Names: Homework Problem # 3 gneous activity and plate tectonics The diagram show
ID: 288597 • Letter: N
Question
Names: Homework Problem # 3 gneous activity and plate tectonics The diagram shown below is a cross section of an Stonehead, a geologist, visited this area and colle volcanoes A, B and C. Unfortunately, he got the samples mixed active subduction zone. Mr. samples was a basalt, the other a rhyolit him figure out which sample came from which volcano? the location of each sample. I you knew that one of the e and the third an andesite, can you help Which of the three samples for volcanoes) do you think is the one is t youngest? Which he oldest? Please give your reasons. (There are no hot spots here, and you cannot rely on erosion to tell which cone is oldest!) a constant rate of subduction that has remained the same until the assume present). Please write all answers neatly on the back or on a separate piece of paper Sea L Trench Continental crust Oceanic crúst 0 km 50Explanation / Answer
In the given tectonic condition , we see a subduction zone with the oceanic plate subducting below the continental plate . As we know the oceanic plate is of high density than the continental plate it subducts below the continental plate . The geologist collected 3 samples from the 3 volcanos A,B,C , but lost the sequence due to artificial shuffling .
We know the 3 sample rock types as Basalt,Andesite,Ryolite .Among the 3 rock types the ryolite is the silicic rock, basalt is the mafic rock and andesite is the intermediate composition of the above . The mafic rock is produced by the melting of mafic source . So the sample A cannot be the mafic composition because it is nearest to the subduction zone/trench . and at such low depths the mafic oceanic crust can't be melted . But due to the friction of the plates here some heat is generated which is capable enough of melting a silicic composition rock i.e, the continental crust(granite-granodiorite) . So at A the volcano must be erupting the ryolitic lava .
Again as we move far from the A site the C site is far enough for the oceanic crust ti reach a depth where the optimum temperature could melt it and from the melt the Site C volcano must be erupted with basaltic composition . and as we know the andesite is intermediate in composition the B site suggests andesitic composition .
AGE :
From these 3 volcanic sites the A is the oldest and subsequently age is younger towards C . I'm saying this because the oceanic plate is subducting below the continental crust . Hence first melting is due to friction to the nearer positions of trench . As the subduction continues the later melting of crust will give rise to subsequent volcanos till C .
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