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LECTURE 10. MATERIALS OF THE EARTH'S CRUST
Structure of the Earth
Studies of seismic waves have revealed that the Earth has
four internal layers - the solid inner and molten outer core,
consisting of iron and nickel; the mantle and crust,
consisting of various types of rock.
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Geography is mainly concerned with the CRUST, because
that is the zone of human habitation. Compared to the
size of the Earth, the crust is small; oceanic crust averages
only about 5 miles thick; continental crust about 20 miles
thick. The base of the CRUST is marked by a sharp
increase in density - known as the MOHO
DISCONTINUITY - this marks the transition from lower
density rocks of the crust to higher density rocks of the
mantle. The upper part of the mantle is rigid and solid together with the overlying crust this forms the
LITHOSPHERE (to about 70 km depth). Below this is the
ASTHENOSPHERE: a weak part of the mantle extending
to about 250 km depth, within which the rock is partially
molten and capable of flowing under pressure.
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MINERALS. As mentioned previously, the crust is made of rock
- but what are rocks made of? The answer is "MINERALS"; a
combination of atoms of various elements. Relatively few
elements make up most of the Earth's crust.
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Atoms of these elements become bonded together in
specific 3-dimensional arrangements to form crystalline
substances. The packing/arrangement of atoms depends
primarily on their size.
Therefore, the type of atoms, type of atomic bonding
and size of atoms determines; MINERAL
COMPOSITION (type of atoms), STRENGTH (bond
strength) and STRUCTURE (arrangement of atoms)
(In other words, the PHYSICAL AND CHEMICAL
PROPERTIES of the mineral).
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Atomic structure of the mineral halite (chemical name = sodium
chloride; common name = table salt). The sodium and chloride
ions (charged atoms) are bonded by opposite electrical charges. The
size of the ions determines how they pack together.
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Most minerals are quite small, but millions of mineral
particles join together to form ROCKS.
Fluorite
Halite
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Calcite
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ROCKS
Rocks are classified by their origin:
1. IGNEOUS: formed from magma (molten rock).
2. SEDIMENTARY: formed from the accumulation
of rock and mineral fragments or precipitation of
dissolved minerals.
3. METAMORPHIC: rocks altered by heat, pressure
and/or chemical action.
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Igneous rocks are formed by cooling of molten rock, which is
referred to as MAGMA (below surface in magma chambers) or
LAVA (on surface). Classification of igneous rocks is based on
TEXTURE (crystal sizes) and MINERAL CONTENT.
Extrusive
Intrusive
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TEXTURE: The size of mineral crystals within an igneous
rock depends on the rate of cooling of the magma or lava.
The rate of cooling is usually very fast for extrusive rocks,
producing fine grains; and very slow for intrusive rocks,
producing coarse grains.
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Intrusive
Extrusive
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Granite is a very common
intrusive igneous rock,
while BASALT is the most
abundant extrusive igneous
rock on Earth, for the
simple reason that all the
ocean floors of the world
are made of basalt (the
reason why will be
explained later in the
section on plate tectonics).
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Basalt is EXTRUSIVE - formed from LAVA erupting
onto the surface of the earth (the ocean floor and land).
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It should be noted that although intrusive rocks form below the
surface, many are later exposed by erosion; for example, the
Sierra Nevada mountains of Yosemite Park are granite.
Half Dome Mountain
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An example of basalt = the Columbia Plateau of Washington/
Oregon; and Hawaii
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The Hawaiian Islands are large basaltic volcanoes that
have grown from the ocean floor to rise above sea level in
the Pacific.
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SEDIMENTARY ROCKS
Rock exposed at the surface of
the Earth is subject to
disintegration and solution by
the elements of the weather
(rain, ice, wind etc.). This
WEATHERING produces rock
and mineral fragments and
dissolved minerals called
SEDIMENT (eg. boulders,
gravel, sand, mud, clay).
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Sediment is often carried by
rivers into lakes and seas
where it accumulates to
form layers. After some
time, these layers are
compressed by the weight of
overlying sediment and
dissolved minerals
precipitate into spaces
between particles forming a
cement - the sediment layers
are thereby LITHIFIED
into solid sedimentary rock
strata.
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Common examples of such CLASTIC sedimentary
rocks are sandstone and shale.
Sandstone (cemented sand particles).
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Shale (cemented clay particles).
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There are also organic
sedimentary rocks made
from the remains of
organisms; for example
many marine organisms
have calcium carbonate
shells, the fragments of
which form limestone.
These shells can also
dissolve and be
precipitated as limestone
without shell fragments.
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The white cliffs
of Dover are
made of
CHALK, which
is a type of
LIMESTONE.
These cliffs
formed as
thousands of
layers of
calcium
carbonate mud
on the ocean
floor - the
layering is
visible.
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Most of Dallas County is on Austin Chalk, shown here at
Tenmile Creek, DeSoto.
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West Dallas County contains Eagle Ford Shale, shown
here near Mountain Creek Lake.
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The Dallas/Tarrant county border is on Woodbine sandstone,
shown here at Arlington. The sandstone contains wood and
oyster fossils.
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Enchanted Rock, in the Hill Country, is a granite dome.
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METAMORPHIC ROCKS
A metamorphic rock is any rock that has been
changed by heat and/or pressure (resulting from earth
movements), or chemical action.
Heat and pressure causes rock to become "PLASTIC",
meaning that atoms and minerals can move around
and slowly slide past each other. The atoms can join
together in new ways to form different minerals and
often the pressure produces a FOLIATED
STRUCTURE.
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1. Granite -> Gneiss
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2. Shale + low intensity metamorphism -> clay minerals change to
tiny mica flakes = slate. Shale + medium intensity metamorphism ->
clay minerals change to larger mica flakes = phyllite.
phyllite
slate
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3. Shale (and other rocks) + high intensity metamorphism ->
schist. Accessory minerals, unique to metamorphic rocks, may
form e.g. below is a garnet mica schist.
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4. Sandstone -> quartzite.
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5. Limestone -> marble.
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