Download 1 Volcano Eruption Styles and Case Examples

Lecture 10: Volcano Eruption Styles and Case Examples Ch. 6: p.141-­‐153; 156-­‐159 / Ch. 7: p.177-­‐189 Stratovolcanoes and Shield Volcanoes: There are two dominant volcano types and their eruptive styles can broadly be classified as either explosive (stratovolcanoes – steep-­‐sided and pointy) or nonexplosive (shield volcanoes – gently-­‐sloped and very broad). However, we should acknowledge that eruption styles are actually a little more diverse than these two descriptions. Explosive Eruptions: In order to differentiate the different types of explosive eruptions, whether related to shield volcanoes or stratovolcanoes, we need to define specific eruptive styles: • phreatic: These are steam-­‐driven eruptions caused by magma interacting with groundwater, lakes, or shallow seas, producing steam and ash. • phreatomagmatic: If magma dominates a steam-­‐rich eruption, it is phreatomagmatic (such as the subglacial eruption at Grimsvötn in Iceland). Such eruptions may produce a circular crater called a maar crater where the steam blasted a hole through overlying lavas. • Strombolian: Named after the volcano Stromboli in Italy. These are also steam-­‐driven eruptions caused by magma interacting with groundwater or sea water, but cause the lava to froth and produce lava bombs and cinders that turn into scoria. • Vulcanian: Named after the volcano Vulcano in Italy. These generally involve viscous andesite magma rich in gases. During an eruption, pulsing gases are explosively released, creating predominantly ash eruptions that produce ash-­‐fall deposits as well as some ash flows. • Peléan: Named after the volcano Pelée in Martinique. Viscous andesite, dacite, or rhyolite magma explosively releases gases, producing large volumes of ash that collapse to form incandescent ash flows. They commonly form large lava domes in the crater prior to the eruption, with occasional collapses producing ash eruptions (e.g., St. Helens). • Plinian: Named after the eruption of Vesuvius in Italy in 79 AD, as described by Pliny the Younger. These are the truly enormous eruptions of ash that are forced ~20 km into the atmosphere as a Plinian column, and may drop ash over a wide area (e.g., the 1980 St. Helens eruption). Other examples include Santorini, Greece (1620 BC) and Krakatoa, Indonesia (1883). Plinian eruptions also commonly result in the collapse of the volcano into the evacuated magma chamber, forming a caldera. Giant Continental Calderas (Super Volcanoes): Volcanoes like Yellowstone are typically very subdued or exist as a basin formed by a giant caldera eruption. These are giant rhyolite volcanoes that erupt huge volumes of ash at infrequent recurrence intervals of perhaps 100s of 1000s of years. The caldera that forms may be >50 km across. 3
Eruption volumes may exceed 1000 km and cause ash falls that cover millions of square km as well as affecting global weather for many years. Yellowstone erupts every 700,000 years or so, the last eruption being 600,000 ybp. Naples and surrounding areas are threatened by the potentially “overdue” next eruption of Vesuvius. Even more horrifying, 2 million people live within the caldera of the rhyolite volcano Campi Flegrei – the Yellowstone of Italy. Eruption Magnitude: Eruptions may be classified according to the Volcanic Explosivity Index (VEI). Giant continental caldera eruptions such as Yellowstone may reach VEI 8. The last VEI 8 that the world experienced was in the Taupo Volcanic Zone in New Zealand in 186 B.C. 1 Yellowstone shows all the signs of building up to its next eruption, including evidence of magma at ~6 km depth, harmonic tremors, temperature increases at geothermal areas, and bulging of the caldera floor. The last eruption produced 1000 times more material than St. Helens in 1980. A modern day eruption of this size would be devastating. Mount St. Helens: First sign of activity: March 23rd, 1980 – lots of earthquake activity below the volcano. Eventually became a phenomenon called harmonic tremors, which commonly precede a volcanic eruption and consist of long-­‐period seismicity. th
March 27 , 1980: emissions of gas and ash followed by the development of a large bulge on the side of the mountain as magma moved into the volcano. The May 18th eruption was initiated by a M5 earthquake. The earthquake triggered a giant landslide (the largest ever recorded) that peeled away the north side of the mountain. The landslide exposed the magma chamber inside the mountain, which exploded due to the sudden release of pressure on the gases in the magma. Huge volumes of tephra were immediately produced. The lateral blast consisted of a surge of hot gases and ash at 350°C that moved at 330 mph. It was followed by a pyroclastic flow of tephra at >700°C moving at 80 mph. The Plinian column rose to a height of about 24 km. The surge reached 20 km north of the volcano, leveling trees across a swath 30 km from east to west. Trees were snapped at their bases and flattened against the ground with their tops pointing away from the volcano like compass needles. 57 people in the blast zone were killed. The pyroclastic flow triggered a debris avalanche which moved 22 km down the north fork of the Toutle River, producing a flow deposit 45 m thick. Material continued downstream in the form of a lahar. The mudflows ultimately destroyed 27 of the 31 bridges along their course. Westerly winds moved ash from the Plinian column 800 km downwind towards the east, producing ash falls up to 13 cm thick. The eruption ultimately removed the top 400 m of the summit. Cascades volcanoes: Related to the subduction zone along the boundary between the Juan de Fuca and the North American plates. Mount St. Helens has erupted 14 or 15 times in the past 4000 years. The last Cascade volcano to erupt prior to Mount St. Helens was Lassen Peak in California from 1914-­‐1917. It had been dormant for 27,000 years. Is there reason to be concerned about the other Cascades volcanoes? Mount Hood: last erupted in 1790s (with activity in 1866 and 1907). Only 40 miles from Portland, OR. In past eruptions, Mount Hood has produced lahars that have reached the outskirts of Portland and have reached the Columbia River. See http://pubs.usgs.gov/fs/2000/fs060-­‐00/. Mount Rainier: last erupted in 1894. Only 55 miles from Seattle, WA. In the past, Mount Rainier has produced lahars that have reached Puget Sound. They happen every 500-­‐1000 years. Time since last event: 500 years. Lassen Peak: a large lava dome inside the caldera of the ancient volcano Mount Tehama, which destroyed itself about 350,000 years ago. A new eruption could threaten towns like Redding, Red Bluff, and Susanville, CA. Crater Lake: represents the flooded caldera of the ancient volcano Mount Mazama, which destroyed itself about 6800 years ago. It was the largest known eruption in the Cascades and spread ash over a huge area. The eruption involved about 35 times more magma than St. Helens in 1980. Although Crater Lake is not expected to produce a similar eruption again in the near future, it is still an active volcano, evidenced by the growth of a lava dome in the caldera and which forms Wizard Island within the lake. 2 Three Sisters: represents three close-­‐proximity volcanoes in central Oregon. Recent INSAR (Satellite Radar Interferometry) data indicate that the underlying magma chamber is inflating. An eruption could be devastating for Bend, OR. Last erupted: ~350 AD (North Sister)/~50 BC (South Sister). Mount Shasta: approximately the size of Mount Mazama before it erupted explosively. It most recently erupted very viscous rhyolite lava and currently has magma below it. Towns at risk: Mount Shasta City and Weed, CA – both built on old debris avalanche deposits. Last erupted: 1786. 3