The volcano is one of the most powerful forces in nature, but the source of its immense power is unseen. This source is the heat of rock 15 miles or more beneath the Earth's surface. This tremendous heat melts solid rock into a white hot mixture of gas and a liquid called. Atma, the gas filled magma is lighter than the unmelted rock, so it rises toward the surface, melting gaps in the rock. As it rises, it enters a magma chamber, a region of magma that hardened after a past eruption. The magma continues to push and melt its way upward. It pushes through channels of weak, fractured rock consisting of hardened magma. Eventually, the magma and gas push up to the top of the volcano and blast out through an opening called the Central Vent. Magma erupts as a fluid called lava or as rock fragments called tephra. Lava can be highly fluid and fast moving or sticky and slow moving. Tephra can be as small as dust or as large as a boulder. One type of volcano, the composite volcano, is formed during successive eruptions of lava and tephra, which build a cone around the central vent. Famous composite volcanoes are found in Japan, Italy, and the United States.