Magnetic+polarity

=Plate Tectonics – //Key Ideas//=

The Earth’s surface is made up of __moving__, solid pieces called **plates** ([|Reference Tables]). Some of these plates are moving together and some are moving apart. //Plate tectonics// is the study of the formation and movement of these plates. · The Earth’s plates are part of the **lithosphere.** The lithosphere considered to be the crust and the upper part of the mantle. · The oceanic crust is mainly made of the igneous rock basalt, while the continental crust is made of the igneous rock granite. · Oceanic crust is __denser__ than continental crust.
 * __What is Plate Tectonics?__**

The lower part of the mantle is called the **athenosphere**. This layer has similar composition to the upper mantle, but it is partially melted. This makes the athenosphere like a very thick “liquid” (you can compare solid rock to very thick mud to give yourself a picture in your mind). · The “liquid” property of the athenosphere allows it to flow very slowly. · You can think of the lithospheric plates as “floating” on the athenosphere. · The heat from inside the Earth causes convection currents to form in the athenosphere. · Where the convection currents rise, new material (molten rock) forces the plates apart. · Where the convection currents are sinking, the plates are moving together. __Diagram__:
 * __How Plates Move__**

1. The shape of the west coast of Africa and the east coast of South America seem to fit together like a puzzle. It is believed that they were once together, but moved apart over time (//Continental Drift//). 2. Similar fossil remains in Africa and South America of a reptile that is found nowhere else in the world. 3. Distinctive rocks were also found in Africa and South America in regions where the two continents were most likely joined together. 4. The majority of earthquakes and volcanoes are found in belts that occur along plate boundaries (where the plates are moving). Ex. the Pacific Ring of Fire. 5. Magnetic polarity reversals are recorded in rocks on the ocean floor as the sea floor spreads apart. During certain periods in Earth’s history, the magnetic fields have reversed (a compass would point south during a reversal). Minerals containing iron found in the igneous rocks on the ocean floor would shift before the magma cooled and hardened. If the polarity changed, the new magma would cool with the minerals shifted in the opposite direction. This change is recorded in the rocks. (LAB) 6. Heat flowing out of the rocks is greatest where the rocks are spreading apart (rising convection current), and decrease as you move away from the center (sinking convection currents).
 * __Evidence of Plate Movement__**

//1.// __Diverging Boundaries__- this is where the plates are moving apart at spreading centers (on the ocean floor). These boundaries form //mid-ocean ridges or rises// (like underwater mountain ranges). Between the peaks of the ridge are valleys called //rift valleys//. Ex. mid-Atlantic Ridge, East Pacific Rise. //2.// __Sliding Boundaries__- as the name implies, this is where two plates are sliding past each other. The sliding movement often causes earthquakes to occur. This happens along **faults**. A fault is nothing more than a crack in the Earth’s crust where movement has occurred. Ex. North American Plate and the Pacific Plate are sliding past each other along the San Andreas Fault in California. //3.// __Converging Boundaries__- this is where two plates are coming together. · //Collision boundary//- this is when the two plates that are moving together are both continental plates. The collision causes the plates to form a single, larger continent and the crust is pushed upward into a mountain range. Ex. Himalayan Mountains (including Mt. Everest). The Indian Plate and the Eurasian Plate are still moving together, so Mt. Everest is still slowly getting taller!! · //Subduction boundary//- this is when one of the plates plunges under (subducts) the other plate. This occurs between two oceanic plates, or an oceanic and continental plate. The most common characteristic of a subduction boundary is the formation of a //deep-sea trench//. These trenches are the deepest locations in the ocean (Ex. the Mariana Trench is approx. 35,000 feet deep! Mt. Everest could be put into the trench and the peak would still be about 1 mile below the ocean’s surface!!)
 * __Plate Boundaries__**

Because oceanic crust is __denser__ than continental crust, the oceanic crust will always subduct underneath the continent.
==Volcanoes always seem to form at subduction boundaries. If two oceanic plates converge, volcanic islands will form. If one of the plates is continental and the other oceanic, the volcanoes will form along the edge of the continent.==