Introduction Chapter 5 Volcanism is the eruption of magma, and associated gases at the surface. Some magma erupts explosively as pyroclastic (fire-broken) rock and other erupts as lava flows. Volcanoes and Volcanism Volcanism A fissure eruption along the East Rift of Kilauea. The fountain is 200 feet high! Introduction How can volcanism be both constructive and destructive? Destructive in that volcanism may destroy houses and farmland and cause injuries and fatalities so it is destructive. Pompeii and Mount Vesuvius Fig. 5.1, p. 109 Introduction How can volcanism be both constructive and destructive? Introduction Some Notable Volcanic Eruptions It is also constructive because it is responsible for the origin of many oceanic islands and oceanic crust as well as critical atmospheric gases. Pompeii and Mount Vesuvius Fig. 5.1, p. 109 Table 5.1, p. 110 1
Volcanic Gases Gases, primarily water vapor, constitute only a small weight-percent of magmas. Lesser amounts of carbon dioxide, nitrogen, sulfur dioxide, and hydrogen sulfide Volcanic Gases Very small amounts of carbon monoxide, hydrogen, and chlorine. Erupted sulfur can have widespread, significant effects on climate. Fig. 5.2, p. 110 Fig. 5.2, p. 110 Lava Flows Lava Flows Low viscosity mafic flow Aa lava is made up of angular blocks and fragments. Pahoehoe lava has a smooth surface much like taffy. Fig. 5.4, p. 112 Lava Flows Top 7 of fresh pahoehoe flow. The top of the flow is vesicular. Why? Recent Lava Flow, Snake River Plain, Idaho 2
Lava Flows Lava Flows Pressure ridges and spatter cones are typical of many flows. Columnar Joints Columnar joints, formed in response to cooling and contraction of lava, form most commonly in basalt and andesite lava flow. Fig. 5.5 a-b, p. 113 Fig. 5.6, p. 114 Lava Flows Pyroclastic Materials Pillow lavas are bulbous, interconnecting masses that result from underwater eruption of basaltic lavas. Pyroclastic materials are solids, including ash, lapilli, blocks, and bombs, that are explosively ejected by volcanoes. Ash is dangerous to aircraft because it fouls jet engines. Fig. 5.7 a-b, p. 116 Fig. 5.8, p. 117 Volcanic Ash, Hawaii A volcano is a hill or mountain that forms around a vent, where lava, pyroclastic materials and gases erupt. Geo-inSight 7., p. 121 3
Calderas A caldera is a large oval to circular volcanic depression that forms when the summit of a volcano collapses into its magma chamber following voluminous eruptions. There are several kinds of volcanoes. Shield Volcanoes Shield volcanoes have gentle slopes, rounded shapes and are composed largely of basaltic lava flows. The Hawaiian volcanoes are shield volcanoes. They are non-explosive and pose little danger to humans. Fig. 5.9 a-e, p. 116 Geo-inSight 1., 3., p. 120 Cinder Cones SP Crater near Flagstaff, Arizona Cinder cones consist of cinder-like pyroclastic materials accumulated as relatively small, steepsloped cones. Geo-inSight 4., p. 121 Fig. 5.10, p. 119 Composite Volcanoes (Stratovolcanoes) Composed of lava flows of intermediate composition, and layers of pyroclastic debris and volcanic mud flows (lahars). They are explosive and are the most dangerous to humans. The volcanoes rimming the Pacific ocean are mostly composite volcanoes. Mt. Shasta, California (a stratovolcano) Geo-inSight 6. & 9., p. 121 4
Stratification of lava and ash layers, near Mt. St. Helens Lava Domes Bulbous lava domes form when viscous, generally felsic lavas are forced up through the conduits of some volcanoes. These volcanoes can erupt explosively and commonly eject nuée ardentes. NW Trip, 2002 http://vulcan.wr.usgs.gov/volcanoes/msh/images/msh04/crater_dome_eruption_may_2008.html Geo-inSight 11., p. 121 Supervolcano Eruptions Nuee Ardentes (pyroclastic flow), Mayon Volcano, Philippines No supervolcanos have occurred within recorded history Geologists, however have found evidence of their existence in the geologic past. Figure 5.13, p. 119 Supervolcano Eruptions Three supervolcano eruptions have occurred in Yellowstone within the past 2 million years. Each resulted in a thick accumulation of rhyolitic magma that erupted as a widespread blanket of ash and pumice. Fissure Eruptions and Basalt Plateaus Basalt plateaus form when fluid mafic lava erupts from long fissures (not vents), known as a fissure eruption. Basalt plateaus are made up of numerous overlapping basalt lava flows. Figure 5.13, p. 119 Fig. 5.14 a-b, p. 119 5
Flood Basalt layers, Grand Coulee Region, Washington State Isla Encantada, west side of Gulf of California Pyroclastic Sheet Deposits Huge sheet-like eruptions of pyroclastic materials, especially ash, can cover large areas when erupted from fissures during caldera formation. These pyroclastic materials may form pyroclastic sheet deposits. Distribution of Volcanoes Volcanoes are not randomly distributed, but occur in well-defined zones or belts. 60% circum-pacific belt, 20% Mediterranean belt, 20% are at or near mid-oceanic ridges Fig. 5.15, p. 124 North America s Active Volcanoes Alaska s Volcanoes Alaska s volcanoes stretch from the mainland of Alaska thru the Aleutian Islands Most are composite volcanoes, some with huge calderas. This volcanic arc is extremely active with many explosive eruptions. North America s Active Volcanoes The Cascade Range The Cascade Range stretches from Lassen Peak in California to British Columbia, Canada. Most of the large volcanoes in the range are composite volcanoes, but there are also two huge shield volcanoes and numerous cinder cones. Geo-inSight 11., p. 121 Fig. 5.12 b, p. 118 Fig. 5.16 a-c, p. 128 6
Plate Tectonics, Volcanoes, and Plutons Igneous Activity at Divergent Plate Boundaries Volcanic activity at or near mid-oceanic ridges is mainly submarine, but in a few places such as Iceland, it occurs above sea level. The volcanoes that form are mostly mafic shield volcanoes. Plate Tectonics, Volcanoes, and Plutons Igneous Activity at Convergent Plate Boundaries The volcanism in the circum-pacific and Mediterranean belts is the result of subduction along convergent plate boundaries. Fig. 5.7a-b, p. 115 Most of the intermediate and felsic magmas produced by the partial melting of the subducted plates. Fig. 5.15, p. 124 Plate Tectonics, Volcanoes, and Plutons Intraplate Volcanism The Hawaiian Islands formed as a series of volcanoes originating from a stationary mantle plume as the Pacific Plate moved over it. What are the most dangerous manifestations of volcanoes? Lava dome eruptions during which huge amounts of pyroclastic materials and gases are ejected are the most dangerous volcanic eruptions. Lahars are also dangerous and they may take place long after an eruption. What are other manifestations of volcanoes? Nuee Ardentes hot, dense clouds of gas and ash that flow rapidly downhill Gases can also be dangerous or even poisonous. Lava flows are the least dangerous to human life (slow) What are other manifestations of volcanoes? Volcanic ash can coat nearby roads and buildings and cause breathing difficulties. Earthquakes often accompany volcanic eruptions and can be destructive. 7
Potential Long-Term Effects Climate can be altered by volcanic eruptions. Release of ash or sulfurous gases high into the atmosphere can cool the climate by absorbing solar energy. CO 2 and water vapor can have the opposite effect. Potential Long-Term Effects Volcanic soils are often enriched in nutrients relative to other soil types, improving i agriculture in those areas. How Large Is an Eruption, and How Long Do Eruptions Last? VEI - the most widely used indication of the size of a volcanic eruption is the volcanic explosivity index. The volcanic explosivity index (VEI) measures the explosive intensity of an eruption using volume of material erupted eruption cloud height Volcanic Hazards, Volcano Monitoring, Is It Possible to Forecast Eruptions? Geologic Investigations can tell us about: The type of past eruptions The size of past eruptions The frequency of past eruptions Fig. 5.18 p. 128 Fig. 5.19, p. 129 Volcanic Hazards, Volcano Monitoring, Is It Possible to Forecast Eruptions? Monitoring volcanoes helps geologists to forecast imminent eruptions Monitoring involves recording and analyzing both physical and chemical changes at volcanoes. Fig. 5.19, p. 129 Volcanic Hazards, Volcano Monitoring, Is It Possible to Forecast Eruptions? Tiltmeters and GPS to detect changes in slope, elevation, and shape Seismometers to detect harmonic tremors and longperiod events. Gas emissions and well water are also measured (content and temperature). Fig. 5.19, p. 129 8