The shape of the ocean floor is determined by processes in the

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The shape of the ocean floor is determined by
processes in the lithosphere (rock layer).
Lets look at these processes today.
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PLATE TECTONICS,
(or the theory of just about everything
in geology..)
The theory The Earth is covered by a series of
rigid plates that move in relation to one
another and interact at the margins of the
plates.
Why is this a theory and not a fact in science?
Facts are observations, theories are attempts to
explain those observations.
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Lets look at some observations
At the turn of the 20th Century, it was
commonly thought that the Earth consisted of a
rigid outer crust with a hot molten interior.
WHY?
1. Temperature increases 3.5C/100m in
mines. 2. Rocks begin melting at about 500C,
so the depth to the fluid interior was about
15 km. (Earths radius is 6371 km).
but the Earth did not deform as much as a
ball of fluid should under the influence of solar
and lunar tides, so scientists were beginning to
think this was wrong.
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Along come seismologists (people who study
earthquakes and waves from earthquakes)
With these newfangled devices called
seismometers, they begin recording two different
types of waves from earthquakes
1) P waves 2) S waves
Particle motion is in direction that wave is
traveling. Traveling pattern of compression and
rarefaction. In and out motion.
Particle motion is perpendicular to direction
that wave is traveling. Sideways motion.
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and guess what?
They found S waves traveling through the
Earths interior (so NO liquid ball!) but not
through a really deep part they called the core.
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The Earth is layered.
Solid inner core
(16 total)
Molten outer core
Solid mantle (83)
Solid crust (1)
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Other observations?
Fit of Continents
He also used fossil evidence for his theory
of continental drift.
Alfred Wegener suggested that continents were
originally all together in a landmass called
Pangaea (1912)
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Fossil Evidence
How can you explain where fossils of land
reptiles are found? (They cannot swim!) and
what about tropical ferns in Antarctica??
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Past climates
Greenland
Maine
Deserts- sand dunes, dry lake deposits
Tropical climates-reefs, coal
Glacial deposits
Geologic deposits indicating past climates make
sense only if continents are reassembled and
moved from where they are today.
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Pangaea
Present day
Why do Africa, India, and Australia have evidence
of glaciers? What climates do these have now?
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Matching Rocks Across Oceans
so geologists made a prediction based on their
theory of continental drift (there was no plate
tectonics then!) that similar rocks should be
found in Africa and the corner of South America
Geologic units on continents on opposite sides
of oceans match!
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At the end of WWII, the geologic community is at
an impasse. There is evidence for continental
drift, but it is largely ignored because there is
NO mechanism to move continents.
but depth sounding begins to provide an ocean
basin profile that does not match with
expectations.
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Not only was the middle of the ocean high, but
the thickest sediments were at the basin margins
(not the middle)
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Distributions of earthquakes seem to mirror
continental shapes
Note how shape of earthquakes in mid-Atlantic
matches that of Africa?
(and these earthquakes fell on the mid-Atlantic
ridge!)
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and what about the volcanos?
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This led in 1960 to the radical idea that the sea
floor was spreading. New crust was added at the
mid Atlantic ridge.
If new crust was being added, then A) the
Earths surface has to expand OR B) we have to
remove crust somewhere else.
Subduction zone - where the crust is removed
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Where would you find the youngest oceanic crust?
Where would you find the oldest oceanic crust?
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Notice oldest oceanic crust is 0.161 B.y.
old, compared to 4.6 B.y. for the Earth
The oldest oceanic crust is often at the trenches!
(Note the time scales the fastest plates move
about as fast as your fingernails grow (10
cm/year))
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Plate Tectonics
Finally, in 1965, Wilson puts continental drift
and sea floor spreading together into the
theory of plate
tectonics
Fossils
Shapes
Climate
Continental Drift
Similar rock types
Sea Floor Spreading
Ridges, trenches
Age of sea floor
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This is nice, Dr. Park, but WHY SHOULD I CARE?????
The tectonic system makes the ocean floor, thats
why!
Lets look at the ocean floor more carefully.
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Why are the continents (and continental shelves)
so much higher than the ocean basin?
Shelf
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What do we know that might help us?
1. Continents are made of many rocks but
compositionally are very similar to granite
(like you see in the Box Springs Mountains).
These are light (less dense)
2. Oceanic rocks beneath the sediments (the
oceanic crust) are made of basalt which is heavy
(more dense).
3. Light things float and heavy things sink.
This is buoyancy and is a consequence of
GRAVITY!!!
IS IT POSSIBLE THAT THE OCEAN FLOOR ISLOWER THAN
THE CONTINENTS BECAUSE THE BASALT IS DENSER AND
THUS FLOATS LOWER THAN THE CONTINENTS???
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!!!
The Earths crust is in isostatic equilibrium
- plates float on the underlying mantle
Isostasy buoyancy. The Iceberg principle-
pressure forces from the displaced water (or
rock) balance the weight of the object)
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Isostasy buoyancy. The Iceberg principle-
pressure forces from the displaced water (or
rock) balance the weight of the object)
Objects higher above sea level must have deeper
roots.
Lets make a prediction continents are higher
and ocean floors are lower. Which should have
deeper roots?
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Continental crust can be up to 65 km thick
Oceanic crust is about 7 km thick
These observations support our hypothesis that
ocean floors are lower because the oceanic crust
is denser.
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SUMMARY
Continental shelves are higher because they are
composed of lighter crust and thus float higher
than the oceanic crust. This is because of
gravity creating buoyant forces (aka isostasy).
Hot magma rising there to form volcanos
along ocean ridge. Buoyancy of the magma
pushes the overlying crust upward.
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LIFE IS TOUGH WHERE PLATES MEET
Three kinds of interactions
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Lets look more closely at the different margins
and how they affect the oceans
Divergent Margin
Magma (molten rock) rises to the surface here and
encounters what?
WATER, that cools and solidifies it!
1. New ocean crust is created (Makes the oceans
bigger).
2. Outgassing - gases added to hydrosphere and
atmosphere
3. Chemicals from mantle added to ocean - black
smokers
4. Heat is added to the ocean.
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Convergent Margin
2. Volcanic islands (like Japan) form.
3. Volcanos provide sediment from erosion
1. Ocean crust is consumed (oceans get smaller.)
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Transform Margin
BORING, MOSTLY!!
1. Crust is neither added or removed at these
margins.
2. No volcanos either, just earthquakes!
Transform margins form because the ocean crust
breaks in different places as it is pulled apart
by subducting plates.
MAGMA DOES NOT PUSH THE PLATES APART!
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What makes the tectonic system run?
To answer this, we need to know a bit more
about what is inside the Earth.
HOW WILL WE FIND OUT?
ONLY DIRECT OBSERVATION!
1. Deepest drill hole 10 km (or not even
through
crust!) 2. Seismic waves 3. Xenoliths
- foreign rocks. Samples of lower
crust and mantle brought up in
volcanic eruptions. 4.
Experimental petrology - take probable types
of rocks and put them under high P, T to
see what forms.
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Actually, 2 different sets of terms for layering
Mechanical Lithosphere- rigid outer layer
Asthenosphere - weak, flowing layer Lower
mantle - rigid lower layer Outer core - dense
liquid Inner core - dense solid
Compositional Crust - granite, light
(continental) basalt, dense
(oceanic) Mantle - Fe, Mg silicates Core -
Fe, Ni
Crust upper mantle lithosphere
(upper 100-200 km)
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and we know that subducting plates go
quite deep into the mantle.
Colder, dense
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Hot stuff (magma) rises at the ocean ridges and
cold ocean plates sink back into the mantle at
the trenches.
WHY?
Buoyancy!!! (due to the force of GRAVITY)
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The tectonic system is driven by gravitational
forces causing hot, less dense magma to rise at
the ocean ridges and cold, denser oceanic
lithosphere to sink at the trenches
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This is differentiation of the Earth that has
continued since its formation. What do you think
will happen when all of the heat is transferred
from the core to the surface of the Earth (most
of this heat is lost to space, by the way)?
Moon
Good news radioactive elements are still
making heat, so we have a few billion years left.
NO MORE MOVING PLATES!
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What have you learned today about the coupling
between the lithosphere and hydrosphere?
1. The shape of the ocean basins is controlled by
the tectonic system shaping the lithosphere.
2. Isostasy (buoyancy) explains why the
continental shelves are so much higher than the
ocean basins and why the ridges are higher than
the surrounding basins.
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3. Volcanic processes at the divergent and
convergent margins of plates contribute
chemicals, heat, and sediment to the ocean (more
on these later).
4. The tectonic movement of plates results in
transfer of heat from the interior to the
surface of the Earth. When the interior has
cooled, the system will likely stop.
5. Ocean floor is oldest at the trenches (or
adjacent to continents) and youngest at the
ridges.
6. Divergent margins create new ocean floor
convergent margins consume it.
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and one last item concerning oceans in the
geologic past.
Do you think that the ocean basins have always
been the same shape?
Where is the Atlantic Ocean?
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The moving plates have changed the shape of
the oceans profoundly
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Deep ocean circulation
As you will see, the continents influence the
circulation patterns in the oceans