Explain how the slab component is generated in island arc igneous rocks. Consider both the fore-arc and back-arc basin environments
Island arcs form as oceanic plate subducts under oceanic plate. Volcanism is concentrated in an arc of volcanoes, generally approximately located above the leading edge of the subducting plate. A trench often forms where the slabs meet and subduction begins. On the non subducting slab a series of basins form, with a fore-arc basin nearest the subduction/trench, then the main arc, and a back-arc basin on the far side (Mitchell and Reading, 1971; Frisch, Meschede and Blakey, 2010).
An island arc subduction zone. Modified from Frisch et al. (2010).
Magmas in island arc settings consist primarily of components from two different origins, the slab component, and the mantle wedge. The mantle wedge may melt due the descent of the slab, giving the main portion of the non-slab component. The slab component is derived from the subducting slab as it descends. This may consist of melting of the crustal portion of the slab, but also melting of the mantle wedge due to addition of water driven off the slab. Since the descending slab is composed of old, cold oceanic crust, there is a slight paradox in the idea of melts forming from it. However, frictional heat, as well as the heat of the underlying mantle, can drive hydrous fluids off the slab. (Machado, Chemale Jr., Conceição, Kawashita, Morata, and Van Schmus, 2003; Kimura and Yoshida, 2006).The addition of water to the mantle wedge results in the lowering of melting temperatures, allowing melts to be formed at much lower temperatures than might otherwise be expected. Also, the circulation of hot fluids allows materials dissolved in them to migrate upwards ...
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Over the centuries, the making of the Big Island as we know it today eventually entailed the growth and conjoining of six separate volcanoes, building all the way up from the seafloor, some 18,000 feet below the ocean’s surface. These volcanoes, from northwest to southeast, are named Mahukona, Kohala, Mauna Kea, Hualalai, Mauna Loa and Kilauea, and become younger as one moves north to south. Mahukona Volcano, just off the Big Island’s northwest coast, was the first volcano to start forming. Now submerged beneath the surface of the ocean because it is sinking into the Earth’s crust under its own vast weight, Mahukona is no longer visible. As the Pacific Plate slowly continued moving northwestward over the hotspot, the location of the rising magma moved relatively southeastward, and through time the rest of the Big Island volcanoes formed along that path.After Mahukona, Kohala Volcano, the precursor to today’s Kohala Mountain, erupted next. As Kohala Volcano emerged from the sea and joined with Mahukona, a much larger Big
and Metamorphic rocks can be found. There are also a lot of crusted plates, and violent
Basalt forms due to the partial melting of the layer of the mantle called the asthenosphere. The asthenosphere is the plastic zone of the mantle beneath the rigid lithosphere. Mantle plumes coming from the mesosphere can cause the asthenosphere to melt with heat or even if pressure decreases, which is called decompression melting (Richard 2011). The magma that forms from this melting is mafic magma that solidifies once it reaches the earth’s surface and cools quickly. The above process mainly occurs mainly during intraplate igneous activity which is the main explanation for volcanic activity that occurs a long distance away from a plate boundary. If the tectonic plate above the mantle plume is moving it can create a string of volcanic activity such as in Hawaii. See Fig 2.
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Four tectonic environments in which magmatism occurs: destructive plate margins, constructive plate margins/divergent plate boundaries, oceanic intraplate and continental intraplate. Arcs are magmatic products of destructive plate margins (stern, 2001) which are referred to as sites where new continental crust is created and old oceanic crust is subducted back into the mantle (Hawkesworth , Hergt, McDermott, Ellam, 1991). An island arc is a form of an arc produced by the subduction of an oceanic plate beneath another oceanic plate (Hu, 2013). Island arcs are characterised by high large ion lithophile elements and low high field strength elements ((Hawkesworth , Hergt, McDermott, Ellam, 1991). New oceanic crust is created at the mid ocean ridges and by mass balance is then destroyed (recycled back into the mantle) at the subduction zones (Jarred, 2003). As it forms it undergoes hydrothermal alteration where the composition is changed from olivine, plagioclase and glass dominated to being dominated by clay minerals i.e. low grade metamorphism (green schist facies) (Jarred, 2003) where it becomes enriched in water, carbon dioxide and incompatible trace elements such as uranium and potassium. As the crust moves away from the ridge it accumulates sediments and becomes colder, older and dense enough to subduct. As the cold slab subducts into the hot mantle, it heats up. The slab goes to greater depths faster than it’s heated therefore pressure increases therefore hydrous minerals start to break down (Aizawa, Tatsumi, Yamada, 1999). The slab undergoes metamorphic reactions from hydrous green schist facies to amphibolite facies and finally to anhydrous eclogite facies at depths of less than 100 km (Ringwood, 1977). ...
In conclusion, Axial Seamount is an underwater volcano on the Juan de Fuca ridge, between the Juan de Fuca and the Pacific tectonic plates. It is one of the most active in the Northeastern Pacific region to this day. It has even been in the news on several occasions. Axial Seamount is fantastic example of how a mid-ocean ridge operates on a diverging plate boundary. However, we must remind ourselves that this is just one of the many interesting and unique volcano type on our home planet we call
Volcanism is a major part of the Galapagos and their formation. The island chain is positioned on the Nazca Plate, which is subducting beneath the South American Plate at a geologically rapid pace of 2.5 inches per year. In addition, this Nazca Plate is located directly on top of the Galapagos Hotspot. It is here that mantle plumes melt Earth’s crust, creating volcanoes as a product. The oldest island was first shaped by this ...
... the the composition of the core and mantle (Merali and Skinner, 2009, p.259), (Tarbuck, Lutgens, Tasa, 2013, p.397-406).
inferred for the reservoir (4). The magma ascent to the surface occurred through a conduit of possibly 70 to 100 m in diameter (5). A thermal model predicts that such a reservoir should contain a core of partially molten magma (6) that can be detected by high-resolution seismic tomography.
The concurrent convective circulations in the mantle leads to some segments of the mantle moving on top of the outer core which is very hot and molten in nature. This kind of movement in different segments occurs as tectonic plates. These tectonic plates are basically seven on the earth surface as major ones, although, several small ones exist also. The plates motions are characterized by varying velocities, this variance results to sub sequential collision of two plates (leading to formation of a mountain in a convergent boundary), drift of two plates (leading to formation of rifts in a divergent boundary), or parallel movement in a transform boundary(Webcache 3).