Matching fossils on continents now located thousands of miles apart. Example = Mesosaurus, a freshwater reptile
Matching mountain ranges
Matching geologic structures including:
Same rocks of same age
Climate change evidence
Glacial deposits at current equator
Fossilized palm trees in Greenland
Map shows why according to the placements of current continents within Pangaea
Wegener not believed
What could possibly force the continents to move across the ocean floor in this way. They would be crushed.
He was a meteorologist,
not a geologist
Developments 1950s and 1960s
World War II submarines using sonar found mountains under the oceans – the mid-ocean ridges
Sea floor drilling showed rocks younger than expected and youngest towards the center of the mid-ocean ridge
Theory of seafloor spreading suggested by Princeton professor Dr. Harry Hess
Seafloor spreading First look at the earth’s layers as shown here.
Structure of the Earth
The Earth is made up of 3 main layers:
Inner core Outer core Mantle Crust
This is where we live!
The Earth’s crust is made of:
thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old
Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young
Evidence for sea floor spreading
Alvin and other submersibles found cracks and lava showing spreading and volcanism at mid ocean ridges and odd life forms
Matthews and Vine’s survey of the Indian Ocean sea floor showed matching stripes of reversing polarities on either side of ridge – what caused these?
Lava spreading during alternating magnetic periods
The Earth’s magnetic field
It is produced by the outer core of the earth which is made of liquid iron and nickel
This moving magnetic material produces a moving magnetic field, which in turn produces a moving electric field. It is a dynamo!
Earth’s magnetic field varies over time and it protects us from cosmic radiation
Seafloor spreading and paleomagnetism
By piecing together this information, we can see how the continents have moved over the past 200 million years, due to seafloor spreading
Sea floor spreading provides the driving mechanism for movement
However, it is not the continents that are moving, but the “plates” of lithosphere “floating” in effect on the asthenosphere
The lithosphere is made up of about 20 plates which move relative to each other in several ways
Let’s look at a generalized sketch
Confirmation of the Theory of Plate Tectonics Evidence supporting the theory of plate tectonics: Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted. The paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart.
Confirmation of Plate Tectonics Theory Paleomagnetism: strips of alternating magnetic polarity at spreading regions. The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.
“ Plates” of lithosphere are moved around by the underlying hot mantle convection cells
Plate Tectonics An overview of the tectonic system.
Sea-Floor Spreading and Plate Boundaries
3 types of boundaries
Convergent – where plates come together. See example next slide
Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov
Mid Atlantic Ridge Mid Atlantic Ridge
As plates move apart new material is erupted to fill the gap
Iceland has a divergent plate boundary running through its middle
North American Plate and Eurasia Plate diverging
Iceland: An example of continental rifting
East Africa Rift Valley
East Africa Rift Valley
Oceanic lithosphere subducts underneath the continental lithosphere
Oceanic lithosphere heats and dehydrates as it subsides
The melt rises forming volcanism
E.g. The Andes
Continent-Oceanic Crust Collision
Deep Ocean Trench
Andes Mountains in South America
Andes Mountains formed along the coast of South America
Nazca Plate dives under the South American Plate
Japan Volcanoes in Japan
Japan formed as an island chain as the Pacific Plate dives beneath the Eurasian Plate Mt Fuji
Aleutian Islands Chain off Alaska
Aleutian Islands formed as an island chain as Pacific Plate dives beneath North American Plate
Forms mountains, e.g. European Alps, Himalayas
Converging Margins: India-Asia Collision
Alps in Europe
Collision of Africa and Eurasia
Interesting plate collision
This picture shows a place in Newfoundland where a massive collision actually forced mantle rock on top of the crust, during the collision that formed Pangaea and the Appalachian mountains. This looks down the old plate boundary.
Mantle rocks are toxic
These rocks have very different compositions than crustal rocks.
They contain heavy metals, which do not support life forms on the earth’s surface, so few organisms live there.
However, in some places their heavy metal concentrations produce rich metal deposits and are mined
Found at spreading centers – either mid ocean ridges or mid continental rift zones
Transform fault boundary
This shows the San Andreas Fault.
It is a transform fault boundary, where the plates move sideways past each other, rather than away from each other (at divergent boundaries), or towards each other (at convergent boundaries)
Transform Fault Boundary
Transform Faults and Seafloor Spreading
Review of different boundaries
Divergent –mid ocean ridge like Iceland or continental rift zone like the African Rift Valley
Ocean/ocean like Japanese Islands
Continent/ocean like Andes and Cascades
Continent/continent like the Himalayas
Transform fault like the San Andreas fault
Hot spots are not at plate boundaries, but give us information about plate motion
What causes plate tectonics?
Convection in the mantle, as the plastic asthenosphere flows, carrying the plates with it.
This is probably aided by slab pull at subduction zones and ridge push at mid ocean ridges and rising plumes in the mantle
This diagram shows several different model hypotheses
Plate tectonics causes volcanic activity
It causes rocks to be tilted
Or even to fold or break
Volcanoes and Earthquakes and Plate Boundaries – GIS activity
Stress and strain
Formation of mountains
Two forces are constantly at work on the earth.
Weathering and erosion tear structures down while
Plate tectonics builds them up
So we have mountains! But they will not last forever.
Mountains form in different ways -Volcanic mountains-
Volcanoes form by subduction and melting of plates
Volcanic mountains form over hot spots
Volcanic mountains form at rift zones
Other mountain types
Folded mountains form From converging continents like the Himalayas
How do these look? Direction?
Fault block mountains form where blocks of rock drop at faults – mostly near plate boundaries, but not always
Uplifted mountains – where large sections of the crust are pushed up, perhaps by magma, or other forces
Just as a boat sinks or rises with changes in weight, so does the crust sink or rise with changes in weight. Plate tectonics builds mountains and the extra weight causes the crust to sink. As erosion occurs the weight of the mountains decreases and the crust rises again. This process is called isostasy or isostatic change
Isostasy Isostasy is balance. A floating object is balanced in the water, like an iceberg. If some of the top melts, the iceberg rises in the water to stay in balance. If you get into a boat, the boat sinks to maintain balance. The same thing happens with mountains. As plate motions push it higher the mountain sinks into the mantle to stay in balance. On the other hand, when the mountain erodes, it will rise in the mantle as the top erodes. So, a mountain may lose 1 meter from erosion, but regain 0.8 meters from rising due to isostasy. It maintains isostatic equilibrium.
This is also a great example of feedback within a system. Is it positive or negative? Figure it out. Remember that maintaining equilibrium is __ feedback because it keeps the system in balance. So, what is it, positive or negative?
This is a good example of Newton’s 3 rd law. For every force, there is an equal and opposing force. In this case the force of gravity is opposed by the buoyant force.
Right! It’s negative.
Stress Due mostly to plate movements, the earth’s crust is under a lot of stress. There are 3 types, shown at the right A occurs where plates pull apart, divergent boundaries, and is called tension B occurs where plates converge, and is called compression C occurs where plates move past each other, at transform fault boundaries and is called shearing
This stress leads to strain on the crust which bends it. –
If it is warm, underground, it can bend. This called ductile deformation. Features are called folds. Upturned folds are anticlines while downturned folds are synclines.
Or, the rock may break, if it is brittle. This causes faults –breaks of the earth.
A fold above and a fault below anticline syncline Stress and strain video Economic value of folds/faults
Faults move in different ways, depending on the type of stress on them. Here are 3 types that form.
1 is a normal fault
2 is a strike slip fault
3 is a reverse fault
2 3 1
These form at different plate boundaries. Can you figure out which forms where? Match them
A. Convergent C. Transform Fault B. Divergent 2 3 3 2 1 1
Can you match the stress and strain? A is 1 st , B is 3 rd , C is 2nd 2 3 1