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3. Mountain Building and Volcanoes
The earth is not growing in size, so if new material is added to the crust at one place it must be consumed at another. At convergent boundaries, plates come together. If these plates are the same densities then a crumpling occurs as the plates 'crash' into one another. The Alps and the Himalayas are examples of mountain chains that are young - in fact, they are still growing! The Mediterranean Sea is narrowing as the African plate moves northward into the EuroAsian plate. Eventually, this Sea will become a lake.At other boundaries however, one plate goes under (is subducted) beneath the other producing a trench. As the plate moves downward toward the earth's core it becomes heated and eventually begins to melt. The crust is the lightest material and when it melts it begins to move upwards through the overlying lithospheric plate. This magma rises through cracks and fissures and eventually will emerge on the earth's surface as a volcano.
