3/7/17. #16 - Case Studies of Volcanoes II. Announcements Monday 2/27

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1 Announcements Monday 2/27 Exam #1: Monday Feb. 27 th, 7:15-8:15 (see web site) Last Names A - N Loomis 141 Last Names O - Z Loomis 151 Bring your student ID An old exam is posted in Compass If you are very ill that evening notify me and Fangruo BEFORE the exam #16 - Case Studies of Volcanoes II Conflict exam a day or two after Format: Some MC, some short answer Web Ex #3 due date changed to Wednesday Mt. Saint Helens, Washington, 1980 Volcano Web Sites Virtual field trip to Mount St. Helens (Stromboli online) Mount St. Helens VolcanoCams (National Park Service) Cascade Volcano Alert Levels (US Geological Survey) Panoramic View - Mount St. Helens (Fullscreen 360) Sakurajima volcano, Japan Osorno volcano, Chile Ashfall Eruption Case History-Mt. Saint Helens Mt. St. Helens (MSH) in SW Washington is one of many volcanoes in Cascade Mountains in NW US, SW Canada; subduction zone volcano Mt. Saint Helens Eruption Major eruption on May 18, Before 1980, it was symmetrical After 1980 eruption, large part of mountain exploded away. 1) Pre - 5/18/80 (~50 days) Gas-charged silicic magma filled area below volcano causing ground to swell ~300 ft. Geologists warned of possible explosive eruption. After Before 1

2 1) Pre 5/18/80 (~50 days) Bulge develops on north side of Mt. St Helens Pre 5/18/80 (~50 days) Bulge develops on north side of Mt. St Helens What type of volcano is Mount Saint Helens? A. Shield volcano B. Stratovolcano C. Lava Lake volcano D. Rift volcano May 18, 1980 Two strong earthquakes (from magma moving) caused largest historic landslide. Landslide released pressure on magma, caused upward and horizontal explosion of volcanic ash. May 18, 1980 Two strong earthquakes (from magma moving) caused largest historic landslide. (a) Landslide released pressure on magma, caused upward and horizontal explosion of volcanic ash. May 18, 1980 (b) Enormous force of horizontal explosion knocked over trees for large distances Effects of lateral blast Vertical and lateral blast 2

3 May 18, 1980 Horizontal explosion knocked over trees for large distances Lateral blast effects - Trees blown down May 18, 1980 Ash blown 10 miles upward darkened skies for 100s of miles and (c) caused ashfall. Vertical blast, ashfall May 18, 1980 Ash blown 10 miles upward darkened skies for 100s of miles. May 18, 1980 Ash blown 10 miles upward darkened skies for 100s of miles (ash carried by jet stream to eastern US). Ashfall was mainly nuisance. Spokane, WA (300 miles away) Ephrata, WA (150 miles away) May 18, 1980 Ashfall was mainly nuisance- some disruption of daily life. May 18, 1980 Hot ash melted snow to produce enormous (d) lahar (mudflow), which created flooding. Lahar (mudflow) 3

4 May 18, 1980 Hot ash melted snow to produce enormous (d) lahar (mudflow), which created flooding. Lahar (mudflow) effects May 18, people were killed, including one geologist (David Johnston). Many more deaths without warnings from geologists. David Johnston Mt. Saint Helens Eruption The Eruption of Mt. St. Helens video (Otto Sieber 13:11-17:20) Which did NOT cause damage and/or disruption during the Mount Saint Helens 1980 eruption? A. Major lava flows that buried towns. B. A major explosive blast that traveled for miles. C. Lahars (volcanic mud flows). D. Ash falls in the region Other Explosively Volcanoes Cinder cone - consists entirely of pyroclastic debris of all shapes and sizes (no lava flows). Small, symmetrical cone; usually ~short lifespan. Other Explosively Volcanoes - Cinder Cone Paricutin volcano in Mexico, 1943; built a 400 m high cone in 9 years then extinguished itself. 4

5 Volcano Size - Shield, Strato, Cinder Cone Cinder cones are much smaller than typical stratovolcanoes, which are much smaller than shield volcanoes of Hawaii. Other Explosive Volcano Features Lava dome - bulbous mass of extremely viscous lava that accumulates around vent, which can get plugged; potential for extremely explosive eruption. Lava Dome at Mt. Saint Helens, 1984 Other Explosively Volcanoes Caldera - rare, but extremely violent eruption that produces huge crater (10's of km wide). Other Explosively Volcanoes Caldera - involve largest volcano explosivity indices (VEI 6); commonly begin as stratovolcano, then enormous eruption blows away top part of volcano. Magma chamber empties, leaving large underground cavity, ground over it collapses. Krakatau (southwest Pacific, 1883) created crater that extended 300 m below sea level, produced giant tsunami that killed >30,000 people Tambora Eruption Pre-telegraph VEI = 7 7 km diameter caldera 41 cubic km 14,000 ft. mountain GONE 1816: Year without a summer Crops ruined in the eastern USA Famine in Europe and India Political instability 5

6 Other Explosive Volcanoes - Calderas Mount Mazama (in southwest Oregon, ~6,800 years ago) created Crater Lake Other Explosive Volcanoes - Calderas Santorini (Greece, 1390 BC) destroyed Late Minoan civilization (legend of lost city of Atlantis?) What causes a Caldera eruption? A. Large amounts of low viscosity magma pours out of the ground quickly. B. A large magma chamber empties and, as it does, its roof collapses. C. A mountain builds up very tall and spews ash out of a crater at the summit. D. All of the above. Yellowstone (northwest WY) 3 large caldera-forming eruptions over past 2 million years (1.9, 1.3, 0.6 m.y.a.) 600,000 year recurrence, eruptions were ~1000 times larger than Mount St. Helens, Yellowstone caldera-forming eruptions were ~1000 times larger than Mount St. Helens Yellowstone continues to show signs of activity with hot springs, geysers 6

7 Yellowstone some consider it supervolcano, difficult to predict next eruption precisely, massive one not expected soon (>10,000 years?) Which is NOT a common negative consequence of large Caldera eruptions? A. Huge output of volcanic ash into the atmosphere. B. Huge lava flows that run across the land surface. C. Cooling of the atmosphere. D. Explosions that devastate large areas of the earth s surface (10 miles or more) 7