Volcanos Flashcards
Why do people live in areas of volcanism?
- Fertile soils
- Flat land, from old volcanic deposits
- Recreation, quality of life
- Often by coast … as are most cities
Extrusive volcanic rocks?
- Rhyolite
- Andesite
- Basalt
Convergent plate boundary volcanos?
Examples: o Andes o Cascades o Japan o Indonesia o Subduction related o Explosive • High viscosity • Pressure builds because of this • Bubbles can’t get out because its viscous • Flash of volume expansion • Explosions • Bubbles at depth, silicate melt with some gas o Or dome forming o Mainly andesite, dacite and rhyolite
o Bubbles
o Buoyancy drives eruption
o Bubbles nucleate and grow as magma rises
o Magma viscosity o Low viscosity (basalt, 102 Pa s) • Hawiaian – bubbles can get out – lava flow o High viscosity (rhyolite, 108 Pa s) • Rhyolitic explosion
o Fragmentation
o Stress due to growing bubbles exceeds strength of magma
o Magma fragments to form ash particles
o Buoyant eruption column
o Ash heats air, efficient entrainment
Divergent plate boundary volcanos?
• 90% volcanism is at ridges
• Constant composition
• Magma extrusion
• Instantly cools due to ocean
• MORB = Mid Ocean Ridge Basalt
• Extrusive - Pillows lavas
• Intrusive - Sheeted dykes
• Morphology of ridge is spreading rate dependent
• Slow > strong - steep – graben (MAR)
• Fast > weak – shallow (EPR)
• Shape of ridge is determine by spreading rate
Basaltic Volcanism:
• Mainly effusive (some explosive episodes)
• Aa, Pahoehoe flows, cinder cones, fissures
• Low viscosity > Fast flows > high volume
• Lava Lakes
Intraplate boundary volcanos?
• Examples
• Yellowstone
• Hawaii
• Two types of lava flow
o Pahoehoe
o A’a - faster
• Canaries
• Magma can interact with surfaces, becomes more rhyolitic
• Iceland – happens to be on spreading ridge – intraplate volcanism controlled by mantle plume
Mantle Plumes and Large Igenous Provinces
o Volcanic Chains (Hawaii)
o Kink = change in direction
o Flood basalts and Large Igenous bodies
o Plume melting temperatures at surface are higher than others
Viscosity in volcanos?
Viscosity is a function of: – Composition – Temperature Magma viscosity o Low viscosity (basalt, 102 Pa s) • Hawiaian – bubbles can get out – lava flow o High viscosity (rhyolite, 108 Pa s) • Rhyolitic explosion High viscosity • Pressure builds because of this • Bubbles can’t get out because its viscous • Flash of volume expansion • Explosions
Primary hazards?
– Lava – Pyroclastic flow (PDC) – Ash fall / tephra – Explosions / lateral blasts – Gas – Earthquakes – Flooding & jökulhlaup
Secondary hazards?
– Lahar – Landslide / debris avalanche / sector collapse – Ash remobilisation – Tsunami – Evacuation – Roof collapse – Acid rain
Hazard assessment questions?
We need to answer the following questions:
– When did the volcano last erupt?
– How frequently has the volcano erupted in the past?
These two tell us about the return period
– What sizes of eruptions have occurred in the past?
This tells us about magnitude
– What types of eruptions have occurred in the past?
This tells us how explosive the eruption will be
– How far have dangerous effects reached?
This tells us the likely area that will be impacted
Volcanic monitoring?
• Core methods
– Seismic
• Measure seismic waves produced by deformation through movement of gas/magma.
• As it rises it exerts pressure on surrounding rocks
• Earthquake activity beneath a volcano almost always increases before an eruption
– Ground deformation
• Upward pressure from rising magma deforms volcano
• The ground can change shape by rising up, subsiding, tilting, or forming bulges
• Supportive methods
– Microgravity, electrical & magnetic studies
– Geochemical monitoring (gas and water)
Probabalistic event trees?
A graphical, tree-like representation of events - branches are logical steps from an event through increasingly specific subsequent events (intermediate outcomes) to final outcomes.
– Focus on the range of outcomes that could result from different combinations or sequences of events.
– Calculate the resultant probabilities of different outcomes
– Can be solved probabilistically, using discrete probability values and/or probability distributions
Warning messages?
– What is threatening?
– Exactly what geographic area is threatened?
– When is the anticipated impact?
– How probable is the event?
– Which routes are to be followed when leaving the area?
– Who is issuing the warning?
– Where can shelter be found?
