Volcanoes

Question 1

Volcanoes on convergent plate boundaries

Answer 1

• continental + oceanic OR oceanic + oceanic
• usually explosive due to high pressure the magma is under
• composite volcanoes, made from ash + lava, are formed from these eruptions

Question 2

Convergent boundary (continental + oceanic)

Answer 2

• denser oceanic plate subducts below continental
• the plate subducting leaves a deep ocean trench
• fold mountains occur when sediment is pushed upwards during subduction
• oceanic crust melted as it subducts + the extra magma causes a build up of pressure
• pressurised magma forces through weak areas in the continental plate = explosive, high pressure volcanoes erupt through the continental plate (composite volcanoes)

Question 3

Convergent boundary (oceanic + oceanic)

Answer 3

• heavier plate subducts leaving an ocean trench —> fold mountains occur
• built up pressure causes underwater volcanoes bursting through the oceanic plate
• lava cools + creates new land = island arcs

Question 4

Volcanoes on divergent plate boundaries

Answer 4

• continental + continental OR oceanic + oceanic
• usually effusive as magma is under less pressure so the lava flows freely
• shield volcanoes, made from mainly lava, are formed from these eruptions

Question 5

Divergent boundary (continental + continental)

Answer 5

• any land in the middle of the separation is forced apart = rift valley
• volcanoes form where the magma rises
• eventually the gap will most likely fill with water + separate completely from the main island
• the lifted areas of rock are known as horsts, where as the valley is known as a graben

Question 6

Divergent boundary (oceanic + oceanic)

Answer 6

• magma rise between the gap left behind by the two plates separating = forms new land when cooled
• less explosive underwater volcanoes formed as magma rises
• new land forming on the ocean floor by lava filling the gaps = sea floor spreading

Question 7

Volcanoes on hotspots

Answer 7

• hotspots are areas of volcanic activity that are not related to plate boundaries
• hot magma plumes from the mantle rise + burn through weaker parts of the crust —> this can create volcanoes + islands
• the plume stays in the same place but plates continue to move, which sometimes causes a chain of islands (e.g. Hawaii)

Question 8

Hazards caused by volcanoes

Answer 8

• lava flows
• lahars
• mudflows
• glacial floods
• tephra
• toxic gases
• acid rain
• volcanic landslides
• pyroclastic flows

Question 9

Lava flows as a hazard

Answer 9

• lava can flow quickly or slowly depending on its viscosity
• silica makes lava viscous + slow, which is common in explosive eruptions

Question 10

Lahars as a hazard

Answer 10

• usually caused by melting ice at high altitudes

Question 11

Mudflows as a hazard

Answer 11

• different to lahars which are volcanic material, mudflows may be triggered by the violent shaking that an eruption brings, or meltwater from the volcanic heat

Question 12

Glacial floods as a hazard

Answer 12

• when temperatures are high from lava, glaciers or ice sheets quickly melt
• large amounts of water is discharged

Question 13

Tephra as a hazard

Answer 13

• any type of rock that is ejected by a volcano

Question 14

Toxic gases + acid rain as hazards

Answer 14

• toxic gases released during eruptions, including CO2 as it can replace oxygen as it is heavier
• acid rain is caused when gases such as sulphur dioxide are released into the atmosphere

Question 15

Volcanic landslides as a hazard

Answer 15

• high velocity flows of debris caused when the energy from the eruptions blow apart rocks + other material sending it down the volcanic slope

Question 16

Pyroclastic flows as a hazard

Answer 16

• clouds of burning hot ash + gas that collapses down a volcano at high speeds
• average speeds of 60 mph but can reach 430 mph

Question 17

How is the magnitude of a volcano measured?

Answer 17

• vulcanicity is measured using the Volcanic Explosivity index (VEI)
• the scale is logarithmic from VEI 2 + onwards
• features considered include how much tephra is erupted, how long it lasts, how high the tephra is ejected etc.
• intense high magnitude eruptions are explosive, calmer low magnitude eruptions are effusive

Question 18

How frequent are volcanic eruptions?

Answer 18

• frequency varies per volcano
• volcanoes are classed as either active, dormant, or extinct
• an estimated 50-60 volcanoes erupt each month
• usually a higher frequency eruptions are effusive, and low frequency ones are explosive

Question 19

How regular are volcanic eruptions?

Answer 19

• regular in that the eruptions on each type of boundary are similar e.g. eruptions on convergent boundaries will be regularly explosive
• sometimes eruptions may be irregular + not fit patterns

Question 20

How predictable are volcanic eruptions?

Answer 20

• regularity of eruptions can help estimate when eruptions will take place (e.g. every 10 years)
• seismic activity, gases releasing, elevation etc. can all indicate an imminent eruption
• but there is no definite predictions to a volcanic eruption

Question 21

Primary + secondary environmental hazards

Answer 21

• primary = ecosystems damaged through various volcanic hazards + wildlife killed
• secondary = water acidified by acid rain, volcanic gases contribute to greenhouse effect (global warming)

Question 22

Primary + secondary economic hazards

Answer 22

• primary = businesses + industries destroyed or disrupted
• secondary = jobs lost, profit from tourist industry

Question 23

Primary + secondary social hazards

Answer 23

• primary = people killed, homes destroyed from lava/pyroclastic flows
• secondary = fires can start which put lives at risk, mudflows or floods, homelessness

Question 24

Primary + secondary political hazards

Answer 24

• primary = govt buildings + other important areas destroyed or disrupted
• secondary = conflicts concerning govt response, food shortages, insurance etc.

Question 25

Lake Nyos 1986

Answer 25

• carbon dioxide had been rising from the old magma chamber into the lake, due to the depth + stillness the lake was in layers with the deepest layer being the most saturated with CO2
• a small earthquake caused the volcanic gas escaped from the lake, flowed down the valley into the nearest town
• speeds of up to 70km/hour + 50m thick
• 1700 people (+3000 cows) died

Question 26

Nevado del Ruiz (Colombia) 1985 CAUSES

Answer 26

• volcano with an amplitude of 5400m + covered with an icecap 30m thick covering 20km2 area
• 1985 = earthquake led to volcanic eruption that threw hot, pyroclastic material onto the ice cap causing it to melt
• condensing volcanic steam, ice melt + pyroclastic flows combined to form lahars that moved down the mountain to the village of Chinchina
• killed over 1800 people + destroyed the village

Question 27

Nevado del Ruiz (Colombia) 1985 SECONDARY HAZARDS

Answer 27

• conditions worsened + further eruptions melted more ice creating larger lahars that were capable of travelling further down the mountain into the floodplain of Rio Magdalena
• within an hour it had reached the city of amero (45km away)
• 22,000 out of 28,000 residents were crushed + suffocated beneath lahars up to 8m thick
• the volcanic eruption was relatively small but the presence of the icecap made the area especially hazardous