Volcanoes and volcanic hazards

Question 1

Volcanism on the earth

Answer 1

• island arcs
• mid-ocean ridges
• oceanic intra-plate islands
• subduction zone volcanoes
• rift zone

Question 2

Melting of mantle material

Answer 2

• lower the pressure by moving mantle material upwards
• change the solidus location (add water)
• raise the temperature (plumes melting the crust)

Question 3

complexities of melting rocks: water

Answer 3

• adding water changes the melting point. water breaks the Si-O bongs
• descending slabs loose volatiles
• from hydrated minerals and decomposition of limestone
• causes mantle melting, leads to island arcs

Question 4

Under continents

Answer 4

• rising basaltic melt encounters continental crust
• thick crust: basalt don’t reach the surface (Andes) basalt under plates the crust and heats the continental rock
• melting produces felsic magma which is buoyant and rises to form shallower magma chambers

Question 5

Complexities of transporting melt

Answer 5

• mantle melt forms at crystal junctions where there is high surface energy
• <60o required for permeability
• less dense liquid flows upwards through the permeable mantle

Question 6

Complexities of the magma chamber

Answer 6

• differentiation occurs within magma chambers
• minerals condense and fall to the floor forming cumulates. this follows Bowens reaction series
• melts become more felsic as crystals accumulate
• volatiles no longer kept in solution (H2O and CO2 start to build up pressure in the chamber)
• pressure can force out magma eruptions

Question 7

Processes in the magma chamber

Answer 7

• Formation of bubbles is very important! Reduces magma density - rises
• Increases viscosity. Less water in the melt - allows silica to polymerize.
• Expanding bubbles cool magma
• Emptying magma chamber decompression.
• More volatiles degassed – faster ascent etc…
• Runaway effect until the magma chamber empties.
• Magma shredded by exploding bubbles if volatile content is very high.
• If the viscosity is very high and bubbles can’t escape it

Question 8

Why do some volcanoes erupt explosively?

Answer 8

• addition of water at subduction zones lowers the melting point and adds vapour to the magma

Question 9

Types of volcano

Answer 9

• Fissure
• Shield
• Dome
• Ash-cinder
• Composite
• Caldera

Question 10

Type of eruption

Answer 10

• Hawaiian
• Strombolian
• Sub-Plinian
• Vulcanian
• Surtseyan
• Phreatoplinian
• Ultraplinian and plinian

Question 11

Volcanic Explosively index

Answer 11

• is a relative measure of the explosiveness of volcanic eruptions
• 0 to 8 (Yellowstone caldera)

Question 12

Volcanic eruptions: death

Answer 12

• volcanoes cause fewer fatalities than earthquakes, hurricanes and famine
• pyroclastic flow cause most mortalities over 50%

Question 13

Laki: Iceland volcano

Answer 13

• around 10 eruptions from 1783-1784
• poisoning by hydrogen fluoride (8 million tons released)
• subsequent famine due to crop failure and animal deaths(20-25% of population died)
• 80% of sheep, 50% of cattle and 50% of horses died
• thick sulphur dioxide haze over Europe
• very cold winter (28 days of continuous frost in Hampshire)

Question 14

Decade volcanoes

Answer 14

• large, destructive volcanoes with a past history of large eruptions that are centred near to areas of high populations

Question 15

Repose time

Answer 15

the time interval between successive eruptions. it can range from minutes to thousands of years

Question 16

Active

Answer 16

volcanoes known to have erupted during historical times

Question 17

Dormant

Answer 17

volcanoes that have not erupted during historical times, but will probably erupt again

Question 18

Extinct

Answer 18

volcanoes that are unlikely to erupt again

Question 19

Monitoring

Answer 19

• tiltmeters can tell you when new material enters the magma chamber
• presence of earthquakes in relation to deformation
• often it is a combination of events that forewarns of an eruption