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Chapter 7: Continental drift/Plate tectonics

Unit 7 of Earth Science
by

Bryce Cushman

on 11 November 2013

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Transcript of Chapter 7: Continental drift/Plate tectonics

Continental drift
Conclusion
Continental drift/Plate Tectonics
Continental drift does not explain everything about how the continents move ... which leads to plate tectonics
And one more thing...
Continental drift has a lot of evidence that supports the theory
Wegener proposed that continents used to be stuck together in one large mass called
Pangaea

The mass broke apart and drifted to their present positions over 200 million years

The evidence that went along with the claims included map projections, fossil records, rock structures and ancient climates.
Similarities between coasts of the South Atlantic.
A better fit is the seaward edge of the continental shelf
Look at fig 10.2 and write whether or not this evidence would make you believe in continental drift
Identical fossil organisms found in South America and Africa
Mesoaurus was found in similar conditions on opposite continents.
Debate - is it possible that the Mesosaurus could travel that far across the ocean? Did it travel across a land bridge? How could we know?
Supercontinents
Evidence
More evidence
100 years ago, scientists still did not understand how the Earth worked. A scientific revolution occured in 1915 proposed by Alfred Wegener called continental drift.
Today, the theory has become plate tectonics.
Rocks must match if continental drift is true. If the continents were once together, the rocks of a region need to match in age in type and should appear on different continents.

Debate - do we find similar structures across the Atlantic ocean, or are they just coincidence that some match up?
Evidence - Rock types
Wegener searched for indications of climates that shared similar changes across the Atlantic.

Data was found indicating that areas of the Southern hemisphere and India were covered with glacial ice. Areas in Africa and South America were covered with sediments from glacial ice.

Debate - could Earth have gone through a cool period that made ice sheets in areas that are presently tropical?

Or is it possible that Pangaea was located near the South Pole
Ancient Climates
Wegener's proposal did not attract attention until his book was translated into english. Most people thought his ideas were crazy, but a few thought it was possible.

The problem with Wegener's ideas were they had no mechanism to make it work. What causes the continents to drift?

Debate - do you agree with Wegener's idea of continental drift? How would you feel if you lived during this time period?
The debate
Write down what you think about continental drift. After writing for 60 seconds, open your book and read pg 190 box 7.1 and write where North America was in the Cretaceous period and give a brief response to what you read (2 sentences). Also look at Student Sometimes Ask on 191.
60 second questions
Earth's major plates
Plate tectonics
7 main plates
Plate tectonics the theory emerges
Plate boundaries
Plate Tectonics
World War II and sonar advanced the study of the Earth's crust far beyond what it used to be.
This brought the
global ocean ridge system
, a network of ridges that spans throughout the major oceans, discoveries of young oceanic crust (180 million years), and thin sediments instead of thousand meter thick sediments
The upper most mantle behaves as a strong, rigid layer known as the lithosphere which is broken into plates.
Are thin in the ocean
Thickness varies from 1-3 km at ocean ridges to 100's km in the deep ocean basin
Are thick at the continents
100 -150 or as much as 250 below older landmasses
Lithospheric plate
Make 2 kinds of crust
Oceanic crust and Continental crust
Without the asthenosphere plates would not be able to move around
Asthenosphere
A weak fluid layer that allows the rigid outer shell to move
Plates move as coherent units relative to other plates at slow rates (cm per year)
Most of the larger plates include an entire continent and some of the ocean floor.
These include North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antartic Plates
An area in the same plate will stay that far away, but areas outside that plate, will change distance
Plates are ultimately driven by the unequal distribution of heat within the Earth. Hot material moves slowly upward
Plate movements
The cooler, denser slabs of ceanic crust descend into the mantle, setting Earth's rigid shell into motion
This creates earthquakes, volcanoes and deform large masses of rock into mountains
All major interactions between plates occur at the boundaries, and this shows the most deformation
Transform fault boundaries
Convergent boundaries
Divergent boundaries
There are 3 main types of boundaries which are distinguished by the type of movement they do
Also called constructive margins
2 plates move apart, allowing new magma and lava to up-well and create new seafloor
Divergent boundaries
Also called desctructive margins
Occurs where 2 plates come together, results in oceanic crust going under continental crust where it is recycled
Convergent boundaries
Also called conservative margins
2 plates grind past each other without production or destruction of lithosphere
Transform fault boundaries
60 seconds write what you think will happen if 2 continents collide
Each plate is bounded by a combination of these 3 margins and have influenced the climate and geography of the region
The plates move and run into each other due to pressure exerted by other plates and the movement of material (think about the rock cycle). The plates are pushed on from all sides.
Plate Boundaries
Divergent boundaries
Convergent boundaries
Transform boundaries
Plate boundaries
As plates move away from ridge axis, the fractures fill with molten rock
This cools and creates new seafloor
This causes adjacent plates to separate
Not only found in the ocean, but also on land
Longest topographic feature on Earth's surface
Usually 1-2 miles high and extends as far as 600 to 2,500 miles side to side
Can contain deep faulted structure called rift valley
Oceanic ridges
Seafloor spreading is responsible for the creation of new seafloor
Divergent boundaries are called spreading centers and move slowly
All of the ocean basins have been created in the last 180 million years
The thickness of oceanic lithosphere is age dependent, the older (cooler) it is, the greater its thickness
Continental rifting
Divergent plate boundaries are also found with a continent, which causes the continent to split into two or more smaller parts
This can cause the continent to thin at the rift and eventually the land area turns into a narrow sea
Not all continental rifts will turn into seas, some end up stopping for an unknown reason.
Thought to begin with an elongated depression called a continental rift (east African rift pg 199)
Even though new lithosphere is being created, our planet is not growing larger
This is due to denser oceanic lithosphere descending into the mantle at convergent boundaries (con = together, verge = move)
Occur where 2 plates move toward each other and the leading edge of one is bent downward, making it slide beneath another
This forms a deep ocean trench
Desctructive boundaries
Also called subduction zones. Subduction occurs because the density of the descending plate is greater than the density of the underlying asthenospher
In general, oceanic lithosphere is more dense than the asthenosphere while continental lithosphere is less dense. Always makes the oceanic lithosphere descend
Oceanic - Continental
Continental boundary always remains floating when it comes in contact with an oceanic plate
Partial melting occurs when water is added to the mix, this generates basaltic rock and is usually found in oceanic plates
When a descending oceanic plate reaches a depth of about 100 km (60 miles), melting begins and can cause mountain ranges
There are three types of convergence zones, oceanic -continental, oceanic-oceanic and continental-continental
When partial melt mixes with continents, you get intermediate rock types like in the Andes and are continental volcanic arcs
These are very similar to oceanic-continental plate margins. Differences stem from the overriding plate
Oceanic-Oceanic convergence
One plate will descend while the other goes above it, causing melting and creates volcanoes on the ocean floor
Can build a chain of volcanoes if subduction continues resulting in a volcanic island arc
When two continents come together, a different result occurs that one plate subducting from another
Continental-continental convergence
The continents are bouyant and do not sink, so the continents collide
These collisions cause continents to buckle and fracture, which causes them to build on to each other and get larger
Occurs when plates slide past each other without production of distruction of the lithosphere

Called a conservative plate margin

Discovered by J. Tuzo Wilson in Toronto and was the first to propose the Earth was made of plates
Transform faults connect other faults together. They join two segments of an ocean ridge together forming a
fracture zone
.

They form about 100 km along the end of a ridge axis. They lie between 2 offset ridge segments

Helps move material toward trenches (fig 7.19 pg 205)
Transform faults
three
two
one
Testing plate tectonics
Evidence
Ocean Drilling
Deep sea drilling project 1968-83
Used to date seafloor
When oldest sediment was plotted against distance from the ridge crest, they discovered that it got older as it increased in distance from the ridge
Oldest was farthest away while youngest was closest tothe ridge
Showed that the thickness of the ocean floor was different along the seafloor with the thickness increasing as you get farther from the ridge
Data also showed that the deep ocean basins were less than 180 million years old
Most of this evidence was already shown, but now there is data that backs it up
Mapping of seamounts revealed linear chains of volcanic structure. Linear items are not found often in nature
Hot spots
The chain consisted of 129 volcanoes and travels 6,000 km (4000 miles) figure 7.21
Called the Hawaiin Island-Emperor Seamount Chain
Hawaii is the youngest volcanic island in the chain with the oldest being 60 million years old
Mantle plume
In the Hawaiian islands, there is a similar increase in age in the islands
Researchers believe that a rising plume of mantle material is located beneath Hawaii, and that this creates melting of the lithosphere
This results in hot spots - areas of volcanism, high heat flow, and crustal uplifting. Most hot spots do not move and the chains of volcanic structures are known as hot spot tracks, showing the direction of the plate
Research suggests that some mantle plumes originate at great depth at the mantle core boundary
Mantle plumes/hot spots
Others suggest it is shallower and originate near spreading centers
Most mantle plumes are long-lived and maintain relatively fixed positions within the mantle
Some hot spots have migrated and could change how we see the Hawaiian islands
Another interesting discovery is that Earth's magnetic field switches polarity, North becomes south and vice a versa
Magnetic reversals and sea floor spreading
The magnetic north pole was thought to have once been hear Hawaii and has wandered to its present location within the last 500 million years.
Do you think there is a simpler explanation to the poles wandering?
Evidence: Paleomagnetism
The Earth has north and south magnetic poles that allign closely with the geographic poles (true north and south)
Earth's magnetic field is similar to that produced by a simple bar magnet. Invisible lines of force pass through the planet and extend from 1 magnetic pole to another
Humans are unable to feel the Earth's magnetic field and but it deflects iron rich minerals and magnetizes them in the direction of Earth's magnetic field
The rocks become magnets and point toward the position of the magnetic poles even if they are moved from their current location
Polar wander is an idea that the magnetic alingment in iron-rich minerals showed that many different magnetic poles once existed.
When rocks exhibit the same magnetism as present day magnetic field it is normal polarity, and when it is opposite it is reverse polatirty.
Figure 7.25 shows the magnetic time scale, telling when reversals are found and how old it is
Look at figure 7.26 and 7.27 and describe what you are looking at.
Using magnetometers (sensitive devices for measuring magnetic fields) scientists discovered alternating bands of high and low intensity stripes on the ocean floor.
Plate tectonics exists, but how does it work?
What drives plate motion?
Scientists haven't been able to accurately describe plate motion.

3 main ideas that have been agreed upon
Have scientist been able to come up with an active model of how it works?
Scientist agree that convective flow in the mantle between warm, buoyant rock rises and cooler, dense material sinkds is the driving force for plate movement
Mantle convection and plate tectonics are part of the same system
Subducting oceanic plates drive cold downward moving plates
Shallow upwelling of hot rock creates new crust and is upward part of convection
The slow movement of Earth's plates and mantle are driven by unequal distribution of heat in Earth's interior
3 main mechanisms for plate motion
Slab pull
Ridge push
Slab suction
Forces that drive plate motion
Main driving force is slab pull. The pulling of the plate as the cold, dense slab sinks into the asthenosphere is slab pull
Causes the material behind it to follow it down
Sinks like a rock due to density differences between asthenosphere and oceanic lithosphere
slab pull
Ridge push causes new material to slide down the ridge and push material in front of it out of the way
Does not contribute as much as slab pull
The fastest moving plates have been measured and include the plates that have the most subducting material, not the plates with the most ridge systems
This is the evidence for slab pull being more of a driving force than ridge push
Ridge push
Slab suction is the pulling of adjacent mantle material toward the trench
Both sides of the trench move toward the subduction zone
Pulls both the subducting and overriding plates toward the trench.

The mantle flow tends to "suck" in nearby plates

Similar to pulling the plug in a bathtub
Slab suction
2 basic models of how convection works in the globe.

Layering at 660 kilometers and whole-mantle convection

660 is called the layer cake and deals with a thin convection zone at 660 km below the crust and than a lower mantle convection zone

Whole mantle convection suggests that subducting oceanic crust causes the entire mantle to convect
Mantle - plate convection
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