Continental Drift andPlate Tectonics

Close examination of a globe often results in the observation that most of the continents seem to fit together like a puzzle: •west African coastline seems to snuggle nicely into the east coast of South America and the Caribbean sea; and a similar fit appears across the Pacific. • The fit is even more striking when the submerged continental shelves are compared rather than the coastlines.

• In 1912 Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a single proto-continent which he called Pangaea (meaning "all lands"), and over time they have drifted apart into their current distribution. • He believed that Pangaea was intact until about 300 million years ago, when it began to break up and drift apart.

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Wegener had four main pieces of evidence.

First he noted the jigsaw fit of South America and Africa, especially, but also elsewhere.

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Wegner also noted that fossils from South America and Africa came from the same extinct animal. Both continents back then had the same climate and vegetation, today that is not the case.

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He found that on both sides of the Atlantic, mountains were the same; both in terms of age and structure.

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He found that ice sheets covered parts of Africa, India, Australia and South America 250 million years ago. How could this happen in places that are so warm today?

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As technology progressed two other evidences were added to Wegner’s Theory.

One is called Sea Floor Spreading which will be talked about in detail in the next few slides.

The other is called Magnetic Signature. Rocks that are formed in Polar regions take on a ‘Polar’ characteristic and rocks formed near the Equator take on an ‘Equatorial’ signature. Huge rocks and mountains with Equatorial signatures have been found in Polar regions and vica versa!

“Advances in sonic depth recording during World War II (SONAR) led to a detailed mapping of the ocean floor. The Ocean Floor in the Mid-Atlantic was found to be spreading apart. Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea trenches and island arcs”

http://www.ucmp.berkeley.edu/geology/tecmech.html

Sea Floor Spreading:

The crust near the continental margins (the shoreline of the continents today) is about 200 million years old. It gets progressively younger toward the mid-Atlantic ridge, where oceanic crust is forming today (red).

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Scientists learned that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased.

http://www.ucmp.berkeley.edu/geology/tecmech.html

Wegener's hypothesis of continental drift lacked a geological mechanism to explain how the continents could drift across the earth's surface.

It wasn’t until the 1960s that the theory of Plate Tectonics was advanced to explain how the continents could separate. A Canadian by the name of Tuzo Wilson played an important part in the development of this theory.

What Tuzo Wilson did was change the way scientists viewed the internal structure of the earth.

A simple look at the Earth’s Interior

A bit more complicated

A closer look

A comparison of the thickness

In order for the theory of plate tectonics to be possible. The crust of the earth called the Lithosphere was subdivided. The upper portion of the Lithosphere was called the Earth’s crust.

• The Crust had to adjust itself based on density – the crust is composed of a dense material mostly found at the bottom of oceans called Oceanic Crust (basalt)

• and a less dense material which we call the Continental Crust (granite). But since there is ‘more’ continental crust, it actually has more weight over the mantle. Hopefully the next diagrams will help!

Isostac(s)yOne interesting property of the continental and oceanic crust is that these tectonic plates have the ability to rise and sink. This phenomenon, known as ISOSTACY, occurs because the crust floats on top of the mantle like ice cubes in water.

When the Earth's crust gains weight due to mountain building or glaciation, it deforms and sinks deeper into the mantle. If the weight is removed, the crust becomes more buoyant and floats higher in the mantle.

This process explains recent changes in the height of sea-level in coastal areas of eastern and northern Canada and Scandinavia. Some locations in these regions of the world have seen sea-level rise by as much as one meter over the last one hundred years. This rise is caused by isostatic rebound.

Both of these areas where covered by massive glacial ice sheets about 10,000 years ago. The weight of the ice sheets pushed the crust deeper into the mantle. Now that the ice is gone, these areas are slowly increasing in height to some new equilibrium level. (PhysicalGeography.net)

http://geog.ouc.bc.ca/physgeog/contents/10i.html

• The Earth's crust is broken into a series of plates (crustal plates) or pieces.• These plates are continually, moving, spreading from the center, sinking at the edges, and being regenerated. • Convection currents beneath the plates move the crustal plates in different directions. • The source of heat driving the convection currents is radioactivity deep in the Earth's mantle.

The main features of plate tectonics are:

As mentioned before there are actually two types of crust:

• Oceanic crust, which is thin and of course found at the bottom of the oceans. It is a compact, thin and heavy crust.

• Continental crust, since it has been exposed to the atmosphere is bulkier (air) and lighter than Oceanic crust.

Convection Currents power the plate movements. Convection currents rise up from the radioactive core, carrying heat to the thin crust of the earth.

http://geog.ouc.bc.ca/physgeog/contents/10i.html

• The currents in the asthenosphere are generated by heat rising to the earth’s surface from the hot radioactive core • At their boundaries, the plates spread apart, converge, and slide past one another

• This makes these areas the most geologically active: earthquakes and volcanoes and mountains

Earth’s Major Plates and their movement

Earthquakes and Volcanoes

The Surface of the Earth without water

http://www.ngdc.noaa.gov/mgg/image/mggd.gif

Click here to go to the actual site and zoom into certain areas

There are four basic Plate movements or boundaries:

1. Divergent: This is where the plates move apart, new magma wells up to the surface, forming new crust. The Mid-Atlantic ridge is a prime example. New land is created

2. Convergent: Two plates come together. Usually one of the plates subducts (goes under) the other plate, creating a Subduction zone. The crust at the leading edge of the subducting plate melts back to magma. The Pacific Rim of Fire is a good example. Land is destroyed – balance.

3. Transform Boundaries: This occurs when two plates rub against each other. This creates tremendous friction which is eventually released in the form of earthquakes. The San Andreas Fault is a Transform boundary.

4. Isostacy(Rebound): Plates moving up or down depending on the weight on the plate. Glaciers and Mountains add weight.

Hot Spots – Hawaii – An area where magma is being released and the ‘volcano’ is not depended on plate movement – an ‘ever erupting volcano’.

The main types of plate movements.

Iceland: On a Divergent Zone

What can happen at a Divergent Zone.

Many things can happen at a Convergent Zone:

Oceanic-Continental CollisionResult - Volcanic mountains or arcs

Oceanic-Oceanic CollisionResult - Island Arcs

Oceanic trenches, which are as deep as 35,000 feet below the ocean surface, are long and narrow, and run parallel to and near the shorelines of the continents. They are associated with and parallel to large continental mountain ranges. There is also a parallel association of trenches and island arcs.

http://www.ucmp.berkeley.edu/geology/tecmech.html

The Pacific Ring of Fire has many trenches

14_10b.JPGAnother look at the famous ‘Ring of Fire’

Continental-Continental CollisionResult - Mountain Ranges

Transform Zone

Result - Earthquakes

The San Andreas Fault, California

Hot-Spot

http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/figures.htm

Finally Canada’s role: The oceanic Juan de Fuca plate is moving beneath the continental North America plate at a rate of about 4 cm/year. Earthquakes occur along part of the boundary between the two plates and Volcanoes occur as well. Mt. St. Helens is a result.

http://www.geog.ouc.bc.ca/physgeog/contents/10l.html

Folding, Faulting and Denudation

Folding is the process that bends and twists rocks, usually due to compression 

Faulting is the process where rocks move past each other along a fracture

Plate boundaries

• There are three types of plate boundaries:

–spreading zones, –transform faults, and –subduction zones

Spreading Zones• At spreading zones, molten rock rises, pushing two

plates apart and adding new material at their edges. • Most spreading zones are found in oceans; for

example, the North American and Eurasian plates are spreading apart along the mid-Atlantic ridge.

• Spreading zones usually have earthquakes at shallow depths (within 30 kilometers of the surface).

• This type of crustal deformation is called tension… pulling apart.

• Tension extends the crust causing it to thin and lengthen. Rifting, like that which created the Great Rift Valley of Africa, is a result of tension

Transform Faults• Transform faults are found where plates slide past

one another. An example of a transform-fault plate boundary is the San Andreas fault, along the coast of California and northwestern Mexico.

• Earthquakes at transform faults tend to occur at shallow depths and form fairly straight linear patterns .

• When plates slide past one another in opposite directions along transform plate boundaries, a shearing stress is created.

• Shearing stress cuts the crust into parallel blocks displacing them horizontally relative to one another.

• Shearing takes place along the San Andreas Fault where the Pacific Plate is moving past the North American Plate.  

                                                       

                          Normal Fault

                                                      

                               Reverse Fault

                                                      

                                

Transform Fault

 

                                      

                                                Thrust FaultStike-Slip Fault

Normal Fault

Reverse Fault Rift Fault or

Graben is produced when tensional stresses result in lowering or sinking of a block of rock

Horst Fault is the reverse of a Graben

Subduction zones• Subduction zones are found where one plate

overrides, or subducts, another, pushing it downward into the mantle where it melts.

• An example of a subduction-zone plate boundary is found along the northwest coast of the United States, western Canada, and southern Alaska and the Aleutian Islands.

• Subduction zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active volcanoes.

• This type of crustal deformation is called compression.

Less common than Ocean Subduction zones are C to C

convergences