Chapter 18

Volcanism and Plate Tectonics

•There are about 485 active volcanoes world wide.•Volcano is a term applied to a structure built around a vent by the material ejected from the vent.•Magma – molten rock located underground.

•The rate at which magma moves is determined primarily by it’s silica content.•High silica content -> thick, light colored, slow moving -> felsic magma.•Low silica content ->thin, dark colored, flows quickly -> mafic magma

Viscosity is a fluids resistance to flow.•The greater the silica content the greater the resistance to flow•The less the silica content the less resistance to flow.

Many gases are dissolved in magma, most important are water vapor, carbon dioxide and sulfur.

Lava – magma that reaches the surface of the Earth.•Felsic lava is thick and viscous•Mafic lava is thin and fluid

The condition and state of lava change as it cools – gases are released.

Felsic magmas result in explosive eruptions.

Mt. St. Helens

Mafic magmas result in flowing type eruptions.

Mt. Vesuvius

Tephra are solid fragments of lava:•Classifications:

•Less than 2mm – ash•Up to 64mm – lapilli•Greater than 64mm – blocks and bombs

•Blocks – solids pieces•Bombs – hardens as it falls

Pyroclastic Flow – tephra and gases that form a dense superheated cloud that can travel down hill at speeds up to 200 mph.

Kinds of eruptions:

1. Rift eruptions – occur along long narrow fractures in the crusts; rift valley.

2. Basaltic flow – occur on land, results in few gasses, forms shield cone – a volcanic mountain w/ broad base and gently sloping sides.

•Basaltic Plateaus may display 6 sided columnar jointing – i.e. – Giants causeway, Devil’s Postpile…

3. Oceanic Spreading Centers ooze lava that cools rapidly forming rounded shapes called “pillow lava”. i.e. – Mid Atlantic ridge and East Pacific Rise.

4. Subduction Boundary eruptions produce magma formed in subduction boundaries. •Explosive eruptions, lava

fragments (tephra), cinder cone mountains with steep sides. i.e. Cascades of Washington & Oregon states, Central America, Andes Mountains in South America.

5. Hotspots are areas of volcanic activity in the middle of a lithospheric plate; the hotspot remains in the same place as the plate continues to move over it. i.e. – Hawaii (active volcano) islands to the north have extinct volcanoes, indicates a steady movement of the Pacific Plate.