Lecture 6// Volcanoes

Question 1

3 types of magma (14)

Answer 1

Basaltic:
-Gently sloped shield volcanoes (ex. Mauna Loa, Hawaii).
-MORs
-Basalt plateaus like Columbia Plateau or Hawaiian Islands(fissure eruptions).
Generated at:
-Oceanic hot spots (stationary mantle plumes beneath oceanic crust).
-Fissure eruptions (flood basalts)
-MORs forming new oceanic lithosphere.

Andesitic:

• Composite volcanoes (ex. Mount St. Helens, Mount Rainier).
• Generates steep-sided composite volcanoes. (lava flows & pyroclastics)

Rhyolitic:

• Rhyolitic caldera complexes (ex. Yellowstone volcano)
• Generated at long-lived continental hot spots.
• Felsic magma pools within the lithosphere and adds hear, causing overlying crust to dome upwards and become stressed.
• Cracks form and extention occurs creating down-dropped blocks and an enormous central caldera.

Question 2

Viscosity (1)

Answer 2

-The resistance to flow, and relates to the number of strong silicon-oxygen bonds that are forming.

Question 3

Basaltic magma (3)

Answer 3

• Mafic with low viscosity = runny and not explosive.
• Erupts often when active with low gas content (0.5%-2%) with silica content of -50%.
• 2 types of lava: pahoehoe and aa.
• Temperature of -1200°C

Question 4

Andesitic magma (4)

Answer 4

• Intermediate with intermediate viscosity = sticky and quite explosive (through erupt infrequently).
• 3%-4% gas content with silica content of -60%.
• Temperature of -1000°C
• Formed adjacent to subduction zones, creating volcanic arcs and island arcs.

Question 5

Andesitic Example: Mount St. Helens (May 18th, 1980) (3)

Answer 5

• Landslide (and pyroclastic flow deposits up to 35 meters thick) and lateral blast followed by a vertical eruption ad lahar (mudflows).
• 57 fatalities who ignored the mandatory evacuation order, the eruption could be heard in the Lower Mainland of BC
• Since then erupting sticky rhyolitic lava that has built a lava dome within the crater.

Question 6

Andesitic Example: Mt. Mazama (now Crater Lake), Oregon (aprrox. 7700 yrs ago) (3)

Answer 6

• The caldera formed from the eruption of Mazama (44x more powerful than Helens).
• Approx. 250 yrs of rain and snow accumulation for the caldera fill to its present-day lake level.
• The lake level is maintained by a balance between precipitation and evaporation.

Question 7

Rhyolitic magma

Answer 7

• Felsic with high viscosity = very sticky and very explosive (through there tends to vast amounts of time between eruptions).
• Formed by hot spots beneath continents creating a rhyolite caldera complex.
• Contains Silica content of >70% and gas content of 4%-6%.
• Temperature of - 800°C

Question 8

Rhyolitic Example (4)

Answer 8

-Yellowstone Supervolcano, Wyoming:

Eruption 1 (640,000 yrs ago):
-1000 km^3 of ash and pumice.
Eruption 2 (2.1 million yrs ago):
-2450 km^3 of ash and pumice.

(Mt. Helens eruption: 2.8 km^3 of material)