2.Nature of volcanic events and features

Question 1

Factors

Answer 1

Type of plate boundary
Nature of magma:
Viscosity- Silica, gas and water content
Magma type- Basaltic, Andesitic, Rhyolitic
Explosivity- Volcanic Explosivity Index

Question 2

Viscosity

Answer 2

Low viscosity= runny magma= shallow sided volcano
High viscosity= sticky thick magma= steep sided volcano
High viscosity causes more explosive volcanoes as gases can’t escape

Question 3

Viscosity (temp)

Answer 3

Higher temperature-> magma more runny

Eg basalt, very runny and buoyant so don’t block up the vent

Question 4

Viscosity (silica)

Answer 4

Higher silica content= thicker magma
Eg Rhyolite= low temps and high silica contents= very thick and sticky= block main vent
Trap gases which build up to produce more explosive eruptions

Question 5

Viscosity (volume of dissolved gases)

Answer 5

Higher dissolved gas content= more runny and buoyant the magma

Question 6

Examples of viscosity

Answer 6

SHIELD VOLCANO-> higher temp= more gases stay dissolved in magma= more runny the magma= gases escape easily= no built up pressure= less violent

This is reversed for composite volcanoes

Question 7

Types of magma

Answer 7

Basaltic
Rhyolitic
Andesitic

Question 8

Balsatic

Answer 8

Hot, low viscosity, easy release of gases, non violent eruptions
Runny lava, low plume of gases, unlikely to penetrate higher than the lower atosphere
Lava spreads far from volcano and cools slowly, volcanic cones are low and wide based
SHIELD VOLCANOES

Question 9

Andesitic

Answer 9

High eruption columns spread gases over large distance
Acidic lava
As eruption decreases it may cause a collapse to the crater which causes pyrostatic flow
Affects regional climate
COMPOSITE VOLCANOES

Question 10

Rhyolitic

Answer 10

Thick viscous magma 
Cools in main vent, plugging it and trapping gases so large build up 
Release is sudden and violent 
Acidic lava 
COMPOSITE EG YELLOWSTONE

Question 11

Magma types based on geological location

Answer 11

Balsatic lavas are formed by melting of oceanic crust and mantle- constructive margins and hotspots

Andesitic are formed by rising balsatic magma mixing with continental crust before reaching surfaces violent eruptions -> largely formed at subduction zones

Rhyolitic lavas with high silica are formed from melting of continental crust so form at destructive boundaries

Question 12

Magnitude/ explosivity

Answer 12

Measured using the volcanic explosivity index
0 non explosive 8 very large
Observed based on: eruption rate (how fast), volume of erupted material, height of eruption column, duration of continuous blasts in hours