Please provide explanations as to why which rock type came from which volcano, a

Question

Please provide explanations as to why which rock type came from which volcano, as well as reasoning for the oldest and youngest.

The diagram shown below is a cross section of an active subduction zone. Mr. Stonehead, a geologist, visited this area and collected three samples from volcanoes A, B and C. Unfortunately, he got the samples mixed up, and now cannot remember the location of each sample. If you knew that one of the samples was a basalt, the other a rhyolite and the third an andesite, can you help him figure out which sample came from which volcano? Which of the three samples (or volcanoes) do you think is the youngest? Which one is the oldest? Please give your reasons. (There are no hot spots here, and you cannot rely on erosion to tell which cone is oldest!). (Hint: Use your knowledge of processes occurring at subduction zones, and assume a constant rate of subduction that has remained the same until the present). Please write all answers neatly on the back or on a separate piece of paper. Sea Level Trench A BC Continental crust Oceanic crust 0km 50

Explanation / Answer

A volcano sample is Rhyolitic magma - 650 to 800oC.

B volcano sample is Andesitic magma - 800 to 1000oC

C volcano sample is Basaltic magma - 1000 to 1200oC

We know that temperature increases with depth in the earth along the geothermal gradient. Magmas are produced by partial melting of the mantle in the asthenosphere, or by partial melting of the base of crust above a down-going (subducting) lithospheric plate, adjacent to convergent plate boundaries. So that based on temperature i decided the samples from each volcanoo. Out of three the youngest volcano is "C".

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