J OURNAL #52 1. What is Wegener’s hypothesis? 2. What is paleomagnetism? 3. Did the earth’s magnetic field always point north?

R ECALL THE LAYERS OF THE E ARTH ??

T ODAY, WE WILL DESCRIBE THE 3 TYPES OF PLATE BOUNDARIES

T HEORY OF P LATE T ECTONICS Evidence supporting continental drift and sea floor spreading let to the development of a theory called Plate Tectonics. Plate tectonics is a theory that explains why and how continents move. It also explains the formation of new features on the Earth’s crust.

H OW C ONTINENTS M OVE The lithosphere is broken up into several blocks called tectonic plates. These plates ride on top of the mantle…like wood floats on water The asthenosphere is a “plastic layer” just below the lithosphere. Plastic rock is under grate pressure and flows slowly, like putty.

H OW C ONTINENTS M OVE Earth’s crust has 2 types Oceanic crust Dense Rich in iron and magnesium Continental crust Low density Rich in silica The continents and oceans are carried along on the moving tectonic plates the same way passengers on a bus are moved.

T ECTONIC P LATES There are 15 major tectonic plates Many plates are bordered by major surface features such as mountains and deep trenches, the boundaries are not always easy to identify. Some plate boundaries are located within continents far from mountain ranges.

E ARTHQUAKES Scientists identify plate boundaries by studying data from earthquakes. When tectonic plates move, sudden shifts can occur along their boundaries. These sudden movements are called earthquakes. Frequent earthquakes in an area are evidence that 2 plates meet in that area. Plate tectonics and Earthquake Video

V OLCANOES The locations of volcanoes also help identify the locations of plate boundaries. Some volcanoes form when plate motions generate magma that erupts on Earth’s surface.

T YPES OF B OUNDARIES There are 3 types of plate boundaries Divergent Convergent Transform

D IVERGENT B OUNDARIES The 2 plates move away from each other Magma from the asthenosphere rises to the surface as the plates move apart. The magma cools to form new rock on the ocean floor forming undersea mountain ranges know as mid-ocean ridges Most divergent boundaries are located on the ocean floor, but some from on continents Example: Red Sea occupies a huge rift valley formed by the separation of the African plate and the Arabian plate.

C ONVERGENT B OUNDARIES 2 plates collide 3 types of collision Oceanic-continental Continental-continental Oceanic-oceanic

OCEANIC-CONTINENTAL Oceanic (denser) subducts or sinks under the continental lithosphere. As the oceanic plate subducts, it heats up and releases fluid into the mantle above, forming magma. The magma rises to form volcanic mountains

C ONTINENTAL - CONTINENTAL Neither plate subducts Edges crumple and thicken causing uplift Large mountain ranges formed (Himalaya Mountains)

O CEANIC - OCEANIC One plate subducts under the other and form a deep-ocean trench. Fluids/Magma release, rise the surface and form an island arc such as Japan

T RANSFORM B OUNDARIES 2 plates slide past each other horizontally Plates do not slide smoothly Plates scrape each other in a series of sudden spurts of motion that are felt as earthquakes. Do not produce magma Example: San Andreas Fault in California

L ET ’ S R EVIEW : 1. What are plate tectonics? 2. What are the 3 types of boundaries? 3. Name two ways that scientists were able to identify the location of plate tectonics?

L ET ’ S R EVIEW 1. What are the 3 types of plate boundaries? 2. Explain divergent boundaries. 3. Explain convergent boundaries 4. Explain transform boundaries. 5. Which type of plate boundary does not produce magma? 6. What are 3 types of convergent boundaries?

L ET ’ S R EVIEW 1. What happens when oceanic and continental plates converge? 2. What happens when 2 continental plates converge? 3. What happens when 2 oceanic plates converge? 4. What is subduction?