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This is a very good question Inkenbrandt. It is quite common for even some graduate students to think that the majority of partial-melting comes from the slab: when it is the the mantle above the fore-arc, in the volcanic region, where most of the partial melting occurs. Lets think about this mathematically: you say yourself that you realize that subduction ...

9

There was an article in that year's Scientific American, which I've read in the Polish edition. I don't have that paper now, but I think this was that one: http://www.scientificamerican.com/article/the-oldest-rocks-on-earth/ The discovery is described in short here: http://news.nationalgeographic.com/news/2014/02/140224-oldest-crust-australia-zircon-science/...

8

First of all, your statement implies that volcanism didn't occur in Troodos. That is not true. Troodos was even referred to as "Troodos Volcano" once in Miyashiro's 1973 article about Troodos. A geological map of Cyprus clearly shows that a large portion of the ophiolite is composed of lavas (volcanic) and dykes (sub-volcanic): (source) Note the red, ...

8

After some brief research, serpentinite diapirism occurs due to deep penetration of seawater at temperature around 100-200$^o$C [1]. To the south of the rift that was forming the ophiolite sequence there was an oceanic plate subducting as African plate moved north. This subducting oceanic plate provided the seawater that serpentinized the harzburgite in the ...

8

Excellent question! Indeed, entire plates have subducted before. One example is the Intermontane Plate (Wikipedia). This plate sat west of North America around 195 million years ago. The plate contained a chain of volcanic islands on its western edge (known as the Intermontane Islands). As the plate subducted under the North American plate, that island ...

7

As a complement to the two other answers, the work of van de Meer et al., 2012 is worth mentioning. In this article, the authors tried to explore the Triassic-Jurassic plate tectonic evolution of the Panthalassa Ocean (currently all remaining oceanic crust in the Pacific Ocean is Cretaceous or younger). They localized a serie of paleo-subduction zones (that ...

7

The end of the Tethys Sea and the initiation of the continent-continent collision that formed the Himalayas is thought to have happened about 65 Ma ago (recent studies suggest maybe only 35 Ma). And since then, that colossal collision have produced an estimated 2,500 km of crustal shortening. That means that 2,500 km of continental surface is gone. A ...

7

Megathrust is not a particularly well defined term but in my experience it generally refers to very large earthquakes that occur on the subduction interface. The subduction interface is the main surface which separates the overriding plate from the subducting plate and is where most of the convergence is accomodated. (image source) Earthquakes occur on ...

6

No. In fact, I don't know why David Hammen didn't say that instead of commenting. The deepest hole that mankind ever dug was barely a scratch on the Earth's surface when you look at its size. They didn't even reach the mantle. The hole is only 12km deep, took about 20 years to create, and is still only 1/3 of the way to the mantle! Also, the hole is nothing ...

6

Fast exhumation is not necessary to retain blueschist mineral assemblage. This is the classical difference between prograde and retrograde metamorphism, and it's not limited to blueschists. Why do we have eclogite on Earth's surface? What about granulites? Amphibolites? In fact, why do we see any metamorphic rocks on the surface instead of just various clays ...

5

I'll add to Richard's contemporary answer and add that we have plenty of evidence for subducted plates in the geological past. The evidence lies in the form of ophiolites. Ophiolites are fossil fragments of oceanic lithosphere, that for some reason (which I will not go over in detail here), survive subduction and become obducted (a fancy antonym for ...

5

The deep zones in the ocean are not where new rock is formed. The trenches are the areas where rock is subducted back into the mantle. Here's a picture to visualise it: USGS/USGov, modified by Eurico Zimbres New rock is formed at mid ocean ridges (for example 12 in the image). Those are areas that are less deep than the rest of the ocean. This rocky plate ...

5

When the subducted oceanic plate slides beneath the continental crust, it causes crustal thickening and sometimes crustal folding. In addition to this, rising plumes of magma are created when the oceanic plate is forced down into the mantle. These are all orogenic (mountain building) processes. We also have mountains built by volcanoes. The friction created ...

5

Short answer: yes, there are other mountains which contains oceanic crust in their lithologies. The process you described as "the upthrust of ocean crust" is called obduction. The resulting rock sequence is called an ophiolite. As you can see here, there is quite a lot of them around the world! One of the most studied is the Oman ophiolite. I have seen the ...

5

Yes, mega thrust earthquake can occur in any point of any subduction zone. However, some areas areas are much more active than others, meaning that the frequency of earthquakes and energy released per event will vary largely from place to place. Convergence rate is one important controlling factor for that, but not the only. Regarding the difference between ...

5

Continental crust is thick and relatively buoyant. Ocean crust is much thinner, and relatively dense. Hence ocean crust "goes down" more easily, and the density can even help pull the ocean slab due to metamorphism. In contrast, the continents are almost like floating scum which won't go down! When they collide, you get faulting and folding, and ...

4

I don't know whether its currently the favoured explanation, but in California people made reference to something informally referred to as the watermelon seed theory . The general idea being that the compression in a subduction zone managed to eject these rocks back up and out of depth by a process like squeezing (and shooting) a wet watermelon seed between ...

4

Ah yes, this is a very good question. The formation of back-arc spreading centers is an open research problem, and happens to be the main topic of my office mate's dissertation! While these aren't exactly "clean" back-arcs, the formation of them is similar, I'm fairly sure. The formation of back-arcs is caused by "Trench Roll-back". Trench Rollback is ...

4

As mentioned in another answer: Continental crust is thick and relatively buoyant. Ocean crust is much thinner, and relatively dense. Let's look at a simulation of what might happen. Here's a figure from "Continental collision and slab break-off: A comparison of 3-D numerical models with observations": Let's look at this in detail. You start by ...

3

Of course, any ultramafic rock will ultimately melt at about 2000 deg C, but long before that there will be some interesting phase transitions, possibly involving serpentine > talc > olivine + orthopyroxene. Various subtle changes occur in the rock during its original hydration from peridotite to serpentinite. There is high- and low-temperature serpentinite....

3

Yes, the situation is much more complex than you make out - the generation of melt is typically caused by partial melting of the mantle wedge located beneath the volcanic arc. This process is mainly affected by the trench geometry (flat or steeply dipping slab), the local geotherm, and volatile content (water, gases etc.) of the slab or subducted material. ...

2

As taken from Wikipedia: There are several factors that control the depth of trenches. The most important control is the supply of sediment, which fills the trench so that there is no bathymetric expression. It is therefore not surprising that the deepest trenches (deeper than 8,000 m (26,000 ft)) are all nonaccretionary. In contrast, all trenches with ...

2

Differences in subduction geometry depend mostly on what type crust takes part in the collision (oceanic to oceanic, continent to continent, oceanic to continent), the speed of the two (or three) colliding parts and the relative movement of the two plates. Many subduction zones share similar characteristics (like back arc compression and volcanic arc) but ...

2

Tohoku is the seismogenic zone, the asperity model explained this small repeating earthquake well. It was said massive earthquake will occur in Nankai Trough. As their expectation was wrong, [Kato and Yoshida, 2011],[Hori and Miyazaki, 2011] created new models. ref Notice japanese source. Here is new model of Sansoken(National institute of advanced ...

2

numerical modelling is a vast field and the tool to address your problem depends strongly on ... your problem and your approach! Are you a master student? a PhD student with very short time to wrap up your thesis? A plumber with an intelectual interest in plate tectonics? In short: Comsol may be an adequate tool. However, with large displacements (larger ...

2

One of the flaws with using subduction zones as dumps for toxic chemicals so they can be broken down by heat and pressure at depth is the rate of sinking of the subducting plate. Subduction is a very slow process, generally between 2 and 8 centimeters per year. Any toxic material is going to hang around the interface for a very long time, with the potential ...

2

10mm a year is a huge volume when you multiply with lithosphere thickness and length of the boundary, but you are right - it's more complex than this. Melting is caused by the interplay of temperature, pressure and chemical composition. In the case of subduction of oceanic crust, it's the mainly the water that have been incorporated in seafloor that lowers ...

1

Both of the processes you describe occur; in some cases such as the Himalayas the plates are not subducted and a moving plate, in this case the Indian plate, bulldozes the continental crust ahead of it into a high mountain range. In most cases, such as the Pacific rim for example, subduction is the norm. Where there is subduction there are usually many ...

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