Volcanoes

About 1900 volcanoes are active or known to have been active About 1900 volcanoes are active or known to have been active. Ninety percent are in the Ring of Fire, a band of volcanoes circling the edges of the Pacific Ocean.

Benefits of volcanoes 1. Provide valuable mineral deposits 2. Fertile soil 3. Geothermal energy 4. Lava flows can build new land  

Most volcanoes occur at plate boundaries, areas where huge slabs of rock meet in the Earth’s lithosphere. Volcanic activity is closely related to plate tectonics.

Cinder Cone Small, steep cone- shaped hill made up of ash and cinder. They are mainly explosive volcanoes but they can issue lava.

Caldera Large oval or circular volcanic depressions formed by inward collapse of a volcano.

Composite (Stratovolcano) Steep-sided, symmetrical cones formed when pyroclastic eruptions alternate with lave flow. (Pyroclastic – explosions of ash, cinder, and rock fragments) Examples – Mt. Fuji in Japan and Mr. St. Helens in Washington

Shield They are saucer-shaped from rapid streams of lava that spread easily and then cool, forming smooth, gentle slopes. The summits are nearly flat.

Submarine They can have many forms, but most are cone-shaped seamounts. They can be active and erupt under water. Hydrophones are instruments that can detect the explosions.

Lava Dome They are formed when lava is so viscous that it mounds up over its vent without flowing away.

Two Important Traits Viscosity Explosiveness

Explosiveness An eruption begins when magma rises toward the surface. Dissolved gases in the magma determine whether the eruption will be explosive or nonexplosive. Higher amounts of gases lead to more explosive eruptions.

Viscosity The measure of a substances ability to resist flow. The lower the viscosity, the more fluid the lava is. The magma’s silica determines its viscosity.