Active volcanism is clearly related to tectonic plate boundaries Active volcanism is clearly related to tectonic plate boundaries. The nature of the eruption is strongly controlled by the source of the magma. Magmas with high silica content and lower melting temperatures tend to produce explosive eruptions. Low silica content and higher temperature magmas tend produce quiescent eruptions.

Active volcanism is clearly related to tectonic plate boundaries Active volcanism is clearly related to tectonic plate boundaries. Note the relationship between volcanism and the location of tectonic plate boundaries. Intraplate volcanic activity is often associated with mantle hotspots (such as Hawaiian Islands).

Active volcanism is clearly related to tectonic plate boundaries Active volcanism is clearly related to tectonic plate boundaries. The nature of the eruption is strongly controlled by the source of the magma. Magmas with high silica content and lower melting temperatures tend to produce explosive eruptions. Low silica content and higher temperature magmas tend produce quiescent eruptions.

Basaltic composition magmas are produced at tectonic plate settings where partially melted asthenosphere rises to the surface. The tectonic plate setting include: sea floor spreading centers, continental rift zones, intra-oceanic plate hotspots, intra-continental plate hot spots, and back arc spreading basins.

Andesitic magma is produced from a partial melting along subduction zones. Introduction of water forced out of the subducting plate lowers the melting temperature of the upper mantle, which rises and partially melts the overlying crust. In an ocean-continental convergent margin it may mix with partially melted continental crust, increasing the magma’s silica content (becomes more felsic). Mount St. Helens dacites are more silica rich than Mt. Rainier andesite, likely due to a continental source.

Rhyolitc composition magmas are produced at tectonic plate settings where partial melting of continental lithosphere occurs, such as continental hotspots (i.e., Yellowstone Park), where the continental crust is being heated closer to the surface by upwelling magma generated from a asthenospheric hotspot.

The three major magma/lava types are largely classified based on silica content and to a lesser extent the other common elements. Note the relationship between SiO2 content and FeO and MgO content of the respective magma types. The silica content and temperature of the lava plays a significant role in its viscosity properties.

Kilauea, Hawaii Snake River Plain, Idaho Basalt flows will travel great distances and slope angles will reflect low viscosity. Gas content readily escapes from low viscosity lavas (e.g., see lava fountain caused by escaping gas in background).

Yellowstone Park Rhyolite/dacite flows will retain steep slope fronts because of high viscosity. Dissolved gas does not escape easily and pressures can build up as trapped gas expands.

Volcanic eruptions can be classified according to the Volcanic Explosivity Index (VEI), which is based on several measurable factors that are directly related to the explosiveness of the eruption, inluding: 1. volume of tephra ejected, 2. cloud column height, 3. eruption type, 4. duration of continuous blast.

Mono Craters, California: VEI - 1-3

Mount Saint Helens: VEI = 5

Mount Pinatubo: VEI 6

Bishop Ash in alluvial fan deposit. Longvalley Caldera: VEI = 7.

Yellowstone Caldera: VEI = 8

The morphology of a volcano is strongly controlled by the viscosity of the eruptive product. The Hawaiian shield volcano shown above is composed of interlayered, low-viscosity basalt flows. Slope angles typcally range between 7°-10° for shield volcanoes.

The Hawaiian Islands and Emperor Seamount chains formed over a mantle hotspot. As the Pacific Plated moved to the northwest, new islands form above the hotspot. The age of the islands becomes progressively older to the northwest. As the plate moves over the hotspot it cools and subsides, submerging the shield volcano.

Most mantle hotspots underlie oceanic crust creating basaltic shield volcanoes, but there are several that underlie continental crust, often resulting in bimodal volcanism (rhyolitic flows overlain by basaltic flows. Why do you think mantle hotspots underlying continental crust result in bimodal volcanism?

Plateau basalts form where basalt flows are erupted along linear rifts in terrestrial settings, such as continental rift zones (e.g., East Africa) or back are basins (e.g., Columbia Plateau).

Low viscosity basalt flows erupted along the East African Rift Zone (shown above) and Snake River Plain (shown on right) travel great distances and create broad lava plateaus.

Strato (composite) volcanoes form along subduction zones where partially melted ocean crust, marine sediments and water-enriched mantle rock rise to the surface to produce andesitic composition volcanoes. The image shown above is a cross-section of the Japanese subduction zone.

Strato (composite) volcanoes form from interlayered pyroclastic and intermediate (andesite or dacite) lava flows. Strato volcanoes can become larger over time with subsequent eruptions.

Strato (composite) volcanoes become large with upbuilding. Eruptions along the flank can occur where extensional cracks can develop as magma upwells within the magma chamber. Over time magma can become more silica-rich through fractionation and the volcano’s life cycle can end with a cataclysmic eruption and emptying of the magma chamber.

The slope angle of the of strato (composite) volcanoes is controlled by the angle of repose for unconsolidated pyroclasts and the viscosity properties of the silica-rich andesite and dacite flows. The volcanoes shown above are part of the Aleutian Island, Alaska.

The Cascade Range is the result of subduction of the Juan de Fuca oceanic plate under the North American plate (continental lithosphere).

Volcanism associated with subduction zones tends to be explosive as the andesitic and dacitic magma has a relatively high viscosity coupled with high gas content.

Crater Lake, Oregon occupies a collapsed caldera of Mt Crater Lake, Oregon occupies a collapsed caldera of Mt. Mazama which had a cataclysmic eruption ~6800 years ago. The lake is over 2000 feet deep. Wizard Island is small dacite dome that formed shortly after the main eruption.

1 4 Crater Lake, Oregon formed from the collapse of Mt. Mazama following a cataclysmic eruption 6800 years ago.

Cinder cones form when gas-charged lava is ejected into air and the large fragments (pyroclasts) cool in the air and fall near the vent. Pyroclasts can range in size from large volcanic bombs to dime-sized lapilli.