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Hi, I'm Emerald Robinson, and in this "What Is" video, we're going to answer the question, "What is a Mountain?" Defining what a mountain is can be tricky, but a good definition is, "any land mass higher and steeper than a hill." Many geographers agree that mountains are a minimum of two thousand feet high, and have a slope that's greater than two degrees. There are many types of mountains, and they are classified by how they are formed. Volcanic and dome mountains are formed by the molten rock, or magma, found beneath the earth's crust. Volcanic mountains are made when erupting magma cools and hardens, forming a cone shape. Dome mountains are made when magma causes the crust to bulge, and then subsides, leaving a rounded formation behind. Japan's Mount Fuji is a famous volcanic mountain, while the Adirondacks in the Eastern U.S. are well known dome mountains. Fold mountains form when the plates that make up the earth's crust collide or pass over top of one another, causing the crust to wrinkle and fold. The Rocky Mountains in the North American west and Europe's Alps are fold mountains. Block mountains are made when pressure on giant cracks in the earth's crust, or "faults," cause slabs of rock to tilt upward and sometimes stack on top of one another.  America's Sierra Nevada Mountains are block mountains. The highest mountain on earth is Mount Everest in the Asian Himalayan Mountains. Its peak reaches 29,029 feet above sea level - and is still growing! If you measure a mountain's might by its area, Hawaii's volcanic Mauna Loa is the world's largest at about seventy five miles across. Both of these, though, are dwarfed by Olympus Mons on Mars, the solar system's highest and largest mountain, over thirteen miles high and about three hundred seventy four miles across.