Volcanoes

Origin of Magma Hot, nearly molten rock in Earth’s asthenosphere can change to a liquid by decompression melting. A buoyant force acts on magma that forms from rock surrounding it.

Origin of Magma Rising magma may reach Earth’s surface if pressure conditions allow and the rock has conduits through which it can flow.

Where do eruptions occur? Hot spots (Yellowstone/Hawaii) Convergent boundary (Mt. Ranier) Rift (African Rift Valley)

Eruptive Products Solids Gases All solid materials expelled by a volcano are collectively called pyroclasts. Gases Volcanoes release a broad variety of superheated gases, the most common of which is water vapor. In addition carbon dioxide and gases composed of sulfur compounds are expelled.

Liquids Lavas can vary considerably in composition, which in turn affects their physical properties. Viscosity is a measure of the resistance of a fluid to flow. Lava can be thick or thin.

Eruptive Styles Eruptive style is strongly linked to temperature and composition and can be linked to the type of plate boundary associated with it.

Thick magma Erupts less easily, causing tremendous pressure. Overlying rock expands. Causes a violent eruption. Abundant pyroclasts. Lava oozes after initial explosion.

Thin Magma Erupt very easily. Produce quiet eruptions of freely flowing lava. Lava can outpour for weeks or months.

Plate Boundary Setting Most of Earth’s volcanoes lie in subduction zones where continental and oceanic materials are being mixed and partially melted.

Hot Spots Hot spots are volcanically active sites that arise in places where large quantities of magma move to the surface in large, column-like plumes. A hot spot under an oceanic plate forms volcanic island chains, such as the Hawaiian islands. Yellowstone National Park is an example of a hot spot under a continental plate.

Hawaiian Islands

Types of Volcanoes Cinder Cone Volcanoes Volcanoes are classified according to their size, shape, and the materials that compose them. Cinder Cone Volcanoes When the primary eruptive products are large fragments of solid material, cinder cone volcanoes form.

Cinder Cone Volcanoes Mauna Kea, Hawaii Craters of the Moon, Idaho

Shield Volcanoes Shield volcanoes erupt with abundant lava flows that can move for kilometers over Earth’s surface before stopping. Shield volcanoes are broad, flat structures made up of layer upon layer of lava.

Shield Volcanoes Iceland Shield Volcano

Composite Volcanoes Volcanoes formed from alternating explosive events that produce pyroclastic materials, and lava flows are called composite volcanoes.

Composite Volcanoes Mayon Volcano Mount Ranier Mount Vesuvius Mount St. Helens