Download Unit VI: Circulation of the Solid Earth

Unit VI. Solid Earth Circulation
A. Structure of the Solid Earth
1. Crust :Thin, brittle, variable
composition, least dense.
a. Continental Crust: older,
thicker, more variable, lots of
water/volatiles
b. Oceanic Crust: thinner,
younger, more uniform
composition.
Unit VI. Solid Earth Circulation
A. Structure of the Solid Earth
1. Crust: Thin, brittle, variable
composition, least dense.
2. Mantle: Uniform composition;
upper Mantle behaves like a soft
plastic (rock that can slowly deform).
3. Core: Very hot, very dense. Outer
core liquid, inner core solid.
Mostly iron (85%) and nickle (6%).
Motion of core creates magnetic field.
Unit VI. Solid Earth Circulation
A. Structure of the Solid Earth
How do we look into the center of the Earth?
By tracking seismic waves produced by
Earthquakes
Volcanic explosions
Large human noises (atomic bombs)
P (compression)-waves can pass through solid
and liquid media.
S (shear)-waves can only pass through solid
media.
P (compression)waves
S (shear)waves
Unit VI. Solid Earth Circulation
A. Structure of the Solid Earth
B. Earth’s Magnetic Field
1. Core is a magnetic dynamo: Transforms
fluid motion (convection) into electrical currents
that create magnetic field.
Unit VI. Solid Earth Circulation
A. Structure of the Solid Earth
B. Earth’s Magnetic Field
1. Core is a magnetic dynamo:
Magnetic mineral grains (in lava or sediment)
align themselves with the Earth’s magnetic field,
preserving a record of the field direction when
they were deposited. Throughout recorded
history, the compass has always pointed North.
But, Earth’s magnetic field occasionally
collapses and reforms in the opposite direction.
These are called magnetic reversals.
Unit VI. Solid Earth Circulation
B. Earth’s Magnetic Field
1. Core is a magnetic dynamo:
Magnetic field occasionally collapses…. and
may reform in the opposite direction.
Magnetic reversals occur every 0.5 to 1 million
years. Most recently, 0.7 Myr ago.
Normal and Reversed Polarity
Unit VI. Solid Earth Circulation
B. Earth’s Magnetic Field
1. Core is a magnetic dynamo: Transforms
fluid motion (convection) into electrical currents
that create magnetic field.
2. Why does the core convect? Density
differences. Radioactivity: Radioactive decay
releases heat energy. Hot core heats the
overlying mantle, so the outer edge core is
coolest. Causes a fundamental instability.
Unit VI. Solid Earth Circulation
A. Structure of the Solid Earth
B. Earth’s Magnetic Field
C. Plate Tectonics
In the 1920s Alfred Wegener proposed that the
continents used to be together based on
similar continental outlines,
distribution of fossils and
mountain chains.
Unit VI. Solid Earth Circulation
C. Plate Tectonics
But there was no physical basis for drift, and his
ideas were dismissed for half a century.
In the early years after WWII, ocean exploration
revealed mountain ranges in the middle of the
oceans (mid-ocean ridges) and ships measured
the orientation of magnetic minerals on the sea
floor.
They found magnetic stripes!
Unit VI. Solid Earth Circulation
C. Plate Tectonics
1. Sea floor spreading: Magnetic stripes
get older away from mid-ocean ridges, proving
that oceanic crust is constantly being formed at
mid-ocean ridges.
Where does all that ocean crust go?
Oceanic crust is destroyed along subduction
zones, where it is recycled back into the mantle.
Unit VI. Solid Earth Circulation
C. Plate Tectonics
1. Sea floor spreading
2. Earth’s Tectonic Plates
3. Plate margins determine earthquakes and
volcanism.
a. Divergent Margins : where Earth’s
lithosphere is being pulled apart. Examples:
Mid-Ocean Ridges
African Rift Valleys
Unit VI. Solid Earth Circulation
C. Plate Tectonics
3. Plate margins
a. Divergent Margins: where Earth’s
lithosphere is being pulled apart.
Commonly associated with mild volcanism,
shallow earthquakes.
Fluid volcanism
Why?
Unit VI. Solid Earth Circulation
C. Plate Tectonics
3. Plate margins
a. Divergent Margins: where Earth’s
lithosphere is being pulled apart.
b. Convergent Margins: where two
plates are forced together (subduction/collision).
Examples?
Pacific Northwest
Aleutian Island Chain off Alaska
Japan, Italy, India
West coast of South America
b. Convergent Margins: where two
plates are forced together (subduction/collision).
Subduction of oceanic plate is commonly
associated with volcanism and deep
earthquakes.
What kind of volcanism?
Explosive volcanism, because heated rocks
have lots of volatile materials in them (water +
water tied to mineral (hydrated)).
When do we get collision, and when
subduction?
Ocean Crust/Continental Crust = subduction
because oceanic crust is more dense
Continental Crust/Continetal Crust = collision
equal densities…neither can easily sink.
Still get subduction, but not of continental crust.
Example: India …. Himalaya
Unit VI. Solid Earth Circulation
C. Plate Tectonics
3. Plate margins
a. Divergent Margins: where Earth’s
lithosphere is being pulled apart.
b. Convergent Margins: where two
plates are forced together (subduction/collision).
c. Transform Margins: Where two
plates slide past each other.
Examples? Southern California and the San
Andreas fault
4. What Drives Plate Tectonics?
In order to get rock to move we need:
• Density differences: Created by an inherent
instability…. Earth is hottest in its interior due
to…radioactive decay.
Liquid core circulates relatively quickly,
heats the lower mantle, which becomes less dense
than the overlying mantle, and SLOWLY starts to
rise. Buoyant.
Rigid lithosphere just along for the ride.
• Plasticity: heat+pressure make rock deformable
Unit VI. Solid Earth Circulation
C. Plate Tectonics
D. Earthquakes: sudden release of stored
energy as a result of rapid movement between
two blocks of rock.
Can only happen where rocks are brittle…..
in the lithosphere.
Unit VI. Solid Earth Circulation
C. Plate Tectonics
D. Earthquakes
E. Volcanos: Explosive vs Fluid
Where they occur and why it matters.
Most occur at Plate Boundaries
Most explosive volcanism occurs along
subduction zones, in continental crust.
Unit VI. Solid Earth Circulation
C. Plate Tectonics
D. Earthquakes
E. Volcanos: Explosive vs Fluid
Hot Spots: Intra-Plate volcanism
• Hawaii: Mid-ocean hot spot
• Iceland: Hot spot on mid-ocean ridge
• Yellowstone: Mid-continent Hot Spot
Iceland: a Hot Spot on a Mid-Ocean Ridge
Yellowstone
The last three Yellowstone eruptions
Caldera forming
Age
ash flow tuff
(millions of
years)
Lava Creek Tuff
Mesa Falls Tuff
0.6
1.3
Difference
Volume
erupted
(km3)
1,000
280
0.6 Myr?
0.7 Myr
0.8 Myr
Huckleberry
Ridge Tuff
2.1
2,450
For our purposes, volcanic eruptions are
most important if they are explosive and
place lots of material in the stratosphere.
But they also pose a very real threat to
nearby, and sometimes very distant,
communities.
Predictive tools are improving, and rarely
do volcanos erupt without warning.