Part 3 Chap 11.3 Mountain fomation

Mountain Building : 

Mountain Building Mountain Formation

Mountain formation : 

Mountain formation Recall the two major types of boundaries discussed in plate tectonics Convergent boundaries Divergent boundaries Of the two types of boundaries, the convergent boundaries are most responsible for mountain building A third type of mountain building is found where there are no boundaries

Mountain formation : 

Mountain formation Convergent mountain building Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediments deposited on land masses Partial melting of mantle rock also provides a source of magma that intrudes and further deforms these deposits

Mountain formation : 

Mountain formation Convergent mountain building Recall the three types of convergent boundaries Ocean-Ocean Convergence Ocean-Continental Convergence Continent-Continent Convergence

Mountain formation : 

Mountain formation Ocean-Ocean Convergence As an ocean plate is subducted under another ocean plate the mantle above the subducting plate is partially melted This produces mainly volcanic mountains in formations known as volcanic island arcs Japan and the Aleutian Islands in Alaska are good examples

Mountain formation : 

Mountain formation Ocean-Continental Convergence Forms similar to ocean-ocean convergence The continental plate, being less dense than the ocean plate stays on top as the ocean plate is subducted

Mountain formation : 

Mountain formation Ocean-Continental Convergence Any sediment scraped from the ocean plate is stuck against the landward side of the trench The accumulation of different sedimentary and metamorphic rocks along with scraps of ocean crust is known as an accretionary wedge

Mountain formation : 

Mountain formation Ocean-Continental Convergence produces mountain ranges composed of two roughly parallel belts The Continental volcanic arc belt A continental volcanic arc develops on the continental block The arc consists of volcanoes and large intrusive bodies mixed with high temperature metamorphic rocks The seaward belt The seaward belt is the accretionary wedge It consists of folded sedimentary and metamorphic rocks

Mountain formation : 

Mountain formation Ocean-Continental Convergence produces volcanic mountains and folded mountains The west coast of North and South America are good examples of this process

Mountain formation : 

Mountain formation Continent-Continent Convergence Continental crust floats too much to be subducted Since neither plate will be subducted they crash together This produces folded mountain ranges The Himalayan mountains between India and Tibet are an excellent example The process is still continuing Mt. Everest grows taller every year because the plates are still colliding

Mountain formation : 

Mountain formation Creation of the Himalayan Mountains

Mountain formation : 

Mountain formation Will the Himalayan mountains always grow? Numerous earthquakes along the southern coasts of India indicate that a new subduction zone may be forming If the new subduction zone forms, the pressure forcing India against Asia will end and the growth of the Himalayan Mountains would cease

Mountain forming : 

Mountain forming Fossils provide a good clue to mountain formation from collisions The fossils in the picture are high in the mountains in Montana, far from any ocean

Mountain formation : 

Mountain formation Forming mountains along divergent boundaries Normally occur on the ocean floor The mountains form a chain that curves along the ocean floor at the ocean ridges The Mid-Atlantic Ridge is the longest mountain range in the world The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains

Mountain formation : 

Mountain formation Non-boundary mountains Some volcanic mountains, fault-block mountains, and upwarped mountains are not formed along plate boundaries The Hawaiian Islands are formed at a hot spot far from any plate boundary The Tetons in WY are an example of fault block mountains in an area undergoing stretching/extension

Mountain formation : 

Mountain formation More about accretion and mountains Many of the mountainous regions rimming the Pacific have been produced through the process of collision and accretion This produces two main landforms Terranes Mountains from accretion

Mountain formation : 

Mountain formation Terranes Any crustal fragment that has a geological history distinct from that of adjoining terranes Come in many shapes and sizes Some can be smaller than volcanic islands while others can be as big as the Indian subcontinent

Mountain formation : 

Mountain formation The west coast of North America is a very good example of terranes The area we know now has formed over millions of years from other land forms crashing into the main continent as the oceanic crust was subducted just offshore

Mountain formation : 

Mountain formation Mountains from accretion Much smaller than those created from continent-continent convergence Evidence for accretion mountain ranges has been found through fossil records and geological differences all along western North America, particularly in Alaska and Canada

Mountain formation : 

Mountain formation Isostasy Mostly we have talked about the horizontal movement of plates Plates, or portions of plates can also move up and down (vertical movement) The concept of a floating crust in gravitational balance is called isostasy Iso means equal Stasis means standing

Mountain formation : 

Mountain formation Isostasy Think of a blocks of wood floating in water Note that the thicker wooden blocks float higher than the thinner blocks Many mountain belts stand high above the surface because they are less dense than the mantle below

Mountain formation : 

Mountain formation What would happen if more wood was added on one of the blocks? The block would sink until a new balance was found, but the top would be higher than the original block alone (and the bottom lower) The process of finding a new level of gravitational equilibrium is called isostatic adjustment

Mountain formation : 

Mountain formation Isostatic adjustment for mountains As more weight is added the crust sinks. As the weight is removed the crust rebounds Evidence of this is found in areas covered by over two miles of ice during the ice ages The weight of the ice depressed the crust by hundreds of meters Since the ice age, the crust in areas has rebounded by as much as 330 meters