Volcanoes and Hazards

Question 1

Where do volcanoes occur?

Answer 1

Mid-ocean ridges
Fixed mantle ‘hotspots’ beneath a moving plate (Intra plate) e.g. Yellowstone, Hawaii
Subduction zones (granitic or andesite magma -containing water) e.g. aleutians, cascades etc.

Question 2

what are different eruptive styles of volcanoes?

Answer 2

flow vs blow

Question 3

what are the characteristics of basalt (mafic) magma?

Answer 3

low in silica, low viscosity, low shear strength -> flows easily and gases can escape easily

Question 4

What type of magma do Hawaiian Style eruptions have?

Answer 4

basalt magma

e.g. Kilauea

Question 5

what are the characteristics of Hawaiian Style eruptions?

Answer 5

Mafic lava flows easily -> thin basalt flows over a broad area -> Can only build up low angle slopes -> Shield Volcano

Fluid basaltic lava effuses or is ejected in a ‘fire fountain’

e.g. ‘Big Island’ Hawaii

Question 6

What type of magma do Strombolian eruptions have?

Answer 6

basalt or andesite magma (mafic/intermediate)

e.g. Stromboli, Italy

Question 7

what are the characteristics of Strombolian eruptions?

Answer 7

mildly explosive
lava bombs and smaller particles ejected up to 10s to 100s of metres into the air
Strombolian eruptions over weeks to months form a cinder cone e.g. Eve cone, Tseax cone, BC

Question 8

what are the characteristics of andesite and rhyolite (intermediate/felsic) magmas?

Answer 8

Intermediate to felsic: high silica content
More dissolved water
More ‘polymerized’
High viscosity -> flow is impeded -> gases trapped

Question 9

What type of magma do plinian eruptions have?

Answer 9

andesite and rhyolite (intermediate/felsic)

e.g. mt st helens

Question 10

what are the characteristics of plinian eruptions?

Answer 10

Andesite & Rhyolite magmas
Viscous, strong, contain more dissolved H2O and other gases -> Form bubbles as magma rises
Degas -> explosive -> magma fragments -> ash
Pyroclastics – hot particles or ‘clasts’
Plinian eruptions and lava flows -> Stratovolcano (steep sides: higher viscosity lava and tephra)

Mt St Helens 1980, Lassen Peak CA 1915

Question 11

How big are Mauna Loa and Mt St Helens?

Answer 11

Mauna Loa: around 200km wide, 4169m tall

Mt St Helens: around 6km wide, 2550m tall

Question 12

What are some examples of volcanic (extrusive) products?

Answer 12

Generally small crystals (or glass), vesicles (gas holes), lava or broken fragments.

Lava–A'a flows (more jagged/rough) and Pahoehoe flows (more smooth)
Gases: H2O, CO2, N2, H2S
Pyroclastics/Tephra
–Ash (fine)
–Scoria (mafic)
–Pumice (felsic)
–Blocks (solid)
–Bombs (molten when ejected)

Question 13

What are the benefits of volcanoes?

Answer 13

Agricultural: volcanic material -> rich soil
Geothermal: electricity production, hot water, spas (health, tourism)
Resources: volcanoes often rich sources of minerals (e.g. gold, silver, copper, S)
Tourism and Recreation

Question 14

What are the 5 volcanic hazards?

Answer 14

(1) Lava flows
(2) Gases
(3) Airfall (ash/tephra)
(4) Pyroclastic flows
(5) Lahars (volcanic mudflows)

Question 15

what is a case study for destructive lava flows?

Answer 15

June 1990 eruption, Hawaii

Question 16

what is a case study for destructive gases?

Answer 16

Tseax Cone, northern BC

Eruption of gases with lava flow ~300 yrs ago killed > 2000 people

Question 17

what are some of the hazards associated with Airfall (ash/tephra)?

Answer 17

Ash -> Health hazard
Airborne ash in atmosphere -> Risks to air traffic
Ash in stratosphere -> Effect on global climate (temporary)
Ash fall -> Risk of building collapse

Question 18

what is a case study for the spread of ash/tephra?

Answer 18

Mt Mazama ash fall –7700 years ago (Crater Lake Oregon). Found in lake sediments throughout Pacific Northwest (as far as Edmonton 1450km away).

Question 19

what are pyroclastic flows?

Answer 19

Ground-hugging currents of gases, ash, fragments

Question 20

what temp. are pyroclastic flows?

Answer 20

Hot: > 400 degrees C

Question 21

what speed are pyroclastic flows?

Answer 21

Fast: 100’s km/hr

Question 22

what is a case study for the hazards of pyroclastic flows?

Answer 22

AD 79:Pompeii destroyed (mt Vesuvius)

Mt Pelee, Martinique 1902: St Pierre

Question 23

what are lahars?

Answer 23

(volcanic mud flows)

snow/ice melt and mix with ash

Question 24

what is a case study for the hazards of lahars?

Answer 24

nov 1985 Nevado del Ruiz, Colombia

23,000 dead

Question 25

what is an example of an indirect hazard from eruption?

Answer 25

eruption triggered tsunamis

e. g.
- Krakatoa, 1883, >40 m tsunami, ~36,000 fatalities
- Anak Krakatau, Dec 2018 ~13m tsunami, >400 fatalities

Question 26

How can we reduce the impact of volcanic hazards?

Answer 26

(1) Hazard assessment
- Use the Geological Record: Style of eruptive activity & related hazards, Frequency of eruptions
(2) Mitigation
- Hazard maps, evacuation routes
- Community planning, education
(3) Geophysical & geochemical monitoring -> Warning

Question 27

what are some examples of mitigation?

Answer 27

Diversion
Cooling (futile)
Education
Evacuation
Risk  e.g. Lahar and debris flow hazard map

Question 28

what are some things you could observe about a volcano which indicate an imminent eruption?

Answer 28

Swelling
Rumbling (earthquakes)
Degassing