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-13 degrees C
The structure is called Cell wall made from cellulose.
At an oceanic/oceanic plate boundary 2 oceanic plates will collide against each other. The older oceanic plate will sub-duct under the newer one. It will sub-duct into magma, and will melt into it. The plate and magma will create a reaction, and will build up pressure. With enough pressure, magma will erupt creating a volcano.