An volcano is a mountain from which lava and hot gasses erupt ... Lava from Nyiragongo volcano, inundates Goma, Congo. Goma airport closed by lava flow ...
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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 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.
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) 1000 600 Eruptions 200 0.1 1 10 100 1000 10000 Duration (days)
How Often do Eruptions Occur? Cumulative number of eruptions With eruptive volumes equal to or Greater than the given amount 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
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
City of Goma Lake Kivu
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 …
Tacoma, Wa If all that iceon Mt. Ranier suddenly melted, where would the water and mud go?
Nevado del Ruis Volcano Note glacier at summit, valley that channels water
Part of town that was in river valley Was innundated by several meters of mud
Reason for Deaths Being buried alive in thick, gooey mud
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)
May 8, 1902: Pyroclastic flow Destroying the town of St. Pierre
St. Pierre After the Pyroclastic flow
(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.
Note location of volcano: island in center of narrow straight between two populated land masses, Java and Sumatra
August 27, 1883: extremely large explosion collapse of the volcanic edifice 30 meter high tsunami hits coast of Java and Sumatra, killing 36,000 people 165 villages totally destroyed