Volacanos

Question 1

What are volacanoes

Answer 1

• These are ruptures in Earth’s surface that allow molten rock (magma) and gases (e.g. silica rich means more violent) to escape to the surface
• May be constructive (adding material to earth surface) and destructive (destroy infrastructure)
• Large eruptions effect:
  1. Gas composition in atmosphere (add C02)
  2. Biosphere- kills organisms
  3. Hydrosphere- may cause tsunami

Question 2

Plate tectonics

Answer 2

• Divergent Margin- ocean ridge and rift volcano formation. Tend to be rich in iron and Mg-> basalt
• Intraplate- hot spot volcanoes which produce basalt
• Convergent margin- subduction volcano production. These are silica rich (as continental material) leading to more explosive volcanoes

Question 3

Volcano Products

Answer 3

• Eruption cloud= Ash and tephra

- Lahar= Rocks + ash + water + debris

Question 4

Volcanic Activity

Answer 4

• Active Volcano: Erupted in historic time- Rangitoto
• Dormant Volcano: One that has not erupted in history but may in future
• Inactive/extinct volcano: Not erupted in history and not expected to erupt again

Question 5

Flood Basalts

Answer 5

• Form due to gigantic volcanic eruptions of low viscosity lava
• Generally erupt due to continental rifting
• Form large igneous provinces
• Cause of mass extinction events- e.g. Dinosaurs

Question 6

Shield Volcanoes

Answer 6

• Formed from low viscous mafic lava- e.g. basalt
• Very gentle slopes (due to low viscosity)
• Build up over successive eruptions and each eruption event is long lived and near continuous
• Effusive rather than explosive eruptions
• E.g. Rangitoto

Question 7

Cinder Cones

Answer 7

• Cinder/scoria cone is steep conical shaped hill of volcanic debris that accumulate around a volcanic event
• Rock fragments=scoria= form as magma explodes into air and cools quickly
• Effusive explosion type of low viscosity magma-basaltic type of eruptions
• Simplest volcano form
• Bowl shaped crater at summit
• Found on flanks of shield, composite and caldera volcano
• Gases may continue to vent through cone long after eruption. Steam oxidises iron in scoria-> turning into red colour
• E.g. Mount Eden

Question 8

Composite Volacano

Answer 8

• Tall conical volcanoes built from different layers of volcanic material
• Explosive eruptions (increased silica)
• Form from felsic magma (rhyolite, dacite and andesite are associated rock types)
• Felsic magma is more viscous (increased pressure), so these volcanoes tend to not produce extensive lava flows
• Ring plain= Deposition of volcanic material around base tends to be present
• E.g. Mt Helens
• Natural discontinuities form- ash deposits, pyroclastic deposits etc

Question 9

Volcanic Domes

Answer 9

• Readily formed from high viscous lava (high silica content so doesn’t readily flow)
• Either basaltic or rhyolitic
• Form after main eruption
• Unpredictable growth and undergo growth, collapse, solidification and erosion
• Angular pieces of dome breaks off (as it grows from repeated eruption). Termed breccia and accumulate around base
• Pressure build up from trapped gases may lead to explosive eruption
• E.g. Mount Tarawera or rhyolitic dome (Pohaturoa rock)

Question 10

Caldera

Answer 10

• Forms due to collapse of volcano after eruption
• Caused by:
  i. Sudden emptying of magma chamber during explosive eruption-> causing collapse of edifice through formation of ring fault
  ii. Gradual emptying of magma associated with shield (low viscosity) volcanoes-> causes edifice
• Emptied chamber cannot support edifice weight
• May cause global temperature drops
• E.g. Lake Taupo or Rotorua volcano

Question 11

Eruption Types

Answer 11

1. Hawaiian: Low viscous lava without much gas
2. Pinian: Large gas volumes and pumice and ash
3. Strombolian: Mixed amount of gases/pyroclastic material with some lava
4. Hydrovolcanic: Highly explosive eruption when hot rising magma comes in contact with water in ground and vaporises it. Gas expansion leads to massive explosion

Question 12

Lava Behaviour

Answer 12

• Lava viscosity will determine how it behaves
• Physical and chemical properties determine the type of lava extruded
• Highly viscous lava (rhyolite, dacite, andecite, trachyte) all show following types of behaviour:
  i. Flow slowly and form semi-rigid blocks, which resist flow
  ii. Trap gases which form vesicles in rock
• High viscous lavas do not flow as a liquid and tend to form tephra or ash deposits
• Low viscous lava (basalt) all show following types of behaviour:
  iii. Flow easily and form channels of molten rock
  iv. Release gases easily as they are formed
  v. Volcanoes tend to be shield type rather than steep cone
• High viscous lavas do not flow as a liquid and tend to form tephra or ash deposits
• Lava tubes may form

Question 13

Pyroclastic Material

Answer 13

• These materials comprise fragments of rock formed by explosive volcanism
• As magma rises, trapped gases expand in viscous magma which causes high pressure explosions
• Classified as airfall deposits and are distinguished by particle size and strength

Question 14

Ignimbrite

Answer 14

• Name given to material deposited as pyroclastic flow
• Produced by explosive eruptions
• Ignimbrite deposits= Volcanic ash (tuff= lithified ash), pumics and lithics
• May be loose & unconsolidated or welded due to heat