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Transcript of Plate Tectonics
Alfred Wegener first coined the term continental drift. He proposed that at some point in Earth's past, all of the continents formed one super-continent and slowly moved apart.
After World War I, we had new technology that we used to start mapping our seafloors. The first thing we found were underwater mountain ranges (mid-ocean ridges) and no one could explain why they were there. It wasn't until 1960 when scientist Harry Hess suggested seafloor spreading.
Evidence for Seafloor Spreading
1. Rock samples from both sides of the spreading ridge match.
2. Younger rocks are found right near the ridge and older rocks further away from it.
3. Magnetism, ocean rocks locked in the direction of magnetism (north or south) and that can be used to track the movement of the seafloor.
The Theory of Plate Tectonics
Combining Wegener's idea of continental drift with Harry Hess' seafloor spreading idea, we discovered the theory of plate tectonics.
Inside the Earth
Geology is the most basic Earth science - it's the study of the rocks and features we can see on land.
When we think about the inside of the Earth, think of an egg. The shell, the whites and the yolk all can represent the different layers the Earth has: the crust, the mantle and the cores(inner and outer).
The earth's crust is the hard rock that sits on the very outside of our globe. There are two different types of crust: continental and oceanic. Continental crust is very thick, and made up of a granite-type rock. Oceanic crust is very dense, thin and made up of a rock called basalt.
The in-between layer of Earth, this layer is a gooey, plastic like material that the pieces of Earth's crust float on.
Earth has two cores: both are made from Iron (and a whole bunch of it). Earth's inner core is solid, forced into this state because of the amount of pressure that sits on it. The outer core is liquid metal, and slowly circles the solid inner core. This flow is what creates Earth's magnetic field, which protects Earth from radiation and space dust.
We refer to the crust and very upper part of the mantle as the
We refer to the upper part of the mantle (below the lithosphere) as the asthenosphere. Asthenos translates to plastic
When looking at a map of the Earth there are certain parts that seem to fit together, a lot like puzzle pieces.
Wegener couldn't explain why the continents were moving. He died before most of the scientific community realized that he had been right.
There were a few "clues" used to pieced together the information about continental drift:
1. The puzzle-like fit of some of the continents.
2. Fossils of animals found on different continents.
3. Fossils of the same plant found on different continents.
4. Fossils of warm weather plants in cold climates and traces of past glaciers in warm climates.
5. Matching rock compositions on different continents
But Wegener still couldn't explain why the continents were moving!
Hess proposed that hot, dense, mantle rises towards the surface at mid-ocean ridges. The hot molten rock flows sideways and cools when it hits the cold ocean water, carrying the seafloor away from the ridge. (Spreading).
Earth's crust is made up of sections called plates. Plates are made of crust and the very upper portion of the mantle. These plates float on the asthenosphere, moving and interacting with the connecting plates.
We still aren't exactly sure what drives plate tectonics, but some ideas are:
1. Convection inside Earth: heat always rises, when it rises it eventually cools and falls back down creating a circular current.
2. Moving convection within the mantle.
Forces that Drives Plate Tectonics
At the edges of each plate, there are interactions between them. Usually it's just two plates, sometimes it can be three though. How they move with/against/away from each other depends on what happens at that boundary.
1. When plates move away from each other: we call that a divergent plate boundary.
-Mid-ocean ridges and rift valleys are features associated with this boundary.
2. When plates move together: we call that a convergent plate boundary.
-This is where subduction occurs. All three types of subduction: oceanic/oceanic, oceanic/continental, continental/continental.
-Subduction is the Earth's way of recycling really old crust.
3. When plates slide past each other: we call that a transform plate boundary.
Read chapter 31. Complete pages 695-698. Review 1-2o; think and explain: 2-8, 10, 12-16; think and solve: 1-3; multiple choice: 1-10.