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11.1 Where Mountains Form Key Idea: Events at plate boundaries and at the continental margins result in the formation of mountains. Objectives: 1. Explain how some of the Earth’s major mountain belts formed. 2. Compare and contrast active and passive continental margins. What is a mountain? A mountain is a large mass of rock that rises a great distance above its base. Most mountains result from forces associated with processes that occur at convergent plate boundaries. Mountain Belts • Look at a physical map of the world. All the mountain ranges that you identify on the map (dark brown areas) have formed as a result of plate tectonics interaction. • Examples: -The Ural Mountains (this is the natural border between Europe and Asia) -The North American Cordillera -The Andes -The Himalayan Mountains, etc. Mountain Belts Mountain Belts • Most of the world mountains form along belts which tend to follow convergent plate boundaries. • These mountain belts are regions where mountains are forming or have formed in the past. • The Himalayan mountains are result of the Indian subcontinent colliding with South Asia. They are young mountains, which are still rising. • The Appalachian or Ural mountains, on the other hand, are very old and they have stopped rising long time ago. They show the suture ( connection line) between older plate tectonics, which have been active in in past. A Model of Mountain Belts Formation Continental Margins • Continental margins are boundaries between continental crust and oceanic crust. There are two types of continental margins: 1. Passive continental margins which do not occur at plate tectonics boundaries. 2. Active continental margins, which occur at the contact of a oceanic plate and a continental plate. Passive Continental Margin Passive Continental Margins • Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal and the earth's weathering and erosion processes are winning. This leads to lots of low-relief (flat) land extending both directions from the beach, long river systems, and the accumulation of thick piles of sedimentary debris on the relatively wide continental shelves. Again South America provides a great example. The Amazon River, whose source is in the Andes Mountains (the active margin) drains east across the interior of South America to the coast, where it enters the Atlantic Ocean and deposits the tremendous volume of sedimentary materials it eroded from the continent Passive Continental Margins • Passive continental margins do not occur at plate boundaries. For example, the along the east coast of North America the continental crust thins out and changes progressively into oceanic crust. Both continental and oceanic crust are the makeup pf the North American plate, which ends at the Mid Atlantic Rift. • There areas are stable, and they accumulate large amounts of sediments. Active Continental Margins • Mountain building takes place near active continental margins. • The active continental margins involve subduction; the oceanic plate goes under the continental plate, and all the sediments are compressed, folded and risen. • Let’s remember that these areas are also accompanied by volcanic activity and seismic activity as well. Active Continental Margins Active Continental Margins • An active continental margin is found on the leading edge of the continent where it is crashing into an oceanic plate. An excellent example is the west coast of South America. Active margins are commonly the sites of tectonic activity: earthquakes, volcanoes, mountain building, and the formation of new igneous rock. Because of the mountainous terrain, most of the rivers are fairly short, and the continental shelf is narrow to non-existent, dropping off quickly into the depths of the subduction trench. Are Passive Margins and Active Margins Related? • Yes, they are! • The passive margins provide the materials that form mountains; a lot of sediments eroded from the continent are carried and deposited at the continental margin. The margin will become heavier and heavier and it will sink. • At some point, the plate will break apart and the oceanic crust will sink and undergo subduction; this area will eventually become an active continental margin. The material accumulated by erosion and transport will become the material that will form the new mountains.