volcanoes

Question 1

what is a shield volcano?

Answer 1

• basalt magma
• runny lava: creates flatter slope as takes longer to cool
• low silica and fewer gases
• found at constructive margins: mid ocean ridges etc
• generally low risk eruptions

Question 2

what is a composite volcano?

Answer 2

• andesite magma
• viscous lava (slow): creates steep sides as takes little time to cool
• high silica and pyroclastic flows (from gases at subduction zone)
• found at convergent margins: subduction zones
• violent, explosive eruptions

Question 3

what are hotspots?

Answer 3

thinning of earths crust where magma pushes through forming shield volcanoes (mantle plume)
hotspot remains STATIONARY- it is the tectonic plates that move meaning volcanoes are formed and then move away.

Question 4

measuring volcanic eruptions

Answer 4

to describe and compare the size or magnitude of volcanic eruptions, the Volcanic Explosivity Index is used. The VEI uses several factors:

• amount and height of volcanic material ejected (ash fall etc.)
• how long the eruption lasts
• qualitive descriptive terms (gentle, explosive etc)

These are then used to assign a number o the eruption. The VEI uses a scale from 0- 8 (each number increases by a factor of ten: 3= 0.1km, 4= 1km and so on)

Question 5

hazards from volcanic eruptions?

Answer 5

1. primary
   - lava flows
   - pyroclastic flows
   - tephra and ash fall
   - gas
2. secondary
   • lahars/ landslides
   • jokulhlaup

Question 6

what are lava and pyroclastic flows?

Answer 6

1. lava flows:
   - streams of lava erupting down sides of volcano
   - can reach up to 1170 degrees
   - move extremely slowly so not a threat to life but destroy everything in their path
2. pyroclastic flows
   - mixture of dense hot rock, lava, ash &gas
   - can reach up to 700 degrees
   - move extremely fast (100km an hour) so huge threat to life

Question 7

what are tephra and ash falls?

Answer 7

1. tephra
   - pieces of volcanic rock that blast into air
   - larger pieces can cause injury or death
   - smaller pieces can travel far
2. ash
   - very disruptive, covers everything
   - causes poor visibility, roofs may collapse under weight etc

Question 8

what is gas?

Answer 8

magma contains dissolved gases that are released into the atmosphere during eruption.
volcanic gases include water vapour (80%) CO2 and sulphur dioxide and once in the air can travel for thousands of km.
eg in 1986 Lake Nyos case study! killed 1700.

Question 9

what are lahars and jokulhlurps?

Answer 9

1. lahars
   - masses of rock and mud that travel quickly down sides of volcano that vary in size and speed
   - largest are 100s of metres wide and runs very fast
   - caused when eruption melts snow/ ice or heavy rainfall during/ after eruption erodes loose rocks/ soil
2. jokulhlaup
   - heat of eruption can melt snow and ice in a glacier causing heavy and sudden floods
   - can be very dangerous- can release when people are unaware

Question 10

predicting eruptions

Answer 10

unlike earthquakes, volcanoes can often be predicted with some accuracy. using equipment that is placed on the volcano (as well as remote technology: GPS etc) volcanos can be monitored for signs that it might erupt:

• small earthquakes (magma rises to surface: breaks the rock creating earthquakes). scientists can detect on seismograms
• changes to surface (magma pushes upwards, building pressure causing surface to swell). GPS measures this
• changes to volcanoes tilt (magama moves inside volcano: changing slope angle/ tilt). tiltmeters are used to measure this
• increase in gas emissions (magma coming closer)

example of successful prediction!! Popcatepeti (mexico) erupted in 2000 but successful studying of long period events meant that 30,000 people were able to be evactuated in time before the eruption.

however can be inaccurate for the exact time of eruption.
long period event: pressure slowly building before eruption