Transcript Volcano Notes
Magma Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases deep beneath Earth’s surface. Pressure and water are two factors that determine whether rock will melt to form magma. A liquid’s resistance to flowing is called its viscosity. Substances such as honey, liquid soap, and motor oil have a higher viscosity than water, vinegar, and gasoline. Rhyolitic magma has the greatest viscosity. Types of Magma Basaltic magma has low silica content and the least explosive eruption. Andesitic magma has intermediate silica content and an intermediate eruption. Rhyolitic magma has high silica content and an explosive eruption; greatest viscosity. Anatomy of a Volcano An opening in the Earth’s surface where magma escapes is called a vent. A bowl-shaped depression around the vent of a volcano is called a crater. If a volcano collapses, the very large crater that is left is called a calderas. Types of Volcanoes Shield Volcano: Mountain with broad, gently sloping sides and a nearly circular base. Shield volcanoes form when layer upon layer of basaltic lava accumulates during non-explosive eruptions. Cinder-cone Volcano Forms when material ejected high into the air falls back to Earth and piles up around the vent. Cinder-cone volcanoes have steep sides and are generally small. Cinder-cone volcanoes are more explosive in nature than shield volcanoes. Composite Volcanoes Forms when layers of volcanic fragments alternate with lava. The magma that forms composite volcanoes commonly contains large amounts of silica, water, and gases. They are larger than cinder-cone volcanoes, and, because of their violently explosive nature, they are potentially dangerous to humans and the environment. Hot Spots Hot spots are unusually hot areas in Earth’s mantle that are stationary for long periods of time, where high temperature plumes of magma material rise toward the surface. Some volcanoes are located far from plate boundaries and these form as a result of hot spots. Where Do Volcanoes Occur? Most volcanoes form at plate boundaries. 80% of all volcanoes are found along convergent boundaries. 15% are found on divergent boundaries. Only 5% occurs far from plate boundaries.