Which force draws the cooler, denser crust downward and away from the mi-ocean ridge?

Island arcs

When the downward-moving slab reaches a depth of nearly 100 km (60 miles), it gets sufficiently warm to bulldoze off its most volatile components, thereby stimulating fractional melting of pall in the plate to a higher place the subduction zone (known equally the mantle wedge). Melting in the drapery wedge produces magma, which is predominantly basaltic in composition. This magma rises to the surface and gives nascency to a line of volcanoes in the overriding plate, known as a volcanic arc, typically a few hundred kilometres behind the oceanic trench. The altitude betwixt the trench and the arc, known equally the arc-trench gap, depends on the angle of subduction. Steeper subduction zones accept relatively narrow arc-trench gaps. A basin may form within this region, known as a fore-arc bowl, and may be filled with sediments derived from the volcanic arc or with remains of oceanic crust.

If both plates are oceanic, every bit in the western Pacific Ocean, the volcanoes grade a curved line of islands, known as an isle arc, that is parallel to the trench, equally in the instance of the Mariana Islands and the adjacent Mariana Trench. If 1 plate is continental, the volcanoes class inland, every bit they practice in the Andes of western S America. Though the process of magma generation is similar, the ascending magma may change its limerick as information technology rises through the thick chapeau of continental crust, or it may provide sufficient heat to melt the crust. In either example, the limerick of the volcanic mountains formed tends to be more silicon-rich and atomic number 26- and magnesium-poor relative to the volcanic rocks produced by sea-ocean convergence.

volcanology

Back-arc basins

Where both converging plates are oceanic, the margin of the older oceanic crust will exist subducted because older oceanic crust is colder and therefore more than dense. As the dense slab collapses into the asthenosphere, notwithstanding, it as well may "scroll back" oceanward and crusade extension in the overlying plate. This results in a process known as dorsum-arc spreading, in which a bowl opens upwards behind the island arc. The crust behind the arc becomes progressively thinner, and the decompression of the underlying drape causes the crust to cook, initiating seafloor-spreading processes, such as melting and the production of basalt; these processes are similar to those that occur at bounding main ridges. The geochemistry of the basalts produced at back-arc basins superficially resembles that of basalts produced at ocean ridges, just subtle trace chemical element analyses tin detect the influence of a nearby subducted slab.

This manner of subduction predominates in the western Pacific Bounding main, in which a number of back-arc basins separate several island arcs from Asia. Examples include the Mariana Islands, the Kuril Islands, and the main islands of Nihon. Nevertheless, if the rate of convergence increases or if anomalously thick oceanic crust (possibly caused by ascension drapery feather activity) is conveyed into the subduction zone, the slab may flatten. Such flattening causes the dorsum-arc basin to close, resulting in deformation, metamorphism, and even melting of the strata deposited in the basin.

Mountain building

If the rate of subduction in an ocean basin exceeds the rate at which the crust is formed at oceanic ridges, a convergent margin forms as the sea initially contracts. This process can atomic number 82 to standoff between the approaching continents, which eventually terminates subduction. Mountain building tin can occur in a number of means at a convergent margin: mountains may ascension as a result of the subduction procedure itself, past the accretion of modest crustal fragments (which, along with linear isle bondage and oceanic ridges, are known as terranes), or by the collision of ii big continents.

Many mount belts were developed by a combination of these processes. For case, the Cordilleran mountain chugalug of North America—which includes the Rocky Mountains every bit well as the Cascades, the Sierra Nevada, and other mountain ranges near the Pacific declension—adult by a combination of subduction and terrane accession. As continental collisions are normally preceded by a long history of subduction and terrane accretion, many mount belts record all 3 processes. Over the by 70 1000000 years the subduction of the Neo-Tethys Sea, a wedge-shaped body of water that was located betwixt Gondwana and Laurasia, led to the accretion of terranes along the margins of Laurasia, followed past continental collisions beginning virtually 30 1000000 years ago between Africa and Europe and between Republic of india and Asia. These collisions culminated in the formation of the Alps and the Himalayas.

Jurassic paleogeography
Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
Adapted from: C.R. Scotese, The University of Texas at Arlington

Mountains by subduction

Mountain building by subduction is classically demonstrated in the Andes Mountains of South America. Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zone, and, therefore, the rocks in this region are warm and weak. Although subduction is a long-term process, the uplift that results in mountains tends to occur in detached episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upwards. For example, rapid uplift of the Andes approximately 25 million years ago is evidenced by a reversal in the flow of the Amazon River from its ancestral path toward the Pacific Ocean to its modern path, which empties into the Atlantic Sea.

In addition, models have indicated that the episodic opening and closing of back-arc basins have been the major factors in mountain-building processes, which take influenced the plate-tectonic development of the western Pacific for at least the by 500 million years.

Mountains by terrane accretion

Every bit the sea contracts by subduction, elevated regions inside the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin. Since the late Devonian and early Carboniferous periods, some 360 million years agone, subduction beneath the western margin of North America has resulted in several collisions with terranes. The piecemeal addition of these accreted terranes has added an boilerplate of 600 km (400 miles) in width along the western margin of the North American continent, and the collisions have resulted in important pulses of mountain building.

continental margin
The broad, gentle pitch of the continental shelf gives way to the relatively steep continental slope. The more than gradual transition to the abyssal plain is a sediment-filled region called the continental rising. The continental shelf, gradient, and rise are collectively chosen the continental margin.
Encyclopædia Britannica, Inc.

During these accretionary events, modest sections of the oceanic crust may interruption away from the subducting slab every bit it descends. Instead of existence subducted, these slices are thrust over the overriding plate and are said to be obducted. Where this occurs, rare slices of body of water chaff, known as ophiolites, are preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on state. A classic case is the Coast Range ophiolite of California, which is 1 of the near all-encompassing ophiolite terranes in Due north America. These ophiolite deposits run from the Klamath Mountains in northern California south to the Diablo Range in central California. This oceanic crust likely formed during the centre of the Jurassic Period, roughly 170 meg years ago, in an extensional regime within either a back-arc or a forearc basin. In the late Mesozoic, it was accreted to the western Due north American continental margin.

Because preservation of oceanic crust is rare, the recognition of ophiolite complexes is very important in tectonic analyses. Until the mid-1980s, ophiolites were thought to represent vestiges of the chief oceanic tract, just geochemical analyses have clearly indicated that most ophiolites form nigh volcanic arcs, such as in back-arc basins characterized by subduction gyre-back (the plummet of the subducting plate that causes the extension of the overlying plate). The recognition of ophiolite complexes is very important in tectonic assay, considering they provide insights into the generation of magmatism in oceanic domains, as well as their circuitous relationships with subduction processes. (See above dorsum-arc basins.)

Mountains by continental standoff

Continental standoff involves the forced convergence of two buoyant plate margins that results in neither continent existence subducted to any appreciable extent. A complex sequence of events ensues that compels 1 continent to override the other. These processes result in crustal thickening and intense deformation that forces the crust skyward to form huge mountains with crustal roots that extend as deep equally 80 km (nigh l miles) relative to Earth's surface, in accord with the principles of isostasy.

The subducted slab still has a tendency to sink and may become detached and founder (submerge) into the mantle. The crustal root undergoes metamorphic reactions that consequence in a pregnant increase in density and may cause the root to also founder into the pall. Both processes result in a meaning injection of heat from the compensatory upwelling of asthenosphere, which is an important contribution to the ascent of the mountains.

Continental collisions produce lofty landlocked mountain ranges such as the Himalayas. Much later on, subsequently these ranges accept been largely leveled by erosion, it is possible that the original contact, or suture, may be exposed.

The balance between creation and destruction on a global scale is demonstrated past the expansion of the Atlantic Body of water by seafloor spreading over the by 200 million years, compensated past the contraction of the Pacific Ocean, and the consumption of an entire sea betwixt India and Asia (the Tethys Sea). The n migration of Bharat led to collision with Asia some twoscore one thousand thousand years agone. Since that fourth dimension India has advanced a further 2,000 km (1,250 miles) beneath Asia, pushing upwardly the Himalayas and forming the Plateau of Tibet. Pinned confronting stable Siberia, China and Indochina were pushed sideways, resulting in potent seismic activity thousands of kilometres from the site of the continental collision.