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Volcanic Island Arc Mountain Building
Divergence, and the creation of new oceanic lithosphere, can go on for tens or hundreds of millions of years. At some point, however, divergence stops and the two continents begin to move back toward each other, initiating the second, closing, half of the Wilson Cycle. This is convergence and a new plate boundary must be created for it. Convergence begins when oceanic crust decouples, that is, breaks at some place and begins to descend into the mantle along a subduction zone.
It is always oceanic crust which decouples and descends into a subduction zone; continental crust is too light to subduct. Subduction zones can form anywhere in the ocean basin. In the Stage E cross section subduction is dipping east, but it could have been west, or any direction. For example, in this detail subduction is toward the west.
There are just two kinds of locations for subduction zones, however, one within an ocean basin (Island Arc type), the other along the edge of a continent (Cordilleran type). Both kinds of subduction cause volcanic mountain building and they are extremely important. Things are heating up now compared to the boredom of Stage A. The island arc type is described below; the Cordilleran type in Stage G.
At a subduction zone oceanic crust dives into the mantle. When oceanic crust subducts it sets in motion a chain of processes which creates several new structural features, and generates a wide range of new kinds of rocks (detail) each reviewed separately below.
At the site of subduction, part of the oceanic crust is dragged down into a trench 1-2 km below the normal ocean floor which is about 5 km deep. The subducting oceanic crust begins its descent cold but heats up as it slides into the mantle. At about 120 km deep rock begins melting to form magma. The magma, hot and of low density, rises toward the surface, forms batholiths, breaks onto the ocean floor as lava and builds a volcano which eventually rises high enough to form an island.
The location of the volcano is called the volcanic front (in three dimensions it is a string of volcanoes all rising above the subduction zone). The area on the trench side of the volcanic front is the forearc, and the area on the back side of the volcanic front is the backarc. A new convergent boundary has been created along the zone of subduction. The ongoing subduction and magma generation eventually builds a volcano perhaps 7-8 km off the ocean floor, and its center (mobile core) is made of many batholiths. All of this has set in motion several more processes.
Fractional Melting and the Creation of New Igneous Rocks:
The mantle rock above the subducting plate selectively melts, and fractionates (or see Igneous Rock Evolution). In fractional melting an igneous rock of one composition is divided into two fractions each of a different composition.
The original rock descending into the subduction zone is the oceanic lithosphere (ophiolite suite) composed of cold basalt and gabbro of the oceanic crust, and peridotite of the upper mantle (detail). As it descends into the mantle it gradually heats because of the geothermal gradient and friction of subduction. But the descending slab also carries a lot of sea water with it and at about 120 km down the water and heat lead to fractional melting of the mantle material just above the subducting slab. As heating progresses only the lower temperature phases (lower on Bowen's Reaction Series) in the rock melt to produce magmas of intermediate composition. And since these are fluid and hot they rise up through the crust to eventually emplace and solidify as intermediate rocks (e.g. diorites, granodiorites, etc). The second fraction is the unmelted residue with a composition more mafic/ultramafic than the original rock. That is, its composition is higher in Bowen's Reaction Series than the original rock.
If time and conditions allow, the fractionation process can continue and the intermediate magma fractionate into felsic magma (typically plagiogranites), leaving behind a magma more mafic than the original intermediate starting rock. Thus, beginning with one (mafic) igneous rock many new igneous rocks can be generated, including ultramafic, intermediate, and felsic (model). Or, felsic continental crust is created from the fractional melting of mafic oceanic crust.
In our subduction zone, the ultramafic residue, being very dense, stays in the mantle, while the hot, less dense, melt rises to the surface where it forms first intermediate and later felsic batholithic magma chambers. From the chamber the magma reaches the surface as lava and forms explosive composite volcanoes, which are dominated by andesite, although it can evolve from mafic, to intermediate, to felsic as the magma fractionates. Hydrothermal metamorphism also occurs when hot lava spills out onto the ocean floor and reacts with cold sea water to form pillow basalts (detail).
As soon as the volcano breaks the surface weathering/erosion processes attack it and form lithic rich sediments (detail) (becoming more feldspar rich as erosion exposes batholiths, or as rhyolites and andesites with feldspar phenocrysts weather) that wash into the sea on all sides. Sediments on the backarc side just spill onto the ocean floor as turbidity currents and stay there undisturbed. On the forearc side, however, the sediments pour into the trench as turbidity currents (underwater avalanches). A trench is like the mouth of a conveyor belt and sediments do not stay there long. Instead they are scraped off the subducting oceanic crust into a melange deposit, or they are partially subducted and metamorphosed. A melange is a chaotic mixture of folded, sheared, faulted, and blueschist metamorphosed blocks of rock formed in a subduction zone. It is also normal, if the climate is right, for reefs to grow around the island. These limestones typically interbed with the coarse-grained lithic breccias and conglomerates eroding from the volcano, and the volcanic sands on the beach. During a volcanic eruption, then, lavas and pyroclastics may interbed with limestones to form a very unusual association of rocks.
Two major kinds of metamorphism are common in a volcanic arc forming a Paired Metamorphic Belt. The first is Barrovian metamorphism (low to high temperature, and medium pressure) formed inside the volcano by heat from the batholiths, accompanied by intense folding and shearing. Because the batholiths are invading mafic oceanic crust these rocks are converted into greenschist (chlorite and epidote rich), amphibolite (amphibole rich), and granulite (pyroxene rich) facies rocks as we get closer to the batholiths and deeper in the crust. Also earlier, now crystallized, intermediate and felsic batholiths may be converted into gneisses and migmatites.
The second metamorphism is high pressure-low temperature Blueschist metamorphism formed in the melange of the trench. It is high pressure because this is a convergent boundary and the trench sediments are being rapidly subducted between two plates. The low temperature is because cool surface rocks are rapidly subducted and do not have time to heat up. These belts of Barrovian and blueschist metamorphism form a Paired Metamorphic Belt, which is always the result of subduction.
Other kinds of metamorphism are also associated with the volcanic arc. At depth along the subduction zone the ultramafic layers of the ophiolite suite undergo eclogite metamorphism, and contact and hydrothermal metamorphism would be common along the volcanic pipes and dikes coming off the batholiths (detail).
Ancient and modern volcanic island arcs are very common. Modern examples are Japan, the Aleutian Islands of Alaska, and the Malaysian archipelago including the islands of Java, Borneo, and Sumatra. Ancient examples are not as obvious because they eventually collide with another island arc or a continent and are hidden, but that is Step F in the model.
Now, step back and look at the whole of Cross Section E. Notice that the ocean basin to the west of the volcanic arc is trapped between the divergent continental margin and the subduction zone. Clearly, if subduction continues the ocean basin between the two will become smaller and smaller until the Westcontinent and the volcano collide. Also the more the continent and volcanic arc move together the more oceanic crust is subducted and destroyed. These ocean basins which will soon disappear in a subduction zone are called remnant oceans.
The fact that subduction zones always create remnant ocean basins means that no ocean basin can survive long in geologic history (see these examples). In fact, the oldest ocean basins we know of are only around 200 million years old (compared to the 4 billion year age of the earth). In contrast, continental crust, because it is too light to subduct, tends to remain around just about forever, excluding weathering and erosion.) Many parts of the continents are three to four billion years old.
Go to Step F - Island Arc-Continent Collision Mountain Building
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