Introducing Volcanic Hazards

Question 1

Where are the volcanoes found?

Answer 1

. Most found at plate margins (constructive and destructive)
. This is either the result of subduction which causes pressure melting of past crust which rises to the surface or because at constructive margins the pressure where the plates move apart is low so the magma rises.

Question 2

Why are some found in the sea (mantle plumes)

Answer 2

In areas such as Hawaii mantle plumes (areas of rising hot magma) cause melting of material in the lithosphere which rises through fissures and causes volcanoes

Question 3

Eruption styles: Icelandic

Answer 3

. Low viscosity and highly effusive
. May have water therefore phreatic
. Lava flows gently from fissures
. Basaltic lava

Question 4

Eruption styles: Hawaiian

Answer 4

. Effusive, minor explosivity, with fire fountain and low viscosity lava flow
. Lava flows gently from a central vent
. Basaltic lava

Question 5

Eruption styles: Strombolian

Answer 5

. Eruption with gas bubbles so eject tephra (ash & lapilli)
. Frequent explosions of tephra and steam
. Occasional, short lava flows
. Thicker basaltic magma

Question 6

Eruption styles: Vulcanian

Answer 6

. Higher gas build up than Strombolian highly viscous lava
. A series of short lived explosions, with tephra including bombs, ash and gases
. Thicker basaltic, andestic and rhyolitic magma

Question 7

Eruption styles: Pelean

Answer 7

. Explosive eruptions with range of tephra with nuee ardent (glowing cloud) pyroclastic flows
. Andesitic and rhyolitic magma

Question 8

Eruption styles: Plinian

Answer 8

. Highly explosive, large eruption column (up to 45km) range of tephra and pyroclastic fallout, including flows
. Torrential rainstorms cause Lahars
. Rhyolitic magma

Question 9

Shield volcanoes

Answer 9

• Gentle slopes and a wide base
• Frequent eruptions of basic lava
• Lava flows at high speeds and for long distances before cooling
• Usually non-violent eruptions
• Non-viscous lava due to basaltic composition
• Found at constructive boundaries
Examples - Mauna Loa & Kilauea

Question 10

Composite Volcanoes

Answer 10

• Steep sided with a distinctive cone shape
• High with a narrow base
• Explosive eruptions of lava & ash
• Layers of alternating ash & lava
• Viscous lava builds in layers and does not flow far from the crater
• Found at destructive boundaries
• Secondary or parasitic cones formed when the main vent gets blocked
Examples - Montserrat, Etna & Mt St Helens

Question 11

Constructive plate margins

Answer 11

. Plates move apart, magma rises up through middle’
. Basaltic magma
. Lava: low in silica, low viscosity (very fluid and runny)
. Little violence in eruption - gases escape easily
. Materials erupted - mainly lava
. Regular eruption time and can be continuous
. Broad base and gentle side and lava only
. Lava plateaus and shield volcanoes

Question 12

Destructive plate margins

Answer 12

. Continental crust and oceanic plate moving towards each other with a subduction zone, over the mantle
. Acidic magma
. Lava: rich in silica, very viscous (slow flowing)
. Potentially explosive eruptions - lava shatters into pieces
. Materials erupted - lava bombs, ash, dust
. Eruptions are from time to time long dormant periods
. Classic cone shape with columns of gas + finer fragments
. Acid lava dome, composite cones

Question 13

Which is the most hazardous volcano?

Answer 13

. Composite
. Explosive eruptions of lava and ash
. Steep sided with non-basaltic magma (very runny)
. Shield volcanoes have gentle slopes and gases escape easily meaning little violence

Question 14

Magma viscosity

Answer 14

. How much a liquid resists flow, how thick it is - result of silica
. More silica = thicker magma - more viscous
. Less silica = thinner magma - less viscous
. High viscosity prevents the gases dissolved in magma from easily escaping - more silica
. Trapped gases = magma foam = more explosive eruption

Question 15

VEI

Answer 15

. The volcano explosivity index
. A scale used to measure the intensity of volcano eruptions.
. Similar to the Richter and Mercalli scales, which measure the intensity of earthquakes
. VEI measures explosivity, volume of ash (tephra), and the height ash reaches into the atmosphere.
. Gives a description of the eruption, a name for the eruption, and the total number of those eruptions in human history
. Allows for comparisons between events to be made
. 0 (non-explosive) - 8 (mega-colossal)
. Bigger frequency, lower magnitude