Transcript PPT - Hss-1.us

Env Sci Week 6 - Geology, Earth and
the Lithosphere
• Geophysical Process of the Earth
Layers of the Earth
• There are two different
ways to describe the
layers of the earth. The
layers can be described
by their chemical
properties or their
physical properties.
Look at the diagram
below: chemically
defined layers are on
the shown the left;
physically defined
layers are shown on the
right. Notice how some
of these layers overlap.
Layers of the Earth
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The Crust is the layer that starts at the solid surface. Oceanic and continental crusts
are different. The depth of the crust has been defined seismologically. The oceanic
crust is about 6 - 7 km thick; the continental crust is about 35 - 40 km. For this class
we will say the average depth is down to about 6 miles (10 km).
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The Mantle is the layer of rock that begins at the bottom of the crust at about 6 miles
(10 km) below sea level and goes down to about 1,800 miles (2,900 km) below sea
level. Since the mantle is so thick compared to the crust, its total thickness can also
be estimated to be about 1,800 miles (2,900 km). (Spelling note: "mantel " is over a
fireplace. Mantle is a covering, such as a loose, sleeveless coat.) The mantle is
composed of an upper and lower layer. The upper mantle is solid, the lower mantle
flows.
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The lithosphere is the rigid outer layer of the Earth consisting of the crust and upper mantle.
The lithosphere is approximately 100 km (62 mi.) thick The lithosphere is the rigid outer layer
of the Earth required by plate tectonic theory. It differs from the underlying layer because of
its mechanical ‘flow’ properties, even though the layers are chemically similar. The
lithosphere responds essentially as a rigid shell whilst the
asthenosphere behaves as a highly viscous fluid
The Core is the layer that begins at the bottom of the mantle at 1,800 miles (2,900
km) miles and goes down another 4,000 miles (6,500 km) to the center of the earth.
The types of rocks are:
• (1) Igneous - Rocks that are formed by first heating the earth's
material (elements) to a molten lava state. The when the molten
lava cools it forms igneous rocks. Today we can see this happening
in association with volcanoes.
• (2) Sedimentary rocks are rocks that form when the rocky and
organic matter caused by erosion settles into layers (sediments)
and over time hardens to form rock.
• (3) Metamorphic rocks are igneous or sedimentary rocks that have
been "changed" from their original form through pressure and heat.
The metamorphic rock is NOT heated enough to make it molten.
• (4) Conglomerates. Some rocks don't fit nicely into the three
categories of rocks. These rocks are a mixture of igneous,
sedimentary and/or metamorphic rocks that are cemented together.
These are called conglomerates.
Mantel
• The Mantle is the layer of rock, which begins at the bottom of the
crust, at about 6 miles (10 km) below sea level down to about to
1,800 miles (2,900 km) below sea level. The boundary between the
earth’s crust and mantle Mohorovicic discontinuity or “Moho.”
• There are several overlapping divisions used to describe layers
within the Mantle:
• (1) Upper Mantle is solid. Similar to the crust but the minerals that
make up the rock material are made up of different elements than
the crust. The distinction between crust and mantle is based on
chemistry, rock types, deformation, flow and seismic characteristics.
• (2) Lower Mantle is plastic rock. The bottom part up mantle is
made up of molten plastic type rock, meaning that is flows.
• (3) Lithosphere: The crust and solid part of the mantle.
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(4) Asthenosphere: The hot, plastic lower mantle and outer
core. Convection cells
within this layer bring heat to the
surface from the earth's molten layer.
Core
• The Core begins at the bottom of the mantle at 1,800
miles (2,900 km) miles and goes down another 4,000
miles (6,500 km) to the center of the earth. Seismic
measurements show that the core is divided into two
parts:
• (1) A solid inner core with a radius of ~1220 km - The
solid inner core was discovered in 1936 by Inge
Lehmann and is generally believed to be composed
primarily of iron and some nickel that is very hot but
made solid by th very high pressure )pressure frizzing).
• (2) A liquid outer core extending beyond it to a radius of
~3400 km.
• (3) Lehman Discontinuity is name for the boundary
between the inner and outer core.
Core and Relationship to the
Magnetic Field of the Earth
• Because electrical
currents can flow in the
molten rock of the
asthenosphere, a great
magnetic field is set up
that surrounds the earth.
Magnets are surrounded
by magnetic fields
described by Magnetic
field lines as shown in
the figure below. The
magnetic field can be
thought of as consisting of
lines of force. The forces
of magnetic attraction and
repulsion move along the
lines of force creating a
North and South Pole.
There are two current
• There are two current theories of how the Eath's magnetic field
developed and is maintained. They are the:
• Dynamo Theory: The theory that explains the origin of the Earth's
main magnetic field in terms of a self-sustaining dynamo. In this
dynamo mechanism, fluid motion in the Earth's outer core moves
conducting material (liquid iron) across an already existing, weak
magnetic field and generates an electric current. This turn generates
the earth’s magnetic field.
• Rapid-Decay Theory: The rapid decay theory states that the
Earth’s magnetic field is a function of how it was made. It states that
over time the electrical current and therefore the magnetic field is
weakening over time. The rapid-decay theory contradicts the
Dynamo Theory. The rapid-decay theory is consistent to the
observed weakening of the magnetic field that we have been able to
observe for about the past 200 years.
Plate tectonics
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Plate tectonics is a theory of
geology that has been
developed to explain the
observed evidence for large
scale motions of the Earth's
lithosphere. The plate tectonic
theory encompassed and
superseded the older theory of
continental drift from the first
half of the 20th century and the
concept of seafloor spreading
developed during the 1960s.
The plate tectonic theory
states that the lithosphere is
broken up into what are called
tectonic plates. The
lithospheric tectonic plates ride
or float on the asthenosphere.
In the case of Earth, there are
seven major and many minor
plates. The figure below shows
the seven major plates and
some of the minor plates.
Types of Plate Boundaries
(Interactions)
Types of Activity along plate
boundaries
• Earthquakes, volcanic activity, mountainbuilding, and oceanic trench formation
occur along plate boundaries. The lateral
movement of the plates is typically at
speeds of 0.65 to 8.50 centimeters per
year. Pangea is the idea that one big
super continent existed and before the
plates moved apart.
Pangea
Earthquakes
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Earthquakes are vibration of the earth that are the result of volcanoes or movement along faults. Faults are
boundaries between two sections of rock that can move relative to one another. The focus of the earthquake
is the point of origin of the earthquake. The epicenter is the point on the surface directly above the focus.
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Earthquakes are any sudden disturbance within the Earth manifested at the surface by a shaking of the
ground. This shaking, which accounts for the destructiveness of an earthquake, is caused by the passage of
elastic waves through the Earth's rocks. These seismic waves are produced when some form of stored energy,
such as elastic strain, chemical energy, or gravitational energy, is released suddenly.
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There are several causes of earthquakes, but all major earthquakes are thought to be caused by energy that
builds up along geological faults caused by large areas of the earth's crust called plates that converge. The
steps of earthquake showin the figure below are (1) geological faults develops - cracks in the earth crust, (2)
energy build up along the cracks, (3) semi-solid matter - matter that becomes semi-liquid such as magma that
allow section of the earth to slip more easily causing the earthquakes.
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21. Earthquakes
21.1 Causes of Earthquakes
Measurements of Earthquakes
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The strength of an earthquake is measure by:
(1) Life and property losses
(2) Instruments - seismographs (Richter scale). A seismograph detects several types
of waves (P, S ans L):
P (Primary): high frequency, pass through the interior and
the core, first to
arrive.
S (Second to arrive) pass through the interior but NOT the core , somewhat lower
frequency - longer wavelength than
L (Last) - low frequency long wave length - travel along the earth surface, last to
arrive.
Richter Scale is a numerical Scales that measure the strength of an earthquakebased upon how far a seismograph trace is displaced. The scale is as follows:
- 0-2 - only seismograph can feel
- 3-5 - people can feel - little if any damage
- 6- destructive earthquakes
- 7 and above very destructive earthquakes
Results of Earthquakes
• The results of an earthquake are
• (1) destruction and disruption of social
structures.
• (2) very large ocean waves called
tsunamis and
• (3) volcanic activity - result of the great
frictional and pressure force and allowing
magma to push to the surface of the earth.
Volcanoes
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Volcanoes occur when magma from the
lower mantle is pushed to the surface.
When the magma reaches the surface in
a volcanic eruption it is called lava. The
following are the key parts of a volcano:
- Magma pocket - main reservoir of the
magma underneath the volcano.
- Pipe: the main channel near the
middle of the volcano that leads from the
magma pocket to the surface.
- Sill: a horizontal layer of magma.
- Dikes: small channels of magma that
do not reach the surface.
- Volcanic cone: The cone shape of the
top of the volcano that we see as a
mountain.
- Crater: the depression at the very top
to the volcano where the magma
reaches the surface and becomes lava.
Land Formations Involving
Increases in Elevation
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Land formations that involve increase in elevation are features such as hills, mountains and
plateaus. The different formations have different processes that from them.
22.2.1 Mountains:
Mountains mean a region of land that is raised rather steeply above the surrounding terrain.
Mountains are normally found in groups or ranges consisting of peaks, ridges, and intermontane
(between mountains) valleys. The following terms describe groups of mountains.
Range -smallest group,
Mountain system -Several closely related ranges in a parallel alignment
Mountain chain -An elongated series of systems forms a mountain chain
Belt or Cordillera - An extensive complex of ranges, systems, and chains is known as a belt,
or cordillera.
Special terms used to describe different mountain formations are:
Peak: the highest point on a mountaintop. (Peak is the same as summit.)
Ridge: elongated peak of a mountain so it forms a line of high elevation.
Buttes: (pronounced like "beaut") A butte is flat-topped hill surrounded by a steep escarpment. An
escarpment is along cliff-like ridge of land. From the bottom of the escarpment, a slope descends
to the plain. The term butte is sometimes used for an elevation higher than a hill but not high
enough for a mountain. Buttes are formed when the land around it erodes away.
Mesa: The term mesa means a broad, flat-topped elevation with one or more cliff like sides,
common in the southwest United States. A mesa is typically larger than a butte. (Remember
that you might build a small town on the top of a mesa, but the top of most buttes would be too
small.)
Plateau: a large, relatively flat highland area within a mountain range.
Types of Mountains:
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Folded Mountains Folded mountains are formed when a mountain is in between two
regions of the earth pushing towards each other. When this happens, the mountain
begins to form as folds in the earth. An anticline is when a fold is folding towards the
sky and the bend is on top. A syncline is a fold that folds toward the ground.
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Fault-Block Mountains - a crack in the crust along which there has been movement
(slipping) may form a fault-block mountain. A force applied against a rock that cannot
relieve the stress by bending causes a fault. The rock cracks and moves in order to
relieve the stress. Types of movement result in: (1) Vertical faulting, (2) Horizontal
faulting and (3) Combined vertical and horizontal faulting.
Dome and Volcanic Mountains - Dome and volcanic mountains have a
characteristic ‘dome’ top. In the USA, the Black Hills of South Dakota offer
excellent examples of dome topped mountains. Erosion is believed to be a
major factor in the shaping of most dome formations. A type of "fold"
pushing up uniformly may have formed the dome. In the volcanic caused
mountains magma is released, in the domed caused mountain the magma
is contained below the surface.
Deposition & Erosion
• Some mountains features are caused by
accumulation or deposition of material
and some by erosion. Volcanoes,
glaciers and sand dunes are examples of
features caused by deposition. Mesa,
buttes and plateaus are examples of
mountain features that are the result of
erosion of sedimentary rock surrounding
a harder rock.
Land Formations Caused By
Water Erosion
• Many features we see are caused by flooding and water erosion.
These features include:
• (1) Valleys: The space enclosed between ranges of hills or
mountains; the strip of land at the bottom of depressions, including
usually the bed of a stream, with frequently broad river (alluvial)
plains on one or both sides of the stream.
• (2) Ravines, Canyons, Gorges and Gullies: Names given to deep
and narrow valleys with abrupt sides. Ravines, canyons, gorges
and gullies are usually the result of erosion by water.
• (3) Delta: a region of land that forms a trough of sedimentation that
occurs when a river enters a sea or lake. The Mississippi River
Delta in Louisiana, U.S.A. and the Nile River Delta in Egypt are
good examples.