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Plate Boundaries M. Billings NSHS What is a plate boundary? Any place where plates “meet”. They are always places of great stress within the crust Plate Movement is driven by convection currents in the mantle There are 3 types of Plate Boundaries!!! 1. Divergent Plate Boundaries • Two plates are moving in opposite directions • Occurs over the Up-welling part of a convection current in the mantle • 2 Sub-Types – Rift Valleys (on continents) – Mid-Ocean Ridges (in the oceans) Step 1: Continental crust is pulled apart causing a valley to form. Continental crust Asthenosphere Step 2: Fault earthquakes Asthenosphere As the valley gets deeper, faults form and earthquakes occur. Lakes often form in low areas. Step 3: As the crust gets thinner, pressure is released from the mantle and magma starts to rise. Volcanoes form. Volcanoes Step 4 Eventually the crust is pulled apart and magma from the mantle forms new ocean floor. All ocean floor is produced this way and forms a mid-ocean ridge which is volcanic. Mid-ocean ridge Black & White Smokers http://rst.gsfc.nasa.gov/Intro/Part2_26x.html Found at divergent plate boundaries, black and white smokers form where cold sea water seeps into the mid-ocean ridge and is heated. Once heated it dissolves minerals. Many unusual lifeforms live on black/white smokers and use these minerals as their food source instead of light. http://www.mnh.si.edu/earth/text/4_3_2_2.html Form on or near midocean ridges Examples of a Divergent Boundary: Mid-Atlantic Ridge – Iceland sits on top of it East-African Rift http://www.indiana.edu/~g105lab/1425chap13.htm 2. Convergent Boundaries • 2 plates are moving toward each other (sometimes called collision boundaries) • Occur where convections currents in the mantle move downward • Sub-Types – Ocean-Continent – Ocean-Ocean – Continent-Continent Volcanic Mountain Chain Ocean-Continent trench Continental Crust Ocean crust Magma forms as the ocean plate melts and rises to from volcanic mountains. Earthquakes Ocean floor is more dense and is pushed back into the mantle forming a trench . The part of the ocean crust that is being forced into the mantle produces a subduction zone. This crust begins to melt and the magma rises to the surface and forms a chain of volcanic mountains. Earthquakes are numerous! An example is the Andes Mountains in South America. Notice the trench to the west in the ocean. Ocean-Ocean Volcanic island arc trench The oldest, coldest piece of ocean crust is pushed into the mantle forming a trench and subduction zone. The magma that forms as this plate melts rises to form volcanic island arcs (chains). Again, lots of earthquakes Japan is an example of an ocean-ocean boundary. The trench lies off the east side of the islands. The islands always lie on the plate that is NOT being subducted. Continent-Continent Non-volcanic mountains Continental Plates collide producing very high non-volcanic mountain chains. The “boundary” between these plates becomes welded –this area is called the suture zone. Earthquakes are numerous. Continental crust is too bouyant to go into the mantle (so there is no subduction). The Himalayas (where Mt. Everest is found) is an example of a continent-continent boundary. 3. Transform Plate Boundaries • Plates sliding past each other • They connect other types of plate boundaries • Not usually convection current connected Fault Because plates are sliding past each other, these boundaries are basically faults and have many earthquakes. Transform boundaries also form between sections of mid-ocean ridges. A fault is not a simple straight line. The earth’s crusts fractures much like a glass plate. So a fault is more like a “zone” of breakage. The San Andreas Fault in California is a great example of a transform boundary! Notice most of the world’s volcanoes are found at plate boundaries! It takes all 3 types of boundaries to keep the crust from becoming too large or small for the earth’s surface. How will plate tectonics change the earth in the future? Observe the continents on the next few slides. How are their locations different? In the Future…?... Scotese.com