1. Earth’s Materials and Processes-Part 9 Plate Tectonics and Continental Drift

2. Plate Tectonics
Theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other.
Plates ‘float’ and move on top of the mantle.

3. Lithosphere and Asthenosphere
The lithosphere is the crust and the upper part of the mantle.
100 km thick.
Less dense than the material below it, so it ‘floats’.
The Asthenosphere is the layer below the lithosphere.
The plates of the lithosphere float on the asthenosphere

4. Types of Plates Ocean Plates: plates below the oceans.
Continental Plates: Plates below the continents.

5. Plate Boundaries
Divergent Boundaries are boundaries between two plates that are moving apart, or rifting.
Rifting causes sea floor spreading, mid-ocean ridges, rift valleys, and fissure volcanoes.

6. Plate Boundaries
Convergent Boundaries are boundaries that occur when two plates are colliding.
There are 3 possible types.

7. Continental Plate and Oceanic Plate
An ocean plate can collide with a less dense continental plate.
A subduction zone forms, where the oceanic plate slides under the continental plate, due to the differences in density.
Volcanoes occur at subduction zones.

8. Ocean Plate and Ocean Plate
An ocean plate can collide with another ocean plate.
The more dense plate slides under the less dense plate, creating a subduction zone called a trench.

9. Continental Plate and Continental Plate
A continental plate can collide with another continental plate.
These create collision zones-a place where folded and thrust faulted mountains form.

10. Transform Fault Boundaries
A boundary between two plates that are sliding past each other.
Earthquakes occur along Transform Faults.

12. Causes of Plate Tectonics
Hot magma in the Earth moves toward the surface, cools, then sinks again.
Creates convection currents beneath the plates that cause the plates to move.
SOURCE OF HEAT=RADIOACTIVE DECAY OF UNSTABLE ISOTOPESRELEASES ENERGY!

13. Theory of Continental Drift
Proposed by Alfred Wegener in 1912.
Basic belief that all of the continents were once joined together in a ‘supercontinent’.
Met much resistance because Wegener did not have a good model to explain how the continents moved apart.
Although the theory was discarded, it introduced the idea of moving continents to geoscience, some of which has been confirmed.

14. Wegner’s Evidence
Wegener used fossils of plants and animals found only in South America and Africa in the Permain period were also found on many other continents.
He matched up rocks on either side of the Atlantic ocean.

15. Currently…
Plate tectonics is now the widely accepted theory that Earth’s crust is fractured into rigid, moving plates.
Scientists discovered plate edges through magnetic surveys of the ocean floor and through the seismic listening networks.
This also shows that the continents have shifted relative to one another, due to the magnetic minerals aligned in rock.

16. Alternating patterns of the ocean floor indicated seafloor spreading, where new plate material is born as molten rock from inside Earth rises through the cracks in the ridges, cools, and forms new oceanic crust (pushing the old crust outward).