Continental Drift

Continental Drift 225 million years ago

Continental Drift 180-200 million years ago

Continental Drift Earth today

Continental Drift • Scientist Alfred Wegner in 1910 hypothesized that the continents were once one large landmass called Pangaea. He believed the continents broke apart and drifted away from each other . This theory is called continental drift. Many scientist did not believe his theory because he COULD NOT explain WHY the continents drifted. • Wegner’s Continental Drift Evidence • Fossil Evidence • Landform Evidence • Climate Evidence

Fossil Evidence for Continental Drift • There are the same fossils from a millions of years ago on continents separated by ocean. • These organisms could not have swam across the ocean. The continents must have been connected at one point. • Landform Evidence for Continental Drift • Mountain ranges on different continents line up. • The continents shape is like a puzzle • Climate Evidence for Continental Drift • Places that are now extremely cold have fossils of tropical plants. This place must have been closer to the equator. • Places now very warm have glacier evidence. They must have been further away from the equator at some point.

So, why did the continents move? • In the mid 1900’s Scientist used Sonar and noticed Ridges on the ocean floor now called (mid-ocean ridges) • How did these ridges get there? Sea-Floor spreading • Sea- Floor spreading- Occurs when the crust on the ocean floor pulls apart, and magma comes through this gap to form a newer layer of crust. • The Mid-Atlantic Ridge (a mid-ocean ridge in the Atlantic ocean) are places where sea-floor spreading takes place.

How does Sea-floor Spreading Happen? • A Geologist named Harry Hess decided the sea floor moves like a conveyer belt, moving the continents with it. • At the mid-ocean ridge there is a crack in the crust, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. As the molten material cools, it forms a strip of solid rock next to the ridge.

How do we know sea-floor spreading occurs? • Evidence from Molten Material • Scientist in the 1960’s found that their was indeed molten material erupting along mid-ocean ridge. They discovered strange rock structures at the sea floor that looked like toothpaste squeezed from a tube. These types of rocks only form when molten material cools quickly in water. • Evidence from Drilling Samples • Scientist drilled in the ocean floor to determine the age of the rocks. The rock further away from the ridge was older than the rocks next to the ridge.

Evidence from Magnetic Strips • Scientist studied the patterns and discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”. They hold a record of reversals in Earth’s magnetic field. • Magnetic reversal- when Earth’s magnetic poles change place. Each “strip” is different than the one next to it. • Scientist know that the Earth Acts like a giant Magnet with a north and a south pole and discover that the poles have reversed themselves. Each new strip on the ocean floor is magnetized either North (N) or South (S)

What happens to the old CRUST? • The ocean does NOT keep getting wider. Instead each ocean has deep-ocean trenches. Here the crust of the ocean floor plunges downward this is called Subduction. • At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle. • The Atlantic Ocean has few deep-ocean trenches, due to the sea floor spreading it is getting wider. • The Pacific Ocean has many deep-ocean trenches, therefore it is getting more narrow.

Sea-Floor Spreading

Sea-Floor Spreading Mid-ocean ridge Oceanic crust Mantle

Sea-Floor Spreading Mid-ocean ridge Oceanic crust Mantle Magma

Sea-Floor Spreading Mid-ocean ridge Sea-floor spreading Oceanic crust Mantle Magma

Sea-Floor Spreading Newly formedoceanic crust Mid-ocean ridge Sea-floor spreading Oceanic crust Mantle Magma

Sea-Floor Spreading Newly formedoceanic crust Old oceanic crust Mid-ocean ridge Sea-floor spreading Oceanic crust Mantle Magma

Sea-Floor Spreading Newly formedoceanic crust Old oceanic crust Mid-ocean ridge Continentalcrust Sea-floor spreading Trench Oceanic crust Mantle Magma

Sea-Floor Spreading Newly formedoceanic crust Old oceanic crust Mid-ocean ridge Continentalcrust Sea-floor spreading Trench Oceanic crust Mantle Old oceaniccrust melts Magma

Sea-Floor Spreading Oceanic crust subducted through forms Mid-ocean ridge Deep-ocean trenches Molten material forms erupts through

Plates • The lithosphere is not one solid layer. The lithosphere is broken down into what we call plates. The plates fit closely together along cracks in the lithosphere. • Each plate contains a continental crust, oceanic crust or both. • The theory of continental drift and sea-floor spreading is combined into one theory called Plate Tectonics. • This theory states pieces (plates) of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the asthenosphere.

Plate Boundaries • Transform- Plates slide by each other • Divergent- plates move apart • Convergent- plates move into each other

Eurasian Plate Eurasian Plate Antarctic Plate Antarctic Plate Plate Tectonics North American Plate Pacific Plate AfricanPlate SouthAmericanPlate NazcaPlate Indo-Australian Plate

Plate Tectonics

What they form How they move Plate Tectonics Movement of Oceanic Plates Converging Plates Diverging Plates Together Trench Apart Ridge

subduction volcanoes trenches Plate Tectonics Converging plates lead to

Stresses at the Boundaries • The movement of the plates creates a powerful force these, forces are called Stresses. • Stresses cause a deformation (change) in the lithosphere. • Three types of stresses • Shearing- rocks sliding by each other (happens at a transform boundary) • Tension-rocks pulling apart (happens at a divergent boundary) • Compression- rocks pushing together (happens at a convergent boundary)

Faults • A fault is a break in Earth’s surface due to the build up of stress where slabs of rock slip past each other. • Three types of Faults • Strike-slip Faults- forms because of Shearing (San Andreas fault in California) • Normal Faults- forms because of tension, the • Reverse Faults- forms because of compression

Faults Before Stress Shearing Tension Compression

Faults Strike-Slip Fault Reverse Fault Normal Fault

Compression Tension Shearing Reverse fault Normal fault Strike-slip fault Faults Stress causes produces produces produces

Continental Drift