Presentation on theme: "Volcanism and Volcanic Hazards Bill Menke September 21, 2005."— Presentation transcript:
Volcanism and Volcanic Hazards Bill Menke September 21, 2005
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?
What is an volcano ? An volcano is a mountain from which lava and hot gasses erupt
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 H 2 0 and CO 2 ) 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.
V EI Descriptio n Plume Height VolumeClassificationHow oftenExample 0 non- explosive < 100 m1000s m3HawaiiandailyKilauea 1gentle100-1000 m10,000s m3 Haw/Stromboli an dailyStromboli 2explosive1-5 km1,000,000s m3 Strom/Vulcani an weeklyGaleras, 1992 3severe3-15 km 10,000,000s m3 VulcanianyearlyRuiz, 1985 4cataclysmic10-25 km 100,000,000s m3 Vulc/Plinian10's of years Galunggung, 1982 5paroxysmal>25 km1 km3Plinian100's of years St. Helens, 1981 6colossal>25 km10s km3 Plin/Ultra- Plinian 100's of yearsKrakatau, 1883 7 super- colossal >25 km100s km3Ultra-Plinian 1000's of years Tambora, 1815 8 mega- colossal >25 km1,000s km3Ultra-Plinian 10,000's of years Yellowstone, 2 Ma
Global distribution of Volcanoes About 1500 volcanoes worldwide
How long do eruptions last? Most eruptions last 10 - 1000 days Less than 20% over within 72 hours Median is 7 weeks (1176 hours) 0.1110 100 100010000 Duration (days) Eruptions 600 1000 200
How Often do Eruptions Occur? Eruptions obey Gutenberg-Richter Statistics: lots of small ones, just a few big ones. The graph at the left are for Explosive eruptions in Kamchatka During the last 10,000 years Cumulative number of eruptionsWith eruptive volumes equal to orGreater than the given amount
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 Zones dewatering of subducted lithosphere (string at 100-150 km depth) suppresses the melting point of mantle rock to produce basaltic magma Continental Collision Strong 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 Hotspots rising mantle plumes lead to melting that produces basaltic magmas and Hawaii- type volcanic islands Continental Hotspots Thick lithosphere leads to greater degree of chemical variability, broader range of lava types.
Hazard 1: Lava Inundation (rare, but it happens) Case of Goma, Congo a 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.
African Rift Valley Continental Divergent Plate Boundary Nyiragongo Volcano
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 that summit glacier …
Crate Lake, Oregon lots of water in that lake...
If all that ice on Mt. Ranier suddenly melted, where would the water and mud go? Tacoma, Wa
Armero, Columbia Town of 27,000 people on the flank of Nevado del Ruiz Volcano Much of the town in a river valley subject to Lahars Nov 13, 1995: 23,000 people died on when the town was innudated by 15 meters of mud
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 spire that was pushed up out Of the crater of Mt. Pele in 1902 as new 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.
Reasons for Deaths drowning Being crushed by floating debris
Hazard 5: Ash Falls volcanic 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 atmosphere reflect sunlight back to space, resulting in net cooling of the Earth’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=6 eruption in 1815. May have been the Largest in 10,000 years Perhaps 90,000 people Died worldwide, from starvation
People and Volcanoes Very local hazards Lava inundation pyroclastic flow regional hazards mudflow Explosion induced tsunami global hazards Global cooling Chemical pollution by ash Island 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