Download Oceanic Crust

Earth Science
And the Ocean
Part I
Plate Tectonics
Structure of the Earth
Plate Boundaries
Driving Mechanisms of Plate Tectonics
Structure of the Earth
The Earth can be
considered as being
made up of a series of
concentric spheres, each
made up of materials
that differ in
composition .
We will Focus on 4
Layers
Crust and Lithosphere
Crust: the outermost layer of the earth, a hard outer shell.
Crust beneath the oceans and the continents is different:
Oceanic crust: relatively thin, varying from 5 to 8 km
(but thinner at Oceanic ridges) yet denser.
Has the average composition of basaltic
rock (Basalt) that is rich in silica and magnesium.
Continental Crust:
thicker than oceanic
crust but it is less dense
Thickness ranges from 20 km to about 75 km (beneath
mountain ranges).
Has the average composition of granitic
rock (Granite).
Lithosphere:
The topmost layer of
the upper mantle.
The Lithosphere is more viscous (fluid) than any
other part of the mantle. This is what the crustal
plates are sitting on.
Due to its elasticity and convection we have plate
movement
The crust and lithosphere are broken up into 25
Lithospheric Plates
USGS
Oceanic Vs Continental
Made of Basalt
Made of Granite
Thinner
Thicker
More Dense
Less Dense
Younger
Older
Continental Crust is _____
Oceanic Crust
1.
2.
3.
4.
Older than
Younger than
The same as
None of the above
Continental Crust is made of
1.
2.
3.
4.
Granite
Feldspar
Basalt
Quartz
Oceanic Crust is ______
Continental Crust
1. Thinner than
2. Thicker than
3. The same as
Oceanic Crust is made of
1.
2.
3.
4.
Granite
Feldspar
Basalt
Quartz
The Mantle includes the Lithosphere.
Upper Mantle or Lithosphere: near its melting point so
that it behaves like a plastic (Similar to Silly Putty ); the
upper mantle material is more fluid
Upper mantle material
MOVES by convection;
transfers heat from within
the Earth towards the
surface.
Lower Mantle: solid
material, rather than
plastic.
The Core: the metallic portion of the Earth; Iron mixed
with small amounts of Nickel. It is at the center
Outer Core: probably liquid (based on studies of shock
wave passage through the Earth).
Inner Core: solid, made up of cooled liquid core material.
In a Nutshell…….
• The Earths interior is composed of 4 main
layers.
• The Crust the outermost layer.
• Oceanic Crust Thinner, Denser, Younger and
made of Basalt.
• Continental Crust Thicker, Older, Less Dense
and made of Granite.
• The Mantle which includes the Lithosphere
which is just under the crust. Largest of the
layers
• It is composed of molten rock which has the
consistency of Silly Putty.
• The Core Fe Ni inner core (solid) and a liquid
outer core also made of nickel and iron.
The Outermost layer of the
Geosphere is the
Inner Core
Outer Core
Mantle
Crust
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In
1.
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3.
4.
There are ____ Crustal Plates
10
1
13
25
0%
25
0%
13
0%
1
0%
10
1.
2.
3.
4.
The Crustal plates sit or “Float” on
top of the
0%
0%
0%
0%
1.
2.
3.
4.
Inner Core
Lithosphere
Crust
Outer Core
The Lithosphere is part of the
0%
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0%
0%
1.
2.
3.
4.
Crust
Inner Core
Outer Core
Mantle
Plate Boundaries
The types of boundary movement between plates are
distinguished by the type of relative plate motion along the
boundary:
Oceanic Ridge – Divergence
Oceanic Trench – Convergence
Transform Margins – Horizontal slip
Oceanic Ridge
More-or-less continuous volcanic mountain chain
throughout the world's oceans.
65,000 km long.
Average width approx. 1,000 km.
Rise up to 3 km above
the surrounding sea
floor.
Average depth approx.
2.3 km below sea level.
A kilometre deep valley
runs along much of the
length of the ridge.
The ridge is a Divergent Plate Margin and divergence
takes place by Sea Floor Spreading.
New crust is added from upwelling magma (molten
rock) from the upper mantle.
Older crust is
pushed laterally
away from the
ridge axis – so that
the sea floor
spreads away from
the ridge axis.
From http://www.uwsp.edu/geo/faculty/ritter/glossary/s_u/sea_flr_spread.html
Oceanic crust becomes older with distance from the
oceanic ridge.
Spreading rates (distance per year that two points on
either side of a ridge move apart) vary:
N. Atlantic Ridge
3cm/yr
S. Atlantic
5cm/yr
N. Pacific
12.5cm/yr
E. Pacific
17.5 cm/yr
http://www.gisdevelopment.net/technology/images/image002.gif
Divergent Boundaries Summary
• Crustal Plates are moving Away from each other
at this boundary.
• Most well known is the “Mid Atlantic Ridge”
• Largest Geological structure on Earth
• Area of sea floor spreading – The Atlantic Ocean
is getting bigger.
• Can be found on land in the country of Iceland
• Definitive Proof of Crustal Plate Movement through Paleo Magnetism of rock.
The mid Atlantic Ridge is a
____________ plate boundary
Divergent
Convergent
Transform
Integral
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1.
2.
3.
4.
The largest geological structure on
Earth is
0%
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0%
0%
1.
2.
3.
4.
The Grand Canyon
Mid Atlantic Ridge
Challenger Deep
Mount Everest
The Atlantic Ocean is getting
Smaller
Bigger
Deeper
Shallower
al
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Sh
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D
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0%
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0%
B
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Sm
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1.
2.
3.
4.
The only place on Land you can
see the Mid Atlantic Ridge is
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0%
0%
1.
2.
3.
4.
Ireland
Greenland
Iceland
Norway
At divergent plate boundaries the
plates are moving
1. Toward each
other
2. Away from each
other
3. Sliding past each
other
4. down
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•
•
•
•
Oceanic- Continental
Oceanic – Oceanic
Island Arcs (talk more about this later)
Trenches
Oceanic Crust – Continental Crust
Basaltic oceanic crust descends beneath lighter
continental crust.
Coastal mountain chains develop due to compressive
forces and volcanics (e.g., the Andes of South
America).
Magma material rises from descending slab and builds
volcanoes in
the rising
mountains.
Crust descends at angles from 35 to 90 degrees.
Crust melts as it descends, beginning at 100 to 200 km
depth and has melted completely by 700 km depth.
The zone over which melting takes place is termed the
Benioff Zone.
Melting crust rises and penetrates overlying crust to
form volcanoes.
Material (sediment and basaltic rock) is scraped off
the subducting crust and accreted to the over-riding
crust – termed the subduction complex.
Oceanic Trenches
Thousands of kilometers long, 50 to 100 km wide and
several kilometers deep (below sea level).
Longest trench: Peru-Chile trench at 5,900 km.
Deepest trench: Mariana trench (western Pacific) at 11
km deep (almost 7 MILES deep)
Trenches are termed Convergent Plate Margins
because they are locations where plates converge on, or
push against, each other.
Where oceanic crust is subducted back into the upper
mantle.
Island Arcs parallel many oceanic trenches: arcshaped chains of volcanic islands (e.g., Japan) due to
the rising magma from melting subducted crust.
When one plate moves under
another plate. This is called
Induction
Deduction
Subduction
Abduction
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1.
2.
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4.
Older Oceanic Crust will move
__________ Younger Crust
0%
0%
0%
0%
1.
2.
3.
4.
Over
Under
Sideways to
Into
Island Arcs form Parallel to
Oceanic Trenches
1. True
2. False
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The deepest trench is the
Marianas Trench
Tonga Trench
Japan Trench
Peru –Chile
Trench
0%
–C
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1.
2.
3.
4.
The longest trench is the
1. Tonga Trench
2. Peru- Chile
Trench
3. Marianas Trench
4. Kermadec Trench
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Continental Crust will subduct
under Oceanic Crust.
0%
0%
1. True
2. False
The Area where the Oceanic crust
is “melted” back into the Magma is
called the
1.
2.
3.
4.
Benihoff Zone
Melt Zone
Magma Deep
Trench Zone
0%
Benihoff Z...
Melt Zone
Magma Deep
Trench Zone
Continental Crust-Continental Crust
Neither plate subducts (both too light).
Compressive forces driving plates fold and thrust the
continental margins forming an extensive mountain
belt (e.g., the Himalayan Mountains).
Convergent Boundaries Summary
• These are areas where plates are moving
toward each other.
• This is an area of Subduction
• The Oceanic plate will move under the
continental plate.
• The Older Oceanic Crust will move under
the Younger Crust
• This is where Trenches are formed
• The Oceanic plates get “recycled” back into
the upper mantle
• Volcanic Island arcs are formed parallel to
the trenches
• The Deepest Trench is the Marianas
Trench which is 11 KM deep
• The Longest Trench is the Peru- Chile
Trench
• Benihoff Zone is the Area of the Mantle
where the Oceanic crust is “Recycled”
back into the Magma.
Transform Plate Margins
Plate margins along which the plates slip by each
other. Termed: Transform Faults
On either side of a transform fault plate motions are in
opposite directions.
Sudden Shifts will result in Earthquakes
Transform faults displace the oceanic ridge.
The San Andreas Fault is a transform fault.
The land east of the fault is on
the North American Plate; the
land west of the fault is on the
Pacific Plate.
The eastern side of the fault
moves southeast and the western
side moves to the northwest.
Total movement along the fault
has been 564 km over the past 30
million years (1.9 cm per year).
The San Andreas Fault is a
__________ plate Margin
Divergent
Transform
Convergent
Invergent
75%
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6%
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Sudden Movement at a Transform
Margin will result in a
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13%
87%
1.
2.
3.
4.
Volcano
Trench
Ridge
Earthquake
But what drives plate tectonics?
Two main hypotheses:
1. Convection Cells within the upper mantle (first
postulated by Arthur Holmes a year before Wegener
died).
and
2. Ridge push and slab pull.
Mantle Convection
Giant convection cells within the upper mantle
drag the plates along laterally.
Where convection rises sea floor spreading takes place.
Where the convection cells descend they drag crust
down, causing subduction
Ridge push and slab pull
Where new, young crust forms its weight pushes down
slope to drive the plates laterally.
Once the crust has cooled, having been pushed away
form the ridge, it sinks into the upper mantle and helps
to pull adjacent crust along.
This pushing and pulling provides the forces that drive
plate tectonics.
Convection is caused by
Movement
Heat
Plates
Margins
63%
31%
6%
ar
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M
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1.
2.
3.
4.
Slab pull would occur at
1. Ridges
2. Trenches
3. Transform
Margins
4. None of the
Above
60%
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Slab pull would occur at
88%
13%
0%
0%
1.
2.
3.
4.
The Ridge
The Trenches
Transform Margins
Volcanoes
Convection can only occur when
you heat gases like air
1. True
2. False
60%
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Fa
Tr
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40%