
• Light colored rocks typically have high
silica content (granite, rhyolite)
• Dark colored rocks typically have
lower silica content (gabbro, basalt)
III. Sedimentary Rocks
• Formed by contributions
from wind, oceans, rivers,
rain runoff and gravity
• Typical process includes:
• Weathering and erosion
breaks down rocks (of any
kind) and moves the pieces
to other locations on
Earth’s surface
• Water currents naturally
sort out the minerals by
their size and weight
(coarse, medium, fine)
• Particles settle and are
deposited
• Compaction and
cementation press the
particles into a new rock
• Classified based on texture,
chemical composition, and
mineralogy
• Major categories:
• Clastic
• Made from other rock pieces
• Subdivided by grain size (fine
sand vs. boulders)
• Further grouped by mineral
content
• Examples include:
conglomerates, sandstones, and
shales
• Chemical
• Precipitated material
• Examples include:
limestone and dolomite
• Organic (biogenic)
• Formed from organic
(or once living)
material
• Example: coal
IV. Metamorphic rock
• Has undergone a structural
and mineralogical change
• Degree of change depends on
the amount of heat and
pressure and length of time
• Classified based on texture
• Foliated -- aligned sheet or
plate-like layered structure
(gneiss and schist)
• Non-foliated – non aligned
layers (marble and slate)
V. Rock Cycle
• Rocks of all three types can be
changed into another type
• A very long process (millions of
years)
• Involves erosion,
sedimentation, uplift, deep
burial, and recrystallization
• Moving tectonic plates create
heat, pressure and chemical
reactions
• Examples of transformations:
• Sedimentary rocks are transformed
into metamorphic rocks, such as
Limestone turning into marble and
mudstone to slate, after thousands
to millions of years of heat and
pressure.
• An igneous rock that reaches
Earth’s surface through the
uplifting of mountains is destined to
break and weather into sediments,
thereby becoming part of the
sedimentary class of rocks.
• Magma from Earth’s interior adds
new igneous rocks through
volcanic eruptions and at mid-ocean
ridges.
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