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 an accretionary wedge that stands above sea level. California’s coastal ranges formed by this process.

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 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 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 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 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 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 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 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 As the crust bent upwards, tensional forces stretched and fractured it, forming the fault-block mountains of the Basin and Range region.