Volcano

What is a Volcano?

A volcano is “a vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure, usually conical, that is produced by the ejected material.”

Magma Sources and Types

• Recall: lava is magma (i.e., molten rock) that flows on the earth’s surface

• Temperature should be high enough and pressure low enough to melt rock

  • Most magma originates in upper mantle (50-250 km deep in Earth)

• Most volcanoes found in one of these 3 settings:

  1. Divergent boundary – oceanic and continental
  2. Convergent boundary – above subduction zones
  3. Hot spots (i.e., not associated with plate boundary)

• Magma composition and physical properties dictate how the volcano erupts

• Composition determined by the rock material that is melted and extent of melting

• Raw materials and melting processes controlled by the tectonic setting

$\to$ See Magma

Styles (and Locations) of Volcanic Activity

• Fissure Eruptions
• Hot spots
• Shield Volcanoes
• Cinder Cones and Pyroclastics
• Composite Volcanoes

Fissure Eruptions

• Eruption of magma from a crack in the lithosphere

• Continental examples are often huge in area

  • E.g., Columbia Plateau of USA
  • E.g., in India and Brazil
  • Mostly dormant/extinct

• Also, in oceans at spreading centers

• ~70 000 km of submerged magmatic fissures

• Most volcanic (igneous) rock formed at spreading centers

Hot Spots

• In the middle of a plate, but not clear on why they occur

• Could be heat from core/mantle boundary or radioactive hot spots Examples include

• Hawaiian Islands

• Galapagos Islands

• Yellowstone National Park, USA

• Majority of volcanoes we are familiar with lie along plate boundaries

Shield Volcanoes

• Mafic lavas (low viscosity) flow easily and far; Not typically eruptive
• Many long and steady lava flows create a broad, shieldlike shape – large area and relatively flat volcanoes
  • E.g., Hawaiian Islands

Cinder Cones and Pyroclastics

• Cinder cones formed by lava that cools into a solid while being ejected ⇒ pyroclastics (bits of erupted volcanic material)
• Eruptions happen because of gases trapped in the lava (more likely from thick felsic lava)
• Cinder cones: Small area; larger height relative to area; fairlly symmetrical

Composite Volcanoes

• Built by multiple layers of material (lava flows and pyroclastics)
• Intermediate type lavas that can flow and trap gas
• World’s most dangerous volcanoes are of this kind
• Larger than cinder cones but smaller than shield

• Lava domes formed from slow-flowing rhyolitic (or andesitic) lavas oozing out of vent and staying close by to cool
• Often formed in statovolcano craters after eruption
• Indicative of explosions and possibility for more

Hazards related to volcanoes and prevention

• Lava
• Pyroclastics
• Pyroclastic flow
• Lahards
• Toxic gases
• Steam explosions
• Landslides / Collapse
• Secondary effects

Lava

• Not most dangerous to life ⇒ slow moving (< few km/h)
• But is to property ⇒ hot (>850 $degrees$) so cause fire, melting or turns to rock
• Prevention: Don’t build near, but often fertile land
• Prevention: Cool down the lava - Heimaey, Iceland, 1973
• Prevention: Explosives to divert lava flow - Mt. Etna, Italy, 1983; Hawaii, 1935

Pyroclasitcs

• Sudden, explosive eruptions that ca spread fast and far
• Range of sizes: Large drop closer to vent; Ash can go 100 - 1000s km
• Recall more felsic lavas are thicker and have more trapped gases

Major hazards due to pyroclastics:

• Reduction in air quality
• Burying of structures
• Disruption to air travel

Pyroclastic Flows

• Mixture of hot gases and fine ash that is much denser than air, very hot (>1000°C), and flows very fast down slopes (>100 kph)
• How it forms: Above volcano hot gas lifts ash and magma droplets to mix with the air to form a cloud
• When too dense the cloud collapses and flows down the slope
• Can also form within in a crater and spill over the sides and down slope
• Prevention: Not be in its path! If volcano is mildly erupting and has slightest possibility of a pyroclastic flow, then leave.

Mont Pelée, Martinique, 1902

• Had been erupting for weeks but not posing danger
• May 8, 1902 – pyroclastic flow leveled the town of St.Pierre and killed up to 40,000 people
• Sole survivor was prisoner in a dungeon jail

Lahars

• Ash and water combine to create a fast-moving (sometimes hot) volcanic mudflow that clogs rivers and destroys much in its path
• Source of water: rain or glacier / snow melt
• Mount Pinatubo, Philippines, 1991
• Flooding may be more common after the lahar as rivers / streams remain clogged.

Toxic Gases

• Water vapor and Carbon dioxide - largerst by volume; not toxic; CO2 can suffocate animals, plants, people
• Carbon monoxide, various sulfur gases, hydrochloric acid, fluorine - poisonous
• Released directly or from magma chambers into lakes, soils, basements

Steam Explosions

• Phreatic eruption – large quantities of water (usually seawater) seep down through rock near to hot magma which turns water into steam and boils up out of a volcano

• Krakatoa, Indonesia, 1883

• Force of 100 million tons dynamite

• Heard 3000 km away

• Dust 80 km into atmosphere

• Ash found in area of 750,000 km2

• Tsunami 40 m high killed 36, 000

Landslides and Collapse

• Volcanoes can become weak and unstable due to weathering
• Result: Landslides or collapses can happen
• Removal of material may lead to an eruption (e.g., Mount St. Helens)
• Seaside collapses/landslides can cause tsuamis (e.g., Mt. Mayuyama, Japan, 1792 killed 15,000 people) Concern that past slides around island of Hawaii are indicative of more slides to come

Secondary effects: Climate and Atmospheric Chemistry

• Dust in atmosphere from large explosions can last months to years

  • Global temperatures dropped by 0.5°C after 1883 Krakatoa eruption
  • 1816 ‘year without a summer’ after Tambora (Indonesia) 1815 eruption

• Sulfuric-rich gases can lead to acid rain

• Sulfur dioxide (SO2) mist can decrease incoming solar radiation

  • Mount Pinatubo, Philippines, 1991 – average global temperature drop of 0.5C
  • Mist had spread worldwide, and USA had unusually cold summer in 1992

Issues in Predicting Volcanic Eruptions

• Chaitén (Chile) erupted in 2008 after nothing for 9400 years
• Pyroclastic flows destroyed the village of Chaitén and was not rebuilt

Volcanic Explosivity Index (VEI)

• Characterize relative size of explosive eruption based on
  • Volume of pyroclastics
  • How high they rose into atmosphere
  • Length of eruption
• Useful for predicting future hazards at a given volcano
• Thought it could be used to predict impact on climate, but SO2 is a key factor

Volcanic Eruption Precursors

• Seismic activity due to volcano
  • Or earthquake opens fissures of dormant volcanoes
• Harmonic tremors – continuous, rhythmic
  • Mechanism not known
• Bulging, tilt, or uplift of volcano surface
  • Eruption timing depends on overlying rocks
• Changing mix of gases emitted – SO2 promising
• Remote sensing of surface temperature may reveal rising magma
• Lives have been saved because of precursors (evacuation)
  • Evacuated 80,000 people before Mount Pinatubo’s big eruption

Canada’s Volcanoes

• 5 active regions in British Columbia and Yukon

• 49 eruptions during the last 10,000 years

• Most recent was 150 years ago at Lava Fork, northwestern B.C.

• Last big explosive eruption was 2350 years ago at Mount Meager (ash later seen in geology)

• Ash layers in Alberta from various volcanic eruptions

• Tiny pieces of igneous glass and pumice

• Mount Mazama - one of the largest eruptions in the last 10,000 years

• Destroyed volcano and left behind Crater Lake in Oregon

• Ash found as far away as Greenland