Transcript Dynamic Ocean Floor

The Dynamic Ocean Floor
Lab #7
Timeline
• 400 million years ago fish evolved
• 200 million years ago Pangaea split apart,
and the start of the Triassic period
(Dinosaurs)
• Broke up into two smaller super
continents, called Laurasia and
Gondwanaland, during the Jurassic
period.
• 150 million years later, the Earth looks like
it does now.
Plate Tectonics
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What exactly is it?
The movement of plates and continents.
Proposed in 1912 by Alfred Wegener.
Not accepted until early 1960s.
Has had a major impact on Earth
Sciences.
• Occurs in the lithosphere by sea floor
spreading
Lithosphere
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Is the outer most shell of the Earth
This includes the crust and the mantle.
Means stone sphere
Continental crust is 35 km thick
Oceanic crust on 5 km thick.
Lithosphere total 100 km thick
Oceanic crust and continental crust sits on
top of the Lithosphere.
Asthenosphere
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Means weak sphere
Second layer of the Earth.
Approximately 600 km thick.
Outer section of this layer is liquid, this allows the
lithosphere to move.
• Convection currents play an important role.
Plate Tectonics
• The plates along the Lithosphere are moving,
and the continents are along for the ride.
• Its known that rocks are younger at the MidOcean ridge then at the continents.
• Why?
• This is verified by magnetic reversal information
preserved in the minerals of the mid-ocean
ridge.
• The first positive proof that the plates were
indeed moving.
Plate Boundaries
• Plate boundaries are found at the edge
of the lithospheric plates.
• Earthquakes coincide with plate
boundaries.
• There are four boundary types and they
are characterized by their distinct motions.
Plate Boundaries
Divergent
Moving apart
Convergent
Moving together
3 Types
Mid Atlantic Ridge
Subduction (#1)
One plate goes below another
Continental Vs. Oceanic
#2
Continental Vs. Continental
Both plates push up Mts. form
Transform # 3
Plates slide past each other
Divergent Boundary
• Two plates move away from one another.
• This is a zone of weakness.
• As two plates move apart at the mid-ocean
ridges, magma from the mantle up wells
through a crack in the oceanic crust and
cooled by the sea creating new ocean
floor.
• Energy is released in the form of
earthquakes.
• Shallow focus earthquakes (less then 100
km in depth) occur in this area.
Mid Oceanic Ridges
Mid-Atlantic Ridge
• Mountain ranges along the ocean floor.
• It has a distance of 600 miles.
• Longer and also higher then any other
mountain range on any continent.
• The ridges are home to Earth's highest
mountains, and deepest canyons.
• The shape of the mid-ocean ridge is
controlled by the rate the plates move
apart.
Older crust is pushed laterally away from the
ridge; thus the sea floor spreads apart.
•Where convection cells ascend sea floor
spreading takes place.
•Where the convection cells descend they drag
crust down, causing subduction.
•This pushing and pulling provides the forces
that drives plate tectonics.
Convergent Plate Boundary - Subduction
• Here one plate is overriding another forcing one into the
mantle.
– Crust descends at angles from 35 to 90 degrees.
– crust older than the Cretaceous period cannot be found in
any ocean basin.
• Deep focus quakes (100-600 km) occur in this area.
• As the magma melts pressure builds up and the result is
volcanic eruptions.
Convergent Plate Boundary - Continental vs.
Continental
• When two continents meet head-on, neither is subducted.
• The crust tends to buckle and be pushed upward.
Convergent Plate Boundary - Transform
• Third type of plate boundary.
• Material is neither created nor destroyed
at these boundaries, but rather plates slide
past each other.
• These are pressure valves
• The location of shallow focus earthquakes
Classic example: San Andreas fault in
California.
Convergent Plate Boundary Review
Evidence for Pangaea - Fossils
Fossils of glossopteris; a tree - like plant from the Permian
Period and dominant plant of Gondwana are found
throughout India, South America, southern Africa, Australia,
and Antarctica.
Evidence for Pangaea – Rock Formation
• Additional evidence for
Pangaea is found in the
geology of adjacent
continents, including
matching geological
trends between the
eastern coast of South
America and the western
coast of Africa.
• The continuity of
mountain chains also
provide evidence for
Pangea.
• One example of this is
the Appalachian
Mountains chain which
extends from the United
States to Ireland, Britain,
Greenland, and
Scandinavia.
(See chart on page 151)
Paleomagnetism
• Ancient magnetism in rocks
• Used to determine the rate and the time the
plates separated.
• As magma rises to the ocean floor and cools,
the minerals align with the Earth’s magnetic
field.
• When the Earth’s polarity changes the mineral
pattern will also change.
• It is estimated that it takes 70,000 years for the
polarity to rotate.
Paleomagnetism
and the Ocean Floor
• See pages 149-150
• Chart on pg 149 shows the pattern of
polarity over the past four million years.
• As new crust on the ocean floor is created
it spreads out evenly on both sides of the
mid ocean ridge.
Determining the rate of
sea floor spreading
• This example is for the Pacific plate pg 150.
• The green area is 2 million years old; thus the rock
needed 2,000,000 years to travel from the mid ocean
ridge to that distance.
• From the ridge to the green area is a distance of 160 Km.
• 1 Km = 100,000 cm
• So 160 Km = 16,000,000cm (mult. 160 X 100,000)
Distance
Rate 
Time
16,000,000 cm
 8 cm/yr
2,000,000 yr
Chap Summary page 155
• In Class:
– Calculate for Atlantic Ocean on pg. 153 and
record values on pg. 153 - #5
• Home Work:
– Questions #1 – 5 & 9 – 12 (Old Book)
– Questions #1 – 5 & 8 – 12 (New Book)