1. Lesson 1: The Continental Drift Hypothesis
The puzzle piece fit of continents, fossil evidence, climate, rocks, and mountain ranges supports the hypothesis of continental drift.
Scientists were skeptical of continental drift because Wegener could not explain the mechanism for movement.
Key Concepts 1

2. Continental Drift Theory proposed by Alfred Wegener
Suggests that continents are in constant motion on Earth’s surface.
Scientists hypothesize that Earth’s seven continents were once connected as a single land mass called a super continent. That land mass began to separate about 200 million years ago. Overtime, the continents have slowly shifted to their present positions by a movement that we call “Continental Drift”
This theory was first proposed by a German man Alfred Wegener who recognized and uncanny congruence (similarity) in the shape of coastlines on opposite sides of the Atlantic Ocean. He noticed that it appears as if South America's east coast would fit snugly against Africa’s west coast – as though one continent had been cut or torn from the other. This observation launched Wegener onto a track to collect evidence to support his idea of all the continents once being a single landmass which he called “Pangaea” meaning “all land”.

3. Climate Clues
Similar sediments deposited by glaciers were found in South America and Africa, as well as India and Australia
Similar glacial groves and scratches discovered below the sediments that were examined.
Wegener proposed that South America, Africa, India and Australia were once located closer to Antarctica 280 million years ago – meaning climate in the Southern Hemisphere was much colder than it is now and that glaciers similar to those that we find in Antarctica once covered a large part of those continents.
Wegener supported this idea by studying sediments deposited by glaciers in South America and Africa, as well as India and Australia.
Also supported because under the sediments Wegener discovered glacial grooves and deep scratches in the rocks that were made as the glaciers moved across the land.

4. Fossil Clues
Fossils of reptile Mesosaurus found in South America and Africa.
Plants and animals that live on different continents are generally unique to that continent alone, but fossils of similar organisms have been discovered on several different continents separated by oceans. - This ideas supported Wegener’s idea that the continents were once connected in the past.
Fossils of the reptile Mesosaurus have been found in South America and Africa. Because Mesosaurus lived in fresh water and on land, it is improbable that it swam between the continents. This fact suggests that South America and Africa were once joined.
Another fossil that supports the existence of Pangaea is Glossopteris, a fossil fern that once flourished in warm tropical climates. Glossopteris fossils have been found in Africa, Australia, India, South America, and Antarctica. It is improbable that Glossopteris could have orginiated independently in so many isolated locations or the seeds could not have traveled the vast distances between the continents thus lending weight to the theory that these continets were once connected and shared similar climates.
Fossils of Glossopteris, a fern, found in Africa, Australia, India, South America, and Antarctica.

5. Rock Clues
Parts of the Appalachian Mountains are similar to mountains found in Greenland and western Europe.
Similar folded rock beds can be found on the southeastern coast of South America, and southwestern coast of Africa.
- Radiometric dating has shown that the rocks have similar age and chemistry
Wegener knew that he needed more evidence to prove his theory, so he began to study the mountain ranges and rock formations on different continents and found many similarities.
The Caledonian Mountain range in Northern Europe and the Appalachian Mountains are similar in age, structure and composition. If you were to superimpose them together they would fit like a puzzle piece.
Similar rock beds can be found on South American and African coastlines. When the rocks were radiometric tested it showed that the rocks were of similar age and chemistry.

6. Lesson 2: Development of a Theory
Seafloor spreading provides a mechanism for continental drift.
Seafloor spreading occurs at mid-ocean ridges.
Evidence of magnetic reversal in rock, thermal energy trends, and the discovery of seafloor spreading all contributed to the development of the theory of plate tectonics.
Key Concepts 2

7. By the 1960s, scientists discovered the process of seafloor spreading.
When the seafloor spreads, the mantle below melts and forms magma.
Magma erupts on Earth’s surface as lava, which cools and crystallizes on the seafloor, forming rock.
As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge.
By the 1960’s scientists discovered a new process that helped explain continental drift. This process is called seafloor spreading and is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.
Process:
When seafloor spreads, the mantle below melts and forms magma. Because magma is less dense than solid mantle material, it rises through cracks in the crust along the mid-ocean ridge. When magma erupts on Earth’s surface, it is called lava. As this lava cools and crystalizes on the seafloor, it forms a tyoe of rock called basalt. Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lava. As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge. The closer the crust is to the mid-ocean ridge, the younger the oceanic crust is.
seafloor spreading: the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.

8. Development of Seafloor Spreading Theory
Volcanic rock on the seafloor contains iron-rich minerals that are magnetic.
Magnetic minerals in cooling lava from the mid-ocean ridge record the direction of Earth’s magnetic field.
Scientists have discovered parallel patterns in the magnetic signature of rocks on either side of a mid-ocean ridge.
The first evidence used to support seafloor spreading was discovered in rocks on the seafloor. Scientists studied the magnetic signature of minerals in these rocks.
Basalt on the ocean seafloor contains iron-rich minerals that magnetic. Each mineral acts like a small magnet. When lava erupts from a vent along a mid-ocean ridge, it cools and crystalizes. This permanently records the direction and orientation of Earth’s magnetic field at the time of the eruption. Scientists have discovered parallel patterns in magnetic signature of rocks on either side of a mid-ocean ridege.

9. Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer - to measure and record the magnetic signature.
They discovered parallel magnetic stripes on either side of the mid-ocean ridge. Each pair of stripes has a similar composition, age, and magnetic character.
The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is carried away from the center of the ridges in opposite directions.
Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer to measure and record the magnetic signature.

10. Lesson 3: The Theory of Plate Tectonics
Types of plate boundaries, the location of earthquakes, volcanoes, and mountain ranges, and satellite measurement of plate motion support the theory of plate tectonics.
Mantle convection, ridge push, and slab pull are the forces that cause plate motion. Radioactivity in the mantle and thermal energy from the core produce the energy for convection.
Key Concepts 3

11. Tectonic Plates
Question: If oceanic crust continues to form at mid-ocean ridges and it is never destroyed, then Earth’s surface area should be increasing correct? However this is not the case. We know the surface area of Earth is not the increasing – due to this thought, the theory of Plate Tectonics “Earth’s surface is made of rigid slabs of rock or plates that move with respect to each other” was born.
Basically the Theory of Plate Tectonics suggests that Earth’s surface is divided into large plates of rigid rocks that moves over Earth’s “Semi-Solid Mantle”
Recall: Earth’s outermost layers are cold and rigid compared to hot interior layers.
The lithosphere is made up of the curst and uppermost mantle. The tectonic plates are large pieces of the lithosphere that fit together like giant jig-saw puzzle pieces.
The asthenosphere the layer below the lithosphere is so hot that it behaves like plastic which allows the plates above to move because the material below it is able to flow.
Interactions between the lithosphere and asthenosphere helps explains Plate Tectonics.
Plate Tectonics: theory that Earth’s surface is broken into large, rigid pieces that move with respect to each other.

12. Divergent Plate Boundaries
Forms where two plates separate and create new crust
What is found there:
- mid-ocean ridges
- rift valleys
When two plates separate and create new oceanic crust, a divergent plate boundary forms.
East African Rift is an example
This process occurs where the seafloor spreads along a mid-ocean ridge. Lava erupts, cool and forms new oceanic crust.
It also occurs in the middle of continents where it is referred to as continental rifting.
Divergent Plate Boundary: boundary between two plates that move away from each other.

13. Transform Plate Boundaries
Forms where two plates slide horizontally past one another.
What is found there:
- Earthquakes
A transform plate boundary forms where two plates slide past each other. As they move past each other, the plates can get stuck and stop moving. Stress builds up where the plates are “stuck”. Eventually, the stress is too great and the rocks break, suddenly moving apart. This results in a rapid release of energy as earthquakes.
The San Andreas Fault is an example. It extends along the coast of California.
Transform Plate Boundary: boundary between two plates that slide past each other.

14. Convergent Plate Boundaries
Forms where two plates collide and the denser plate sinks below the least dense plate.
Three types of interactions
What is found there:
Subduction zones
Volcanoes
Mountains
Subduction: process that occurs when one tectonic plate moves under another tectonic plate.
Convergent plate boundaries form where two plates collide. The denser plate sinks below the less dense (more buoyant) plate in the process called subduction.
Convergent Plate Boundary: boundary between two plates that move toward each other.

15. Convergent Plate Boundaries
Ocean to Continent
Denser ocean plate subducts under the less dense continental plate.
What is found there:
Deep ocean trenches
Volcanoes
When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the edge of the continent. This creates a deep ocean trench. A line of volcanoes forms above the subducting plate on the edge of the continent.
An example is a the volcanic mountain Mount Rainier in the Cascade Mountains.

16. Convergent Plate Boundaries
Ocean to Ocean
Denser “older” ocean plate subducts under the less dense “younger” ocean plate.
What is found there:
Deep ocean trenches
Island Arc
When two oceanic plates collide, the older and denser oceanic plate will subduct beneath the younger oceanic plate. This creates a deep ocean trench and a line of volcanoes called an island arc.
An example is the Caribbean Island Arc.
Imgarcade.com

17. Convergent Plate Boundaries
Continent to Continent
Neither plate subducts because both plates have the same density.
Both plats uplift and deform
What is found there:
Mountains
When two continental plates collide, neither plate is subducted because the plates are both equally dense or rather have the same density. Instead of subducting, both plates uplift and deform. This creates huge mountains.
An examples is the Himalayas.

18. Plate Motion
Tectonic plates move because of convection currents that occur in the asthenosphere.
How It Works:
Hot mantle rises up and comes in contact with the Earth’s crust
Thermal energy is transferred from the mantle to the crust
Mantle cools, becomes denser and sinks
The tectonic plates move because of convection currents occurring in the asthenosphere. When materials such as rock are heated, they expand and become less dense.
How it works:
1: Hot mantle rises upward and comes in contact with Earth’s crust which is much cooler.
2: Thermal energy is transferred from the hot mantle to the colder surface crust above.
3: As the mantle cools it becomes denser, and sinks forming the convection current.
Convection currents in the asthenosphere act like a conveyor belt moving the lithosphere plates above.

19. Evidence of Plate Tectonics
The theory of plate tectonics provides an explanation for why earthquakes and volcanoes occur in certain places. Because plates are rigid, tectonic activity occurs where plates meet. When plates separate, collide, or slide past each other along plate a boundary, stress builds. A rapid release of energy can result in earthquakes. Volcanoes form where plates separate along a mid-ocean ridge or a continental rift or collide along a subduction zone. Mountains can form where two continents collide.
What do you see? Most volcanoes and earthquakes occur near plate boundaries.

20. Vertical Motion: Balance in the Mantle
Earth’s plates float on the surface of Earth’s mantle for similar reasons as icebergs float on the surface of water.
Plates of continental crust are less dense than the mantle – part of continental crust floats above the mantle
The continental crust displaces some of the mantle below until an equilibrium is reached.
Continents rise above the seafloor because continental crust is made of rocks that are less dense than Earth’s mantle. Continental crust displaces some of the mantle below it until an equilibrium or balance is reached.
Isostasy: the equilibrium between continental crust and the denser mantel below it.
A continent floats on top of the mantle because the mass of the continent is equal to the mass of the mantle it displaces.
Isostasy: the equilibrium between continental crust and the denser mantle below it.

21. Vertical Motion: Subsidence and Uplift
20,000 years ago, much of Earth’s crust was covered by ice more than 1 km thick
Twenty thousand years ago much of the Earth’s crust was covered by ice and glaciers more than 1 km thick. The weight of the ice pushed down on the crust, forcing it to sink into the mantle in a process called subsidence.
Subsidence: Downward vertical motion of Earth’s surface.
When the ice melted and the water ran off, the isostatic balance was upset. In response, the crust moved upward to restore it.
Uplift: upward vertical motion of Earth’s surface.
Figure: The weight of a glacier pushes down on the land. When the glacier melts, the land rises until isostasy is restored.
Subsidence: downward vertical motion of Earth’s surface
Uplift: upward vertical motion of Earth’s surface

22. Horizontal Motion: Types of Stress
A force acting on a surface is called stress.
Types of Stress
Compression: stress that squeezes against a surface.
Tension: stress that pulls something apart.
Shear: parallel forces acting in opposite directions
Compression, tension, and shear stress cause rocks to change shape as plates move horizontally.

23. Horizontal Motion: Types of Strain
Strain: a change in the shape of rock caused by stress.
Types of Strain
Elastic Strain: does not permanently change or deform a rock’s shape
Occurs when stresses are small or rocks are very strong
Plastic Strain: permanently changes a rock’s shape
Occurs when rocks are weak or hot
Rocks can change when stress acts on them. A change in the shape of rock caused by stress is called strain.
There are two main types of strain:
Elastic Strain: does not permanently change, or deform rocks – when stress is removed the rocks return to their original shapes. This occurs when stresses are small or rocks are very strong.
Plastic Strain: creates a permanent change in the a rock’s shape – even if the stress is removed, the rocks do not go back to their original shapes. This occurs when rocks are weak or hot.

24. Horizontal Motion: Deformation in the Crust
In the upper crust, the rocks are cold, and the forces cause the rocks here to break rather than to change shape.
Failure: when strain breaks rocks rather than just changing their shape
Forms fractures or faults
In the hotter lower crust and upper mantle, rocks tend to deform plastically like putty. Compression thickens and folds layers of rocks. Tension stretches and thins layers of rock.
In the colder, upper part of the crust, rocks can break before they deform plastically. When strain breaks rocks rather than just changing their shape, it is called failure – when rocks fail, fractures or faults form.
Stress is an applied force on a material and deformation is the amount the material changes shape.

25. Landforms created by
Compression
Tension
Shear Stress
Mountain Ranges
Volcanic Arcs
Mid-Ocean Ridges
Continental Rifts
Transform Faults
Fault
Zones
Ocean Trenches