Chapter 13 - Plate Tectonics
I. What is Plate Tectonics?
Topic 1: Moving Plates Cover the Globe
- Scientists know that Earth's surface consists of a number of rigid, but moving, pieces called plates.
- Plate tectonics is the study of the formation and movement of Earth's plates.
- Earth's surface is divided into 12 major plates and several minor plates.
Topic 2: How Thick are the Plates?
- The crust and the upper portion of the mantle are very similar in rock composition and physical properties.
- Together they make up a single solid layer called the lithosphere. (It is about 100 km thick.)
- The lithosphere is rigid but broken into plates but they can move with respect to one another.
- The lithosphere's composition is mainly igneous rock basalt.
- The continents however are more like the igneous rock granite.
- Granite is lighter than basalt so the continents occur as pieces embedded in the more dense lithosphere.
Topic 3: Why Do the Plates Move?
- Plates move because they sit upon a layer called the asthenosphere (a layer on top of the mantle).
- Rock is partially melted and rocks can flow very slowly. Convection currents are caused by this flow.
- Materials are constantly being pushed up and this causes the plates to move apart.
- Plates come together when there is a cool, sinking movement of the rocks.
II. Evidence for Plate Tectonics
Topic 4: Africa and South America
- Scientists have suggested that the west coast of Africa and the east coast of South America were once one large continent that seemed to "fit" together.
- This theory is called continental drift.
- This theory was proposed by a German scientist named Alfred Wagener in 1912.
- This idea was controversial and debated until the 1960's.
Topic 5: Earthquakes and Volcanoes
- Scientists have noted that earthquakes occur in limited belts and that the same belts are where most of the volcanoes also occur.
- The belts where earthquakes and volcanoes are located mark the location of the plate boundaries.
- The boundaries are places where one plate is moving relative to another plate.
- 90% of all the world's earthquakes occur in the active belt around the Pacific Ocean.
- Mt. St. Helens, a volcano in Washington state was the result of subduction boundary volcanism. It is along the active belt that surrounds the Pacific Ocean.
Topic 6: Magnetism
- Some rocks contain minerals that are magnetic.
- Spreading centers are where lava wells up from deep within Earth and moves away from the boundary equally in both directions. As the lithospheric plates move apart, they carry their continents with them.
Topic 7: Heat Flow and Seafloor Elevation
- Heat flow is a measure of the amount of heat leaving the rocks of the lithosphere. Areas of spreading centers have unusually high values for heat flow.
- Spreading centers have higher elevations than the rest of the seafloor. Elevation decreases away from spreading centers as the rocks cool and contract.
III. Kinds of Plate Boundaries
Topic 8: Diverging Boundaries
- Diverging boundaries or spreading centers are places where two lithospheric plates are moving apart.
- Most diverging boundaries have mid-ocean ridges, which have deep valleys along their entire length. These valleys are called rift valleys. These are both the boundary between the lithospheric plates and the place where new rocks form and push the older oceanic crust aside.
- Examples of mid-ocean ridges include the mid-Atlantic Ridge and the East Pacific Rise.
Topic 9: Sliding Boundaries
- A fault is a break or crack in Earth's crust along which movement has occurred.
- In California, the North American Plate and the Pacific Plate are sliding past each other along the San Andreas Fault.
- The average rate of movement along this fault is 5cm/year.
Topic 10: Converging Boundaries: Collision
- A converging boundary is when two plates come together. If the converging plates are both carrying continents, the continents may be "welded" together into a single large continent. This kind of boundary is called a collision boundary. When this happens, a mountain range is formed.
- The Himalayan Mountains are an example of a collision boundary that is still forming today. Where the Indian subcontinent is welded to the Eurasian continent, there is the largest mountain range in the world along with a large number of earthquakes.
Topic 11: Converging Boundaries: Subduction
- A subduction boundary is a form of converging boundary that occurs when one plate plunges down under another overriding plate. This causes a deep-sea trench (deepest depths of the ocean).
IV. Continental Growth and Plate Tectonics
Topic 12: The Craton
- Cratons are ancient continent cores. They are usually the oldest and most altered rock of the continent.
- The North American craton is exposed at the surface in most of eastern Canada. Geologists call it the Canadian Shield.
Topic 13: Sources of Growth Material
- There are 3 ways Earth material is added to continents which causes further development.
1. Deep-sea sediments
2. Volcanic rock
3. Sediments deposited by rivers
Topic 14: Growth by Thin-Skinned Thrusting: The Southern Appalachians
- Thin-skinned thrusting is the pushing of thin, horizontal sheets of rock from continental margins over great distances along nearly level fault systems.
- The stacking and shuffling of these thin sheets on the continental margins result in continental growth. (This has been observed in many of the world's mountain ranges, including the Appalachians.)
Topic 15: Growth by Terranes - Western North America
- A terrane is a large block of lithospheric plate that has been moved, often a distance of thousands of kilometers, and attached to the edge of a continent.
- The attachment of terranes may have been the primary method of continent growth in western North America.
-There are three characteristics that identify a terrane.
1. Each terrane block is bounded on all sides by major faults.
2. The rocks and fossils found in them do not match those of neighboring terranes.
3. The magnetic polarity found in the terrane does not match that of neighboring terranes.