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Section 10-1 • What is a volcano? • Describe the process that causes a volcano. • Define vent and crater. • Explain the three kinds of places that volcanoes are likely to be found. • A volcano is an opening in Earth’s surface that forms a mountain when layers of lava and volcanic ash erupt and build up. • Most of Earth’s volcanoes are dormant, which means that they are not currently active. • Active volcanoes sometimes spew steam, smoke, ash, cinders, and flows of lava. What Causes Volcanoes? • Volcanoes occur when magma, which is less dense than the rock around it, is slowly forced upward towards the Earth’s surface. • After many thousands or millions of years, lava flows out through an opening in Earth’s surface. As the lava flows, it cools and becomes solid, forming igneous rock. • A vent is an opening in Earth’s surface through which lava, ash, cinders, smoke, and steam can flow. • A crater is the steepwalled depression around a volcano’s vent. Crater • Volcanoes can occur at divergent boundaries which are areas where Earth’s plates are moving apart. • Where the plates separate, they form long, deep cracks called rifts. Iceland was made from a rift on the Mid-Atlantic Ridge. • Volcanoes can also occur at convergent boundaries which are areas where Earth’s plates move together. • When plates converge, one plate slides under another plate where the plate melts and is forced upward in a volcano. • Examples of volcanoes formed by convergent boundaries are found around the Pacific Plate, which is known as the Pacific Ring of Fire. • Lastly, volcanoes can occur at hot spots, areas in Earth’s mantle that are hotter than neighboring areas, forming melted rock that rises toward the crust. • The Hawaiian Islands were formed from hot spots.