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Questions to think about
• Which is the most dangerous volcano in the world?
• What group of people are most endangered by volcanoes
Summary
• What is a volcano?
• Where does lava come from and why do volcanoes erupt?
• How is size quantified?
• Where are volcanoes occur?
• How frequently do volcanoes erupt?
• How do volcanoes cause damage?
But I have a semantic problem here …
• But I don’t like the word ‘mountain’, because sometimes lava erupts from places that have little, if any, topographic relief
• And I don’t like the definition leaving out the notion of temporal persistence, that the eruptive activity is long-lasting
So maybe I should use the phrase …
Volcanic Center
• A region of the earth in which lava and hot gasses have persistently erupted from the ground over many thousands, or even millions, of years.
• A single volcanic center may include several related and closely-spaced volcanoes, or it may contain none.
What is Magma ?
• Molten rock with in earth– Chemical composition matters!
Low silica rocks (e.g. basalts) very fluid
High silica rocks (e.g. rhyolites) very viscous
– Concentration of gasses (main H20 and CO2) matters!
Gasses under pressure cause explosions
Gasses are poisonous
Volcano Size
The Volcanic Explosivity Index, or VEI, was proposed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions. It is a 0-to-8 index of increasing explosivity. Each increase in number represents an increase around a factor of ten. The VEI uses several factors to assign a number, including volume of erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), height of eruption column, duration in hours, and qualitative descriptive terms.
VEI
Description
Plume Height
Volume Classification How often Example
0non-explosive
< 100 m 1000s m3 Hawaiian daily Kilauea
1 gentle 100-1000 m 10,000s m3Haw/Strombolian
daily Stromboli
2 explosive 1-5 km1,000,000s m3
Strom/Vulcanian
weekly Galeras, 1992
3 severe 3-15 km10,000,000s m3
Vulcanian yearly Ruiz, 1985
4 cataclysmic 10-25 km100,000,000s m3
Vulc/Plinian 10's of yearsGalunggung, 1982
5 paroxysmal >25 km 1 km3 Plinian 100's of yearsSt. Helens, 1981
6 colossal >25 km 10s km3Plin/Ultra-Plinian
100's of years Krakatau, 1883
7super-colossal
>25 km 100s km3 Ultra-Plinian1000's of years
Tambora, 1815
8mega-colossal
>25 km 1,000s km3 Ultra-Plinian10,000's of years
Yellowstone, 2 Ma
How long do eruptions last?
• Most eruptions last 10 - 1000 days
• Less than 20% over within 72 hours
• Median is 7 weeks (1176 hours)
0.1 1 10 100 100010000
Duration (days)
Eru
ptio
ns
600
1000
200
How Often do Eruptions Occur?
Eruptions obey Gutenberg-RichterStatistics: lots of small ones, justa few big ones.
The graph at the left are forExplosive eruptions in KamchatkaDuring the last 10,000 years
Cum
ulat
ive
num
ber
of e
rupt
ions
With
eru
ptiv
e vo
lum
es e
qual
to
or Gre
ater
tha
n th
e gi
ven
amou
nt
Divergent Plate Boundaries
Mid-ocean ridges
strong mantle upwelling to ‘fill in hole’, so lots of basaltic magmatism (almost all of which is under water)
Continental Rifting
Thicker crust suppresses mantle melting somewhat, but can add volatiles
Convergent Plate Boundaries
Subduction Zonesdewatering of subducted lithosphere (string at 100-150 km depth) suppresses the melting point of mantle rock to produce basaltic magma
Continental CollisionStrong thickening of crust can cause once-shallow, water-rich crustal rocks to be buried. They can heat up and melt to produce granitic melts
Mantle Plumes
Oceanic Hotspotsrising mantle plumes lead to melting thatproduces basaltic magmas and Hawaii-type volcanic islands
Continental HotspotsThick lithosphere leads to greater degree of chemical variability, broader range of lava types.
Hazard 1: Lava Inundation(rare, but it happens)
Case of Goma, Congoa city with a population 500,000, located near Rwanda border, on shore of Lake Kivu, affected by lava from Nyiragongo Volcano
Jan 17, 2002, 45 people died, 50,000 displaced. Eruptions with deaths in 1977, too.
Reasons for deaths
Burns from coming in contact with lava or hot rock, or from buildings set afire.
High concentrations of poison gases that are being emitted from the lava.
Destruction of infrastructure that supplies vital services (water, etc)
Hazard 2: Mudflow = Lahar
Mt. Rainier, Washington State
Lots of water locked up in thatsummit glacier …
Hazard 3: Pyroclastic Flow= Nuee’ Ardente
Hot gasses and dust “burp” out of the volcano’s summit and cascade downhill at speeds of 50-100 mph.
St. Pierre, Martinique
• Town of 29,000 people on the shore of a volcanic island in the Carribean
• May 8, 1902: Pyroclastic flow from Mt. Pele volcano destroys the entire town, leaning only 1 survivor (a guy in jail)
(Aside) … The famous Pelean Spine,350 meters high, was a rock spirethat was pushed up outOf the crater of Mt. Pele in 1902 asnew lava rose beneath the volcano.
Hazard 4: Tsunami
Explosive volcanism at volcanic island caused a tsunami in nearby water
Local tsunamis, like the one following the Krakatau eruption, are the most common. But ocean-crossing tsunamis are possible. The ca. 1625 BCE eruption of the Agean island of Thera is an example.
Note location of volcano: island in center of narrow straight between two populated land masses, Java and Sumatra
• August 27, 1883:
extremely large explosioncollapse of the volcanic edifice30 meter high tsunami hits coast of
Java and Sumatra, killing 36,000 people
165 villages totally destroyed
Hazard 5: Ash Fallsvolcanic ash = fine rock particles
Ash falls over broad area, like snow
weight of ash collapses house
ash makes road impassible
ash-covered grass poisonous
to livestock (e.g. Iceland, 1783)
larger chucks of rock (=bombs) also fall
Luzon, Phillipines
• Near Mt Pinatubo
• 800 people killed by housing collapse when ash covered their houses
• Plenty of warning, but ash does not initially appear all that dangerous. But wait till it’s a meter thick! An it becomes very heavy and slick during the rain!
Hazard 6: Global Cooling
Aerosols (particularly sulfate) injected high is the atmospherereflect sunlight back to space, resulting in net cooling of theEarth’s surface
Unseasonably cold temperatures case crop failures
1816: The Year without a Summer
June 9-10, 1816: On the 9th, frost was reported as far south as Worcester, Massachusetts and on the 10th to East Windsor, Connecticut.
July 6, 1816: Temperatures in the 40’s F range were reported in Connecticut at both Hartford and New Haven. Robbins in East Windsor noted temperatures almost cold enough for a frost.
Tambora volcano (Sumbawa,Indonesia)
Extremely large VEI=6eruption in 1815.
May have been theLargest in 10,000 years
Perhaps 90,000 peopleDied worldwide, fromstarvation
People and VolcanoesVery local hazards
Lava inundationpyroclastic flow
regional hazardsmudflowExplosion induced tsunami
global hazardsGlobal coolingChemical pollution by ashIsland collapse
Questions to think about
• Which is the most dangerous volcano in the world?
• Versuvio, because of its proximity to Naples, Italy• And because of the very large eruption that
occurred in 79 CE
• What group of people are most endangered by volcanoes
• Volcanologists … I know several of whom have died, and nearly all of us have had close calls