CHAPTER 5 Volcanoes and Volcanism Introduction About 550

CHAPTER 5 Volcanoes and Volcanism

Introduction About 550 volcanoes have been active in historical times. About 12 or so erupt each year; most eruptions are small and go unnoticed

Introduction Volcanism is the eruption of magma and associated gases at the surface. Some magma erupts explosively as pyroclastic ("fire-broken") rock and others erupt as lava flows.

Introduction How can volcanism be both constructive and destructive? Volcanism may cause injuries and fatalities, and destroy property. Pompeii and Mount Vesuvius Volcanism is also responsible for the origin of many oceanic islands, fertile farmland, and critical atmospheric gases. Fig. 5. 1, p. 111

Volcanoes and Volcanism Types of Volcanoes Not all volcanoes erupt from giant cones, some erupt from fissures (fractures) in the Earth's crust. Volcanoes were once active on all the inner planets of our Solar System and our Moon. Today, volcanoes are still active on Earth and possibly on Venus. Jupiter's moon, Io, has active sulfur volcanoes. Triton, a moon of Neptune, probably has volcanoes that erupt liquid methane (natural gas or CH 4).

Volcanoes and Volcanism Volcanic Gases, primarily water vapor, constitute only a small portion of magmas Lesser amounts of carbon dioxide, nitrogen, sulfur dioxide, and hydrogen sulfide Very small amounts of carbon monoxide, hydrogen, and chlorine Fig. 5. 2, p. 112

Volcanoes and Volcanism Volcanic Gases Most volcanic gases dissipate without harm to humans Chlorine gas may contribute to the deterioration of the ozone layer, which protects organisms from ultraviolet radiation (Geo. Focus, pp. 122 -123) Sulfur gases from large eruptions may cool global climates for 1 -2 years Fig. 5. 2, p. 112

Volcanoes and Volcanism Low Viscosity Mafic Lava Flows Mafic lavas may flow through near surface lava tubes Mafic lava flows may reach speeds of 50 km/hr Fig. 5. 4, p. 113

Volcanoes and Volcanism Mafic Lava Flows on Land Aa lava flows consist of angular blocks and fragments. Pahoehoe lava flows have a smooth surface, much like taffy. Fig. 5. 5, p. 114

Volcanoes and Volcanism Underwater Mafic Lava Flows Pillow lavas are bulbous, interconnecting masses that result from underwater eruption of basaltic lavas. Fig. 5. 6 a, p. 114

Volcanoes and Volcanism Lava Flows Columnar Joints Columnar joints form in response to the cooling and contraction of lava. They most commonly occur in basalt andesite lava flows. Fig. 5. 7, p. 115

Columnar joints in a basalt lava flow at Devil’s Postpile National Monument in California. The rubble in the foreground is collapsed columns. Surface view of the columns from (b). The straight lines and polish resulted from abrasion by a glacier that moved over this surface. As lava cools and contracts, three-pronged cracks form that grow and intersect to form four- to seven-sided columns, most of which are six-sided. Stepped Art Fig. 5 -7, p. 115

Volcanoes and Volcanism Pyroclastic Materials Pyroclastic materials are magma fragments that are explosively ejected by volcanoes, most are solid Ash < 2 mm in diameter (smaller than sand grains) Lapilli, 2 -64 mm Bombs, partially molten, >64 mm Blocks, solid, >64 mm

Volcanoes and Volcanism Pyroclastic Materials Volcanic ash, unlike larger pyroclastic materials, may be transported by winds over long distances. Ash is unhealthy for lungs and dangerous to aircraft because it fouls jet engines. Ash falls settle out of the atmosphere. Ash flows are turbulent clouds of ash and gas that travel close to the surface during pyroclastic eruptions.

Types of Volcanoes A volcano is a hill or mountain that forms around a vent, where lava, pyroclastic materials and gases erupt. Fig. 5. 12 c, p. 119

Types of Volcanoes 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. Fig. 5. 8, p. 116

Types of Volcanoes Shield volcanoes have gentle slopes and are largely composed of basalt with possibly some mafic pyroclasts. The Hawaiian volcanoes are shield volcanoes. They are usually non-explosive and pose little danger to humans. Fig. 5. 9, p. 117

Types of Volcanoes Shield Volcanoes The largest volcano in the Solar System, Olympus Mons on Mars, is a shield volcano. Fig. 5. 10 b, p. 117

Types of Volcanoes Cinder Cones Cinder cones consist of cinders and other pyroclastic materials. They accumulate as steep-sloped cones that rarely exceed 400 meters high. Fig. 5. 11, p. 118

Types of Volcanoes Composite Volcanoes (Stratovolcanoes) Composed of layers of lava flows, 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. Most composite volcanoes have intermediate compositions. Fig. 5. 12 a, b, p. 119

Types of Volcanoes Some Notable Volcanic Eruptions, Many Involving Deadly Composite Volcanoes Table 5. 1, p. 120

Table 5. 1, p. 120

Types of Volcanoes Composite Volcanoes (Stratovolcanoes) Lahars are mudflows consisting of mixtures of liquid water and pyroclastic materials. They may be hot or cold, and often form from rain during eruptions. Fig. 5. 13, p. 119

Types of Volcanoes Composite Volcanoes (Stratovolcanoes) Lahars may also form without an eruption if a submit of a volcano heats up, catastrophically melts snow and ice, and the resulting water mixes with pyroclastic materials from previous eruptions. Fig. 5. 13, p. 119

Types of Volcanoes Lava Domes Bulbous lava domes form when viscous, usually felsic or intermediate, lavas are forced up through the conduits of some volcanoes. These volcanoes can erupt explosively and commonly eject nuée ardentes (hot clouds of pyroclastic materials and gas). Fig. 5. 14, p. 120

Types of Volcanoes Lava Domes Nuée ardentes are hot clouds of pyroclastic materials and gas that are released by explosive eruptions. In 1902, a nuée ardente engulfed the city of St. Pierre on Martinique in the Caribbean. 28, 000 people were killed. Fig. 5. 15, p. 121

Types of Volcanoes Supervolcano Eruptions No supervolcano eruptions have occurred within recorded history Erupt hundreds of square kilometers of material and produce huge calderas Three supervolcano eruptions have occurred in Yellowstone within the past 2 million years Figure 5. 16, p. 124

Other Volcanic Landforms 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. Fig. 5. 17 a, b, p. 124

Other Volcanic Landforms Pyroclastic Sheet Deposits Huge sheet-like eruptions of pyroclastic materials, especially ash, can cover large areas when erupted from fissures during caldera formation.

Volcano Belts Volcanoes are usually not randomly distributed, but occur in welldefined zones or belts along plate boundaries. Fig. 5. 18, p. 125

Volcano Belts About 60% circum-Pacific belt, 20% Mediterranean, belt, 20% are at or near midoceanic ridges Fig. 5. 18, p. 125

North America’s Active Volcanoes Alaska’s Volcanoes Alaska’s volcanoes stretch from the mainland of Alaska through the Aleutian Islands. Fig. 5. 18, p. 125

North America’s Active Volcanoes Alaska’s Volcanoes Most are composite volcanoes, some with huge calderas. This volcanic arc is extremely active with many explosive eruptions. Fig. 5. 18, p. 125

North America’s Active Volcanoes The Cascade Range: Lassen Peak in California to British Columbia, Canada Mostly composite volcanoes, but there also two huge shield volcanoes and numerous cinder cones Fig. 5. 19 a-c, p. 127

North America’s Active Volcanoes The Cascade Range: Lassen Peak in California to British Columbia, Canada Results from the subduction of the oceanic Juan de Fuca Plate underneath the continental North American Plate Fig. 5. 19 a-c, p. 127

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. Fig. 5. 6 a, p. 114

Plate Tectonics, Volcanoes, and Plutons Igneous Activity at Convergent Plate Boundaries The volcanism in the circum-Pacific and Mediterranean belts results from subduction. Most of the magmas are produced by the partial melting of the subducted plates. Fig. 5. 18, p. 125

Plate Tectonics, Volcanoes, and Plutons Intraplate Volcanism Mafic hotspot volcanism The Hawaiian Islands formed as a series of volcanoes originating from a stationary mantle plume as the Pacific Plate moved over it. Fig. 5. 4, p. 113

Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions What are the most dangerous manifestations of volcanoes? Lava dome eruptions with huge amounts of pyroclastic materials and gases are the most dangerous volcanic eruptions. Lahars are also dangerous and they may occur without an eruption. Fig. 5. 20, p. 129

Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions 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 VEI measures the explosive intensity of an eruption using: volume of material erupted eruption cloud height Fig. 5. 21, p. 130

Fig. 5. 21, p. 130

IC SM C LY TA C A EX PL O SI VE VEI 4 -7 VEI 3 -5 G EN TL E VEI 1 -3 Plinian Vulcanian Strombolian Stepped Art Fig. 5 -21, p. 130

Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions Is It Possible to Forecast Eruptions? Monitoring volcanoes helps geologists to forecast imminent eruptions Fig. 5. 22, p. 130

Fig. 5. 22, p. 130

Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions Is It Possible to Forecast Eruptions? Monitoring involves recording and analyzing both physical and chemical changes at volcanoes Tiltmeters and geodimeters to detect changes in slope, elevation, and shape of the volcano Fig. 5. 22, p. 130

Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions Is It Possible to Forecast Eruptions? Monitoring involves recording and analyzing both physical and chemical changes at volcanoes Seismometers to detect harmonic tremors Gas emissions are also measured Fig. 5. 22, p. 130

End of Chapter 5