At depths of km the oceanic crust begins to melt. The resulting magma, along with the water mentioned earlier, rises up into the wedge of mantle above. This rising material then lowers the melting point of the hot mantle wedge, so in turn parts of that melt.
This new mantle-wedge melt is what rises to the surface and forms the volcanic arc. On the way it may be further modified by melting and then mixing with the crust it is intruded into. The end result is that mantle melting ends up producing rock with a very different composition, called andesite.
This process is worth studying in detail as it is one of the main engines of continental crust formation, producing the stuff that most of you are currently sitting on. Over time, volcanic arcs have been the major mechanism for turning mantle rocks into continental crust.
Subduction is involved in not one but two interlocking cycles of creation and destruction. Oceanic crust is created, but it is destined soon to return to the mantle at subduction zones, to make space for newer crust. Squeezing out of the water the crust gained from the oceans helps create new continental crust. Eclogite plays an important role in both cycles — it helps pull the mid-ocean ridges apart by slab pull and it sweats out the fluids that kick-off the creation of volcanic arcs.
I talk of destruction, but our crust still lives on, transformed. By now it has travelled km down, after about 10 million years of subduction. I found the paper Metamorphic chemical geodynamics of subduction zones an invaluable recent summary of the details of metamorphism within subduction zones and how it drives melting. The link is to a freely available copy. Helens and the drama caused indirectly by devolatilisation reactions in eclogites. Nice and well-written overview.
Do you say that water flows down along the plate from trenches? I thought that water goes down with the plate because the plate contains lots of hydrous minerals which will later release water during metamorphism to initiate partial melting and subduction related volcanism. Glad you liked it! You pick out a fact that surprised me when I read up on it. As the plate bends, it fractures and this allows water to enter down fault planes into the subducting plate, deep enough to reach both the crust and the lithospheric mantle.
As you said, this then creates hydrous minerals serpentine which are carried down into the subduction zone. In fact this mechanism may get more water into the plate forming hydrous minerals than the hydrothermal alteration at mid-ocean ridges. You really make it seem so easy with your presentation but I find this topic to be actually something which I think I would never understand. It seems too complicated and extremely broad for me.
I am looking forward for your next post, I will try to get the hang of it! What category of rock will I become because of this? Depending on the temperature of the subduction zone greenschist or blueschist after this eclogite. Map and features associated with African continental rift zones. As oceanic lithosphere sinks back into the asthenosphere it carries large quantities of seawater and sediment with it. As it sinks, the increased heat and pressure forces water and gases out of the rock.
Does the existence of young oceanic crust suggest an extensional or a compressional tectonic regime in back-arc basins? An extensional tectonic regime. New ocean crust is only produced when lithospheric plates move apart. The presence of an extensional regime in the back-arc region basin may appear counter-intuitive because where two plates are converging the dominant tectonic regime should be compressional.
The mechanisms that give rise to back-arc tension may relate to convection in the asthenosphere underlying the back-arc region. Alternatively, it has been suggested that old, dense slabs may subside into the mantle at a faster rate than the plate is moving, causing the trench to migrate towards the spreading centre euphemistically called 'slab roll-back'.
This gives rise to an extensional regime not only in the back-arc basin but also across the whole arc, even to the extent of suggesting that back-arc basins may originate as arcs that have been split by extension as a consequence of slab roll-back. All of the major features of an oceanic destructive boundary are included in Figure 18, which is an idealised cross-section through an oceanic island-arc system. As you work through this course you will need various resources to help you complete some of the activities.
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For instance, a mid-ocean ridge system in Panthalassa—an ancient ocean that surrounded the supercontinent Pangaea —contributed to shallower oceans and higher sea levels in the Paleozoic era.
Panthalassa was an early form of the Pacific Ocean, which today experiences less seafloor spreading and has a much less extensive mid-ocean ridge system. This helps explain why sea levels have fallen dramatically over the past 80 million years. Seafloor spreading disproves an early part of the theory of continental drift. Supporters of continental drift originally theorize d that the continents moved drifted through unmoving oceans. Seafloor spreading proves that the ocean itself is a site of tectonic activity.
Seafloor spreading is just one part of plate tectonics. Subduction is another. Subduction happens where tectonic plates crash into each other instead of spreading apart. At subduction zones, the edge of the denser plate subduct s, or slides, beneath the less-dense one.
The denser lithospheric material then melts back into the Earth's mantle. Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remain constant. Earth's newest crust is created at sites of seafloor spreading—red sites on this map. Map courtesy NOAA. Triple Junctions. Also called the geosphere. Mid-Atlantic Ridge. Paleozoic Era.
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