Volcanoes

Question 1

volcano

Answer 1

location where molten rock or pyroclastic material erupt through a vent. Often, but not always, a conical mountain

Question 2

crater

Answer 2

depression overlying a volcanic vent

Question 3

extrusive igneous rocks

Answer 3

• Form from fast cooling of molten rock (lava) at the earth’s surface OR
• Fragmentation and fast cooling of molten rock as it explovely erupts forming “pyroclasts”
• Fast cooling = small crystal size; fine-grained (microscopic)

Question 4

explosive vs. effusive

Answer 4

1. explosive (BOOM):
   - High viscosity magmas (like peanut butter)
   - felsic – high silica content (strong chains of silica tetrahedra)
   - Magmas that are gas-rich (full of volatiles)
   - Ex. Mt. St. Helens
2. effusive (fizz… non-explosive):
   - Low viscosity magmas (like honey)
   - Low silica content
   - Gas poor
   - Dissolved gases – bubbles escape “vesicles” - lava fountains may form
   - Ex. Kilauea

Question 5

Pahoehoe vs. A’a

Answer 5

• Pahoehoe = smooth, ropey lava type

- A’a = sharp, clinky lava type

Question 6

pillow lava

Answer 6

• Errupts underwater

- “skin” cools immediately, inflates with lava like a balloon until too cool -> “pops” and another baloon formed

Question 7

pyroclasts (5 types)

Answer 7

• “fire piecies” ejected out of a volcano during eruption
• Types:
• Ash: <2mm
• Lapilli: 2-64mm
• Bombs: >64m
• Pumice: floats on water, felsic
• Scoria: sinks, mafic

Question 8

pyroclastic deposits

Answer 8

• Pyroclastic flows (these kill)
• Pyroclastic fall
• Spatter (scoria)
• bombs

Question 9

gases and volcanoes

Answer 9

• Gases associated with volcanoes: H2O, CO2, N2, SO2, CL2, H2
• Effects of these gases: global warming, acid rain -> global cooling, destruction of ozone

Question 10

types of volcanoes

Answer 10

• shield volcanoes
• cinder cones
• composite volcanoes/Stratovolcanoes

Question 11

shield volcano

Answer 11

• Gentle slopes
• Basaltic
• Can be very big
• Associated with hot spots (anomalously warm regions of mantle)
• Generate hot, mafic (low silica, low viscosity) magmas
• Ex. Muana Loa

Question 12

cinder cones

Answer 12

• 100s of meters high
• Layers of pyroclastic ejecta (scoria, bombs)
• Mostly mafic
• Some lava
• Angle of repose = 30-40 degrees
• May occur on their own or on the flanks of one of the larger types of volcanoes
• Ex. Paricutin, Mexico – turned from a gassy crack in the earth to a full-blown volcano in 9 years

Question 13

composite volcanoes (Stratovolcanoes)

Answer 13

• Kilometers across, several kilometers high
• Quite steep – classic volcano shape
• Interbedded lava flows, pyroclastic flows, lahars, shot through with sills and dykes
• Full range of chemical compositions (mafic, intermediate, felsic) and volcanic products including lahars
• Ex. Mt. St. Helens, Mt. Baker, Mt. Garibaldi
• Form pyroclastic flow: hot pyroclastic material flowing downslope under gravity
• Creates hot gas (500 degrees celcius) that moves at supersonic speed (500km an hour)

Question 14

Mt. St. Helens

Answer 14

• Magma built up into volcano, earthquake happened, summit slid away and created a landslide, depressurization of magma -> giant explosion
• Explosivity: 8x all explosives in WW2
• Part of Cascadia subduction zone which is part of Paficic Ring of Fire (along with Mt Baker)
• Now there is a new dome growing in the crater

Question 15

Order of events generating felsic lava into a composite cone

Answer 15

• Migration of water into mantle above subduction zone
• Melting in the mantle above subduction zone
• Magma rising through mantle
• Underplating
• Partial melting of continental crust
• Generation of intermediate and felsic magmas
• Migration of magma towards the surface
• Eruption of viscous magma forming composite volcano

Question 16

calderas

Answer 16

• Large collapse depression (approx 1 km across) from aftermath of “super volcanoes” - hundreds to thousands of cubic km of pyroclastic material erupted, highly explosive
• Form from collapse of overlying landmass into the magma chamber
• Ex. Crater Lake, Oregon

Question 17

volcano explosivity index (VEI)

Answer 17

• Considers total volume of material erupted explosively
• Scale 1-8 (factor of 10 for each unit -> VEI 2 is 10x more explosive than VEI 1, and so on)
• Ex. Kilauea, HI – VEI 1
• Ex. Mt. St. Helens – VEI 5
• Ex. Krakatoa & Vesuvius – VEI 6
• Ex. Toba Lake Supervolcano in Indonesia – VEI 8

Question 18

toba lake supervolcano

Answer 18

Exploded 75,000 years ago – likely severely impacted global climate, may have almost wiped out humans, and may have reduced our genetic diversity by killing so many of us

Question 19

relationship between plate tectonics and volcanoes

Answer 19

Plate tectonics eplains volcanoes – volcanoes happen along boundary lines