Untitled Volcanoes PDF

Shield Volcanoes Characteristics of Shield Volcanoes Fluid Basalt- gentle sloped shape Often quite large Mauna Loa, Mauna Kea, and Iceland Broad Shape Basalt/Low Silica Content. Formation and Eruption The low viscosity of basaltic magma allows it to travel long distances,...

Shield Volcanoes Characteristics of Shield Volcanoes Fluid Basalt- gentle sloped shape Often quite large Mauna Loa, Mauna Kea, and Iceland Broad Shape Basalt/Low Silica Content. Formation and Eruption The low viscosity of basaltic magma allows it to travel long distances, creating wide, shield-like structures. Eruptions are generally non-explosive, producing lava flows rather than pyroclastic materials. The Pahoehoe (ropy) surface form is a common feature of these volcanoes, indicating fluid lava movement. Visual Representation This diagram illustrates the structure of a shield volcano, highlighting the vent and lava flow patterns. Cinder Cone Volcanoes Characteristics of Cinder Cone Volcanoes Form Pyroclastic fragments Often steep, small, and symmetrical May form abruptly Formation Process Cinder cones form rapidly, often in a single eruptive event, and can be found on older lava terrains. The steep slopes are a result of the accumulation of volcanic debris around the vent. An example is the Sunset Crater in Arizona, which showcases the typical features of a cinder cone. Composite Volcanoes (Stratovolcanoes) Characteristics of Composite Volcanoes Layers of lava pyroclastic form hard surface, and resist erosion Steep Sided profile Tall Shape/Andesite, high silica content Notable Examples Mount St. Helens in Washington is a well-known stratovolcano that erupted explosively in May 1980. Other examples include Mount Rainier in Washington and Mount Vesuvius in Italy, which famously buried Pompeii in 79 AD. The eruptions of these volcanoes can produce significant ash flows and pyroclastic flows, posing risks to nearby communities. Volcanic Explosions and Their Effects Caldera Caldera- Collapsed, evacuated magma chamber –Steep-walled –Circular –40+ km in diameter Examples Yellowstone – 3 calderas –Last eruptions (1.9 mya) -2,500 km3 of pyroclastic materials (2500 larger than Mt St Helens), –(0.6 mya) – 1,000 km3 of ash & debris Yosemite’s Long Valley (Bishop Tuff) –170 times larger than Mt St Helens –1994 CO2 releases denote magma activity Pyroclastic Flows Pyroclastic Flows – column of pyroclastic materials and gas falls to ground –Nueè ardente – “glowing cloud” –Hot (1,000oC or 1,800oF) –Fast (700 kph or 450 mph) -Far-reaching (100 km or 62 mi) Fluidized masses of rock fragments and gases that move rapidly in response to gravity Very fast, from 10 to 30 m/s to 200 m/s Distance: 50 to 200 km Ash Flow Tuff Ash flow tuff – post flow the ash may weld / compact into a solid mass Examples Mt Vesuvius – stratovolcano near Naples, Italy –Pompeii & Herculaneum (79 AD) –5-8 meter ash flow –Intermittent activity in early – mid 1900s –Magma still underlies Vesuvius Risk Assessment and Prediction Regional prediction – areas most likely Subduction zones Spreading centers Hot spots Violence - related to crustal domain Continental (granitic/intermediate) – more violent Basaltic – tend to be less violent Short-term prediction – looking for time and place of eruption Changes in shape EQ swarms Gas / ash emission Temperature anomalies Examples Mt St Helens, 1980 –`78 Crandall & Mullineaux –1980 Earthquake swarms, gas, ash –USGS predictions averted even larger losses Montserrat, 1997 pyroclastic flows –1992 earthquake swarms –1995 steam /ash –predictions kept fatalities down to 19 Volcanic Eruptions and Global Climate Pinatubo, 1991 Ash & sulfur clouds Solar radiation reaching Earth declined by 2-4% Global cooling – high alt dust reflects radiation Permian extinction, 225 mya 90% of marine species, 65% reptile & amphibians Siberian flood basalts Mantle Carbon  Basalt eruption 0.44% weight/weight CO2 120 mya – mid-Cretaceous superplume in Pacific Ocean – lowered temperatures +10-15°C Volcanics linked to both warming & cooling