Volcanoes Chapters 6 & 7, Hyndman & Hyndman with input from Richard Sedlock Department of Geology San Jos é State University

Origin of “Volcano” Word is from the Island of Vulcano off Sicily Ancient people from the area believed that Vulcano was the chimney of the forge of Vulcan (blacksmith of the Roman gods) They thought hot lava fragments and clouds of dust erupting from Vulcano came from Vulcan's forge as he beat out thunderbolts for Jupiter (king of the gods) and weapons for Mars (the god of war)

VOLCANOES form where molten rock is vented at Earth’s surface. Where do volcanoes form in the context of plate tectonics? Volcanoes aren’t equally dangerous.... how do their hazards differ, and why? BIG Questions

What comes out of a volcano? Ash Fragments of solidified magma Jagged pieces of tiny rocks and glass magnified 200 times

What comes out of a volcano? Gas Most common: H 2 O CO 2 SO 2 HCl

What comes out of a volcano? Lava

Characteristics of volcanic rocks basalt contains the least silica erupts at the highest temperature lowest viscosity (the least resistance to flow basalt lava moves over the ground easily, even down gentle slopes dacite and rhyolite lava tend to pile up around a vent form short, stubby flows or mound-shaped domes Illustration by J. Johnson from

lower strata (layers) have hollow “well” for magma chamber streams of magma rise from magma chamber, through the strata some streams push up strata layers other streams move to the top opening(s) of the volcano raised opening that emits magma called the cone stream that moves to the top of the cone called the Central Vent volcano emits magma and a stream of tephra (ash and rock) packets/volcanoes/poster/poster.html

Mauna Loa (Hawai’i): A typical shield volcano (also the largest volcano on earth) Characteristics: broad gentle slopes formed from basalt lava of low viscosity eruption is mostly lava rather than pyroclastic material eruptions are not explosive (unless water gets into vents) lava pours out of vents or as fountains Photograph by J.D. Griggs on 25 February 1983

Mt. St. Helens: A typical composite volcano (stratovolcano) Characteristics steep sides, symmetrical cones formed from magma of moderate viscosity (andesite) alternating layers of lava flows, volcanic ash, fragments of debris erupts explosively repeatedly from summit crater often tall and snow covered, therefore subject to mudlfows (lahars)

Mt. St. Helens after its 1980 eruption

Explosive Non-explosive (“Clogged”) (“Runny”) Andesitic composition Basaltic composition Composite volcanoes Shield volcanoes Volcanologists recognize many types of volcanoes, but we only need to worry about two:

Volcanic Hazards Lava flows Ash fall Pyroclastic flows Mudflows Volcanic Gases Tsunami

Ash Composed of bits of pumice less than 2 mm across Can drift long distances on the wind Can stay suspended in atmosphere blocking sun and ruining crops Crop and roof damage - 20 cm can cause roofs to collapse Ash eruptions generate “volcano weather” – rain Irritates lungs and eyes - long term exposure can cause lung disease Reduced visibility making evacuation difficult Planes flying into plumes of ash can have engine trouble Mt. Pinatubo

Mount St. Helens Ash Fallout

Plate-tectonic setting of volcanism Explosive (andesitic) volcanoes form at subduction zones.

Plate-tectonic setting of volcanism At spreading centers, low pressure triggers mantle melting—fluid basaltic magma rises.

Plate-tectonic setting of volcanism Within plates, rising plumes of hotter mantle feed hot spots; varied volcanoes result (basaltic on Hawaii).

Evidence of Volcanoes in our own Backyard Calistoga, Petrified Forest Redwood trees, 8 ft in diameter Blown over and covered with ash from volcano 3 million years ago molecules of silica in the ash replaced molecules of wood wood turned to solid stone