Stage E - Arc Mountain Building


 
 Creating a Convergent Boundary: Volcanic Island Arc Mountain Building

 
Divergence and the subsequent creation of new oceanic lithosphere can continue for tens or hundreds of millions of years. When divergence halts, and the two continents begin to move back toward each other, the second half of the Wilson cycle is initiated. This is termed convergence and a new plate boundary is created. 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. 
     
There are two types of locations where subduction zones form; one within an ocean basin (Island Arc type) and the other along the edge of a continent (Cordilleran type). Both kinds of subduction result in volcanic mountain building.  Subduction into the mantle may create several new Structural Features, and generate a completely new suite of rocks.  Each are reviewed separately below.

~Structural Features~
 
At the site of subduction, part of the oceanic crust is dragged down into a trench 1-2 km below the ocean floor (5 km deep). The subducting oceanic crust begins to heat up as it slides into the mantle and at about 120 km, rock begins to melt, forming magma. The hot magma (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 and it typically forms a String Of Volcanoes 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 is 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

~Fractional Melting and the Creation of New Igneous Rocks~
 
The mantle rock above the subducting plate selectively melts and fractionates (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 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. DioritesGranodiorites, 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. 
     
The fractionation process may continue and the intermediate magma alter into Felsic Magma (typically Plagiogranites), leaving behind a magma that is 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).
     
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. 
 
~Sedimentary Processes~
 
When the volcano breaks through to the surface, weathering/erosion processes begin immediately. Lithic sediments are formed, becoming more feldspar-rich as erosion exposes batholiths, or as Rhyolites and andesites with feldspar phenocrysts weather and wash into the sea. Sediments on backarc side spill onto the ocean floor and remain undisturbed. On the forearc side, however, the sediments pour into the trench as turbidity currents (underwater avalanches) 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.

~Paired Metamorphism~
 
Two types 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 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.

~Remnant Oceans~
 
In Cross Section E, the ocean basin to the west of the volcanic arc is trapped between the divergent continental margin and the subduction zone. If subduction continues the ocean basin between the two will become smaller and smaller until the West continent and the volcano collide. As the continent and volcanic arc move closer together, the more oceanic crust is subducted and destroyed. These ocean basins which will soon disappear in a subduction zone are called remnant oceans. Therefore,  no ocean basin can survive for very long in geologic history. The oldest ocean basins 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, is much older; many parts of the continents are three to four billion years old. 
 
Contributed by Lynn Fichter 
Monday, August 25, 2014
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