Mount Etna formed when the oceanic, African plate subducted under the continental, Eurasian plate. The African plate eventually reached a depth of about 100 kilometers. The temperature and pressure then became large enough that the thin layer of water bound in the minerals became unstable and was released into the mantle. The water the mineral contained, was also released, decreasing the melting temperature of the mantle sediments. As a result, small portions of rock began to melt where the oceanic plate subducted. The melting rock rose, and formed a little mountain, which grew and became a volcano.