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Volcanic Features Information Volcanic Features Information Caldera A caldera is a bowl-like volcanic feature usually formed by the collapse of land following a volcanic eruption. They are sometimes confused with volcanic craters. The word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking pot". In some texts the English term cauldron is also used. A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption. If enough magma is erupted, the emptied chamber is unable to support the weight of the volcanic cone above it. This causes a roughly circular fracture, the ring fault, to develop around the edge of the chamber. The collapse may occur as the result of a single large eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds or thousands of square kilometers. Lava Dome In the study of volcanoes, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow leak of thick (high viscosity) lava from a volcano. The geologic chemistry of lava domes can vary from basalt to rhyolite although most preserved domes tend to have high silica content. The characteristic dome shape is caused by high viscosity that prevents the lava from flowing very far. This high viscosity can occur in one of two ways: by high levels of silica in the magma, or by fluid magma giving off gases that are dissolved in it. Pyroclastic Flow A pyroclastic flow is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are a common and devastating result of certain explosive volcanic eruptions. Lahar Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 meters (33 feet) in diameter. Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Blue Group: Mt St Helens Volcanic Eruption 1980 Where is the volcano? Mt St Helens (8365 ft , 22,550m) is in the Cascade Volcanic Range in Washington State, USA. The Cascade Range has 160 volcanoes and Mt St Helens is considered the most active, and lies 100 miles south of Seattle and 50 miles from the coast at Portland Why is there a volcano here? Mt St Helens is a stratovolcano within the Cascade Volcanic range in Washington State and had been dormant for 100 years prior to 1980. As an Eastern remnant of the Pacific Plate, the Juan da Fuca plate, has subducted beneath continental North America the Cascade volcanic chain has formed. The Mt.St.Helens volcano has been active for around 40,000 years but the now damaged cone we see today has formed over the last 2,200 years Eruption Style The volcano is highly explosive and erupts in the Plinian style like Vesuvius. Sudden violent explosions are accompanied by towering clouds of hot ash, dust and rocks thrown high into atmosphere. Pyroclastic flows and volcanic mud flows called Lahars flow down the mountain at speed and consume the surrounding landscape. Lavas from basalt to andesites have been erupted forming layers of ash and lava. In many stratovolcanoes a central summit crater has dome of intermediate lava type which is squeezed out of the main vent, similar to Montserrrat. In 1980 this did not occur and instead a crypto dome formed (crypto means hidden) causing the Northern slope of the volcano to bulge and grow. From mapping the deposits around the volcano it seems that lateral blasts and side domes were not unprecedented. Sequence of Events On May 18 1980 Mt.St.Helens produced an enormous horizontal (sideways) blast which completely destroyed everything for 20 miles north of the volcano. The elevation of mountain peak decreased from 2930m to 2550m and a gaping crater 1.5km wide was created. The sequence of events that occurred had not been witnessed before as until then geologists had always seen volcanoes erupt upwards but this was different this was sideways! In March 1980 earthquakes and a steam eruption column alerted the United State Geologic Survey (USGS) that the volcano was certainly waking up. Throughout the spring the mountain continued to splutter and steam the northern side of the mountain was bulging out by 1.5m/day. A 5 miles exclusion zone was placed around the volcano and famously Harry Truman owner, of the Spirit lake lodge resort, refused to leave. On the morning of May 18 Geologist David Johnson was on a ridge 10km north of the volcano when a 5.1 earthquake struck. Suddenly the bulge on the north side of the volcano slid downwards in a massive landslide at 300km/hour and crashed into Spirit lake causing an enormous wave of water the rest of the landslide ploughed down the Tootle river valley and mixed with vast quantities of lake water and ice . Davids last message was to his colleagues in the nearby Vancouver USGS base ( not Canada) "Vancouver,Vancouver .. this is it!" The landslide uncorked the magma in the crypto dome and released the pressure. The gas in the magma instantly expanded and shattered the rocks at supersonic speed creating a lateral of blast ash and hot rocks which overtook the landslide at reached speeds of 1,000km/hour (670 miles/hour) . The blasts power has been estimated at 500 times that of Hiroshima. 600km2 of forest was blown away and trees combed down like hair, sap exploded in trees in the intense heat. Trees up to 20 miles away were engulfed in the blast 4 times the distance of the exclusion zone. The third string to Mt.St.Helens bow was a 19km (12miles) high eruption cloud which powered vertically out of the volcano in a matter of minutes. The cloud turned day to night and ash fall was heavy. Lightning crackled around the cloud. Mudflows or Lahars formed within minutes of the eruption. lake water, melted glacier ice, hot ash and debris all combined to send hot ( 29-33C) cement like flows down the Toutle river. Pyroclastic flows started to form from the crater just after the blast as fresh magma came into contact with the air. Material appeared to overflow the craters edge and flow down the Toutle valley as a grey turbulent cloud. Harry Truman and David Johnson along with 55 other people did not survive. Both have had ridges named after then within the Mt.St.Helens national monument. The area within the monument is being left to recover naturally. Within 8km of the volcano all trees were obliterated, from 8km to 30km flattened and outside this dead but left standing. Outside the boundary logging companies have replanted trees. Mount St. Helens showed significant activity on March 8, 2005, when a 36,000-foot (11,000 m) plume of steam and ash emerged. A new dome is growing and will eventually fill the 1980 crater. Effects and Aftermath Rescue helicopters looking for survivors were grounded or could not land due to ash. Ash removal from roofs and roads was costly and lengthy costing 2 million dollars just in one town(Yakima) Hazard Management All volcanoes in the Cascades are monitored by the Cascades Volcanic Observatory CVO part of the United States Geological Survey USGS from their base in Vancouver, not Canada but Washington State USA. On March 27th 1980 a phreatic eruption signaled the beginning of an active phase. The CVO moved in a team of Geologists to monitor the volcano. Small eruptions of steam and ash blasted from the old summit dome continued and attracted attention from the press and tourists. Evacuation plans were prepared and roads closed leading to the volcano. As the mountain swelled with magma, and bulged outwards at a rate of 1.5m/day, it was obvious that the Spirit Lake area to the north of the volcano needed to be evacuated. The Washington State Governor placed a 5 mile red zone around the summit only Harry Truman an 83 year old resort owner refused to move. When the volcano erupted on May 18th the lateral blast extended 20 miles 4 x the exclusion zone. Red Group: Kilauea Volcano Hawaii Where is the volcano? Located in the south of the Big Island within the Hawaii Volcanoes National Park. Kilauea sits on the shoulder of Mauna Loa and the summit is only 1247m (4091ft). There are no steep cinder cone slopes here as this is a shield volcano and slopes are very low angled and gentle due to successive lava flows. Kilauea has been active for 23,000 years and has been continuously erupting since 1983 ; 90% of the volcano is less than 1,100 years old. The summit caldera is around 11 miles in circumference and contains the Halema'uma'u crater (active 1974), the home of Pele, and the Kilauea Iki (little Kilauea) crater which was active in 1959.
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