11.3 Mountains and Plates Mountains and Plates
Mountains and Plates: Mountains and Plates • Mountain building still occurs in many places worldwide. • The jagged mountain peaks of the Grand Teton Range in Wyoming began to form about a million years ago and is still rising to this day. • In contrast, older mountain ranges, such as the eastern Appalachians, are deeply eroded.
Convergent Boundary Mountains • With the development of the theory of plate tectonics, a widely accepted model for mountain building became available. • Most mountain building occurs at convergent plate boundaries.
Convergent Boundary Mountains • Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediments deposited at the edges of landmasses. • The partial melting of mantle rock also provides a source of magma that intrudes into and further deforms these landscapes.
Convergent Boundary Mountains Ocean-Ocean convergence: • The convergence of two oceanic plates mainly produces volcanic mountains. • Recall that this process occurs where oceanic plates converge in a subduction zone. • The result of this is the formation of a volcanic island arc on the ocean floor.
Convergent Boundary Mountains Ocean-Continental Convergence: • The convergence of an oceanic plate and a continental plate produces volcanic mountains and folded and faulted mountains. • Mountains develop in two belts that run parallel to the edge of a continent. • Continental volcanic arcs form when an oceanic plate is subducted beneath a continental plate.
Convergent Boundary Mountains Ocean-Continental Convergence: • The belt of mountains that is created is made up of volcanoes and intrusive igneous rocks mixed with metamorphic rocks. • A prime example is the Andes Mountains in South America. The Andes formed through the subduction of the Nazca Plate beneath the South American Plate.
Convergent Boundary Mountains • Another process forms a belt of coastal mountains made up of folded and faulted rocks. • During subduction, sediment is eroded from the land and scraped from the subducting plate. • This sediment becomes stuck against the landward side of the trench.
Accretionary wedges • Along with scraps of oceanic crust, the sediment forms an accretionary wedge. • A long period of subduction can build anaccretionary wedge that stands above sea level. • California’s coastal ranges formed by this process.
Continent-Continent Convergence Continent-Continent Convergence: • At a convergent boundary, a collision between two plates carrying continental crust will form folded mountains. • The reason for this is the continental crust is not dense enough, compared with the denser crust of the mantle, to be subducted. • An example of such a collision began about 45 million years ago when India collided with the Eurasian Plate to form the Himalayas.
Continent-Continent Convergence Continent-Continent Convergence: • Before this event, India was part of Antarctica. It slowly moved thousands of kilometers north of millions of years. • The result of this collision was the formation of the Himalayan Mountains. • Most of the oceanic crust that separated these landmasses was subducted, but some was caught up in the collision zone , along with the sediment along the shoreline.
Continent-Continent Convergence Continent-Continent Convergence: • Today, these sedimentary rocks and slivers of oceanic crust are elevated high above sea-level. • The closing up of the ocean between India and the Eurasian plate is an example of how plate motions can destroy a sedimentary basin.
Divergent Boundary Mountains Divergent Boundary Mountains: • Most mountains are formed at convergent boundaries, but some are formed at divergent boundaries, usually on the ocean floor. • These mountains form a chain that curves along the ocean floor at the ocean ridges. • This mountain chain is over 70,000 kilometers long and rises 2000 to 3000 meters above the ocean floor. Sea-floor spreading produces Ocean mountain chains
Divergent Boundary Mountains Divergent Boundary Mountains: • The mountains that form along ocean ridges at convergent plate boundaries are fault-block mountains made of volcanic rock. • The mountains are elevated because of isostosy. Rock at the ridge is hotter and less dense, so it rises higher than older, colder oceanic crust.
Non-Boundary Mountains Non-Boundary Mountains: • Some mountains occur well within plate boundaries. • Volcanic mountains at hot spots, as well as some upward mountains and fault- block mountains, can form far from boundary plates. • The Hawaiian islands are a well known example of volcanic mountains at a hot spot.
Non-Boundary Mountains Non-Boundary Mountains: • Mountains formed by upwarping and faulting include the southern Rocky Mountains. • The southern rocky Mountains began to form about 60 million years ago with the subduction of an oceanic plate more than 1600 kilometers away.
Non-Boundary Mountains Non-Boundary Mountains: • At first, compressional forces deformed the crust. Than the subducting plate separated from the lithosphere above. • This allowed hot rock to upwell from the mantle, pushing up the crust and forming the southern Rockies.
Non-Boundary Mountains Non-Boundary Mountains: • As the crust bent upwards, tensional forces stretched and fractured it, forming the fault-block mountains of the Basin and Range region.