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6 Volcanoes: Tectonic Environments and Eruptions Hyndman/Hyndman Natural Hazards and Disasters, 3rd Edition

Cascade Range Volcanoes are Active
Until 1970s, Cascade Range volcanoes thought to be extinct or dormant Rule of thumb: if volcano had significant glacial erosion, it had not erupted since end of last ice age and probably wouldn’t erupt again eruptions of Mount Lassen (northern California) seen as exception

Cascade Range Volcanoes are Active
With understanding of plate tectonics came realization that Cascade Range volcanoes sit over active subduction zone, therefore volcanoes are potentially active Active volcano is one that is likely to erupt again, ever Volcanologists don’t consider volcanoes to be dormant – only active or extinct, on geologic time scale

Introduction to Volcanoes: Generation of Magma
Volcano: cone-shaped hill or mountain formed at vent from which molten rock or gases reach Earth’s surface and erupt Magma: molten rock before it erupts Lava: magma after it reaches Earth’s surface Volcanoes only form at settings where magma is generated at depth and can rise to surface

Distinction between liquid, solid and gas Substances change from one to another with change in temperature and/or pressure Melting temperature depends on pressure and availability of water

Figure 6-1: Spreading Molecules
Molecules in a solid are tightly packed, while molecules in a liquid are held together loosely, and molecules in a gas can spread out to fill a container. Fig. 6-1, p. 135

Solid Liquid Gas Figure 6-1: Spreading Molecules
Molecules in a solid are tightly packed, while molecules in a liquid are held together loosely, and molecules in a gas can spread out to fill a container. Stepped Art Fig. 6-1, p. 135

Magma rises through crust because less dense than surrounding rocks Magma sometimes breaks off and incorporates pieces of adjacent rocks Magma chamber: mass of molten magma that rises through Earth’s crust, often erupting at surface to build volcano

Magma Properties and Volcanic Behavior
No two volcanoes exactly alike No two eruptions exactly alike (even at same volcano) Events of eruption depend on How fluid magma is (viscosity) Quantity of water vapor, other volcanic gases (volatiles) Type and amount of magma (volume)

Melting Temperature of a Rock
Depends on depth and amount of water Rock may melt by Increase in temperature Decrease in pressure Addition of water (shifts melting curve to lower temperatures)

Figure 6-BTN1: Melting Temperature of a Rock The melting temperature of a rock depends on its depth within the Earth and the amount of available water. As shown in the following illustration (see green arrows), a hot rock deep within the Earth (e.g., a rock above a subduction zone at “A”) may melt as a result of an increase in temperature, a decrease in pressure, or the addition of water that shifts the melting curve to lower temperatures. Newly formed magmas rising toward the surface may not always reach it. The magma may crystallize by cooling or loss of water. A water-rich magma must lose water and begin to crystallize as it rises to lower pressures. Examples: Most basalt magma (almost dry) forms at high temperatures deep in the mantle. At a mid-oceanic ridge, hot mantle peridotite rises, and the new lower pressure causes it to melt to form basalt magma. Deep in a subduction zone, water boils off the subducted slab and lowers the melting temperature of overlying hot mantle peridotite. The newly formed basalt magma rises into the continental crust, heats it, and partly melts it with the little available water to form granite (rhyolite) magma. Both basalt and rhyolite magmas rise to erupt in stratovolcanoes, the large, steep-sided volcanic cones like those of the Cascades Fig. 6-BTN1, p. 136

Viscosity: resistance to flow High viscosity magmas are thick and pasty Depends on chemical composition, internal arrangement of atoms and molecules Silica tetrahedra bonds are strong  rigid silicate structures  higher viscosity Differences in viscosity mainly due to differences in amount of silica Higher percentage of silica  higher viscosity

Figure 6-2: Molecules Unstick
Four oxygen atoms (blue) surround each silicon atom (gray) to make a silica tetrahedron (shape outlined by dashed lines). Other silicon atoms share oxygen atoms (blue-green atom shared) to form mineral molecules. A water molecule reacts with a silicate structure and its shared oxygen atoms, breaking some of the strong bonds. Fig. 6-2, p. 137

Broken silicate structure
Between tetrahedra Broken silicate structure Formerly shared oxygen atoms Break bonds 1 hydrogen attaches to a shared oxygen, breaking shared bond Added water molecule: (1 oxygen + 2 hydrogens) Hydrogen Figure 6-2: Molecules Unstick Four oxygen atoms (blue) surround each silicon atom (gray) to make a silica tetrahedron (shape outlined by dashed lines). Other silicon atoms share oxygen atoms (blue-green atom shared) to form mineral molecules. A water molecule reacts with a silicate structure and its shared oxygen atoms, breaking some of the strong bonds. Oxygen Stepped Art Fig. 6-2, p. 137

Basalt magma: brown to black, low viscosity, around 50% silica (fewer silica tetrahedra) Andesite magma: intermediate, around 60% silica Rhyolite magma: white or pale shades, high viscosity, around 70% silica (rigid frameworks of silica tetrahedra)

Table 6-1, p. 137

Temperature affects viscosity As magma cools, more bonds form between atoms and molecules  magma becomes more viscous Temperature of oC: basalt lava pours downhill, spreads across flat ground Temperature of oC: rhyolite magma erupts with dull red heat

Figure 6-3: Red-hot Lava A. Lava erupting as a bright-red curtain from a Hawaiian rift zone. B. Lava flowing downslope as a fiery stream. Fig. 6-3, p. 138

Volatiles: dissolved gases in magma Water vapor, carbon dioxide, other gases Water at high temperatures of magma expands violently to form steam when reaches low pressure near Earth’s surface At 20 km depth, rhyolite magma can hold 2% water vapor At Earth’s surface, magma can hold almost no gas – comes out of solution and forms bubbles

Volatiles: dissolved gases in magma Bubbles of volcanic gas can escape easily from low viscosity basalt magma Bubbles of volcanic gas remain trapped in high viscosity rhyolite magma Rhyolite magma explodes into clouds of steam, foamy pumice and white rhyolite ash

Figure 6-4: Pressure Release
and (b) represent steam separating from a magma in open bubbles. (c) and (d) show that the bubbles grow and the magma begins to froth, expand, and rise. That pushes magma upward, and its pressure decreases, launching a chain reaction of increasingly rapid bubble formation that leads to an explosive eruption. Fig. 6-4, p. 139

(d) Numerous steam bubbles (c) (b) Few (a)
Figure 6-4: Pressure Release a) and (b) represent steam separating from a magma in open bubbles. (c) and (d) show that the bubbles grow and the magma begins to froth, expand, and rise. That pushes magma upward, and its pressure decreases, launching a chain reaction of increasingly rapid bubble formation that leads to an explosive eruption. (b) (a) Stepped Art Fig. 6-4, p. 139

Volume determines magnitude of eruption Viscosity and volatiles determine eruptive style Composition and volume of magma in subduction-zone volcanic eruption depend on: Rate of subduction Temperature and water content of descending slab Composition, temperature and water content of overlying crustal rocks Ease with which magma can rise through crust Other intangible factors

Tectonic Environments of Volcanoes
Plate boundaries are common locations of change in temperature, pressure or water content Most volcanoes are along plate boundaries – subduction zones or spreading zones Other volcanoes are above hotspots (not at plate boundaries) Very few volcanoes at continent-continent collisions or transform boundaries Spreading zones typically have peaceful eruptions Subduction zones typically have violent eruptions

Spreading Zones Mid-oceanic ridges (as in Pacific or Atlantic) erupt basaltic lavas onto adjacent ocean floor – little threat or significance to humans, except in Iceland where Mid-Atlantic Ridge extends above sea level Fissure along center of Iceland opens and erupts every few hundred years, most recently in 1783 At oceanic ridges, peridotite from mantle rises to partially melt and form basalt magma Basalt lava erupting into water forms pillow basalt

Spreading Zones Flood basalt flows on continents have volumes hundreds of times bigger than ordinary basalt flows, covering thousands of square kilometers Continental rifts (Basin and Range in Nevada, Rio Grande Rift in New Mexico, East African Rift Zone) spread much more slowly than oceanic rifts, produce less magma Lava erupts along rift-zone faults Forms small volcanic cinder cones

Subduction Zones Locations where oceanic plates slide under oceanic or continental plates Widespread, most active volcanism Most spectacular, most hazardous Cold ocean-floor lithosphere descends into warmer mantle Ocean-floor rocks contain water that begins to boil off at depths of about 100 km Water rises into mantle rocks under overlying plate, changes melting temperature

Subduction Zones Mantle peridotite melts to form basalt magma
Basalt magma rises through overlying crust – can melt granitic rocks to form rhyolitic magma Basaltic magma and rhyolitic magma may both erupt Basaltic magma and rhyolitic magma may mix to form andesitic magma Water still contained in andesitic or rhyolitic magma may cause it to erupt violently

Hotspots Far fewer in number but large in volume
Within tectonic plates at random locations Active hotspot volcano lies at end of series of older inactive volcanoes Source of magma is in relatively stationary asthenosphere, under moving lithospheric plate Hotspot under oceanic lithosphere: Basalt magma from mantle peridotite Peaceful eruptions Hotspot under continental lithosphere: Basalt magma mixes to form rhyolitic or andesitic magma Dissolved water in magma drives explosive eruptions

Volcanic Eruptions and Products
Characteristics of volcanoes (size, steepness and eruption products) depend on: Magma volume Viscosity Volatile content Eruption products include: Lava Pyroclastic materials (ash, pumice, pyroclastic flow deposits) Lahars (volcanic mudflows)

Figure 6-6: Lavas, Ash, and Mud
Most volcanoes produce a broad range of hazards. Fig. 6-6, p. 141

Table 6-2, p. 141

Nonexplosive Eruptions: Lava Flows
Basaltic magma usually erupts as lava flow May spill out from central crater or pour from rift Hawaiian-type lava Pahoehoe: smooth ropy or billowy surface Aa: rubbly and clinkery surface, very sharp edges

Figure 6-7: Lava Types Smooth-topped pahoehoe lava in cross section (bottom) and flowing on Kilauea (top). A ragged, clinkery-looking surface of “aa” lava in cross section (bottom) and from Mauna Loa Volcano, Waikoloa, Hawaii (top). Fig. 6-7, p. 142

Explosive Eruptions: Pyroclastic Materials
Fragments of solidified magma from explosive eruptions More viscous, greater gas content  more likely to explode As it approached surface, steam bubble froth trapped in viscous magma expands to form foam of glassy bubbles: pumice Bubbles continue to expand: pumice bursts into pyroclastic ash particles and volcanic gases

Finer fragments of pyroclastic material deposited as air-fall ash Heavier pyroclastic fragments may collapse downhill as pyroclastic flow Travels distances up to 20 km at high speeds Destroys everything in path Ash and pyroclastic deposits on flank of volcano may mix with water from rain or melted snow to form lahar or mudflow

Figure 6-8: Saint Helens Blows
A. This explosive eruption of Mount St. Helens on July 22, 1980, pushed an ash cloud to a height of 14 kilometers, dwarfing the mountain visible in the lower right. B. A fragment of volcanic ash from the main Mount St. Helens eruption in 1980, magnified about 3,000 times. Fig. 6-8, p. 143

Size of ash eruption depends on Amount of magma Magma viscosity Water-vapor content Volcanic Explosivity Index: size, volume, violence

Table 6-3, p. 144

Styles of Explosive Eruptions
Phreatic eruptions Violent steam-driven eruptions generated by vaporization of shallow groundwater – no magma Phreatomagmatic eruptions Magma incorporates groundwater Water-rich, extremely dangerous eruption

Strombolian eruptions Expanding steam bubbles blow magma to cinders and blocks, fall around vent to form cinder cone Stromboli, off west coast of Italy Vulcanian eruptions Dark eruption clouds blow out blocks of volcanic rock Mostly ash falls, some pyroclastic flows, lateral-blast eruptions Vulcano, off north coast of Sicily

Pelean eruptions Violent rhyolitic or andesitic eruptions with high columns of ash that collapse to form pyroclastic flows Mount Pelee in Martinique, West Indies Plinian eruptions Powerful continuous blasts of gas, carrying huge volumes of pumice high into atmosphere Silica-rich ash falls, pyroclastic flows, pumice flows Ejection of huge volume of magma may cause ground surface above magma chamber to collapse down, forming caldera

Types of Volcanoes From size and slopes of volcano, can infer magma composition, volatile content Type of volcano reflects eruptive style, associated hazards, risks

Table 6-4, p. 146

Figure 6-11: Volcanoes in All Sizes
The different types of volcanoes have dramatically different sizes (or volumes). Mauna Loa, the giant shield volcano in Hawaii, is roughly 220 kilometers in diameter and rises some 9,450 meters above the seafloor; the 4,169 meters above sea level is less than half its total height. The gentle slopes of Mauna Loa are typical of giant shield volcanoes. A typical subduction-zone volcano such as Mt. Rainier, on a base near sea level is only 2,000 to 3,000 meters high. Cinder cones are still smaller. Fig. 6-11, p. 146

Shield Volcanoes Persistent basalt eruptions of low viscosity basalt eventually build gently sloping pile of thin flows Supposed resemblance to Roman shield Flows characterized by low viscosity, low volatile content, broad and gently sloping sides, large to giant volumes Lava does not flow from peak but from three eruptive rifts radiating from central summit

Figure 6-12: A Volcano Sags and Splits
Mauna Loa, the dominant volcano on the Big Island of Hawaii, erupts along three spreading rift zones. The sagging flanks (three arrows) of the crudely three-sided volcano maintain spreading at the three rifts, permitting basalt lavas to erupt and flow, like dark fingers, downslope from the rifts. Fig. 6-12, p. 147

Shield Volcanoes Mauna Loa and Kilauea: Basalt Giants over an Oceanic Hotspot Mildest of eruptions, 33 times since 1843 and continuously at Kilauea since 1983 Three stages of activity Long series of eruptions below sea level building broad base of volcano, little strength Basalt lava flows build main mass of volcano As volcano moves off hotspot, smaller and less frequent eruptions

Shield Volcanoes Mauna Loa and Kilauea: Basalt Giants over an Oceanic Hotspot Activity at Kilauea moved from Kilauea crater to Pu’u O’o crater on East Rift Southern flank of Kilauea has broken along a rift and is slowly sliding toward ocean Impending eruptions announced with swarms of small, shallow earthquakes, harmonic tremor of magma movement, inflation of volcano summit Rarely dangerous – exception of hot gas surge that killed some of King Keoua’s army in 1790

Figure 6-13: Frequent Eruptions
This map of Kilauea shows the summit caldera, the southwestern and eastern rift zones that erupt most of the lava flows, and major fault zones on which the south side is sliding toward the ocean. Note that almost all eruptions occur along rifts along the main ridges, with lava flows spilling down their flanks. The flows in 1986 destroyed subdivisions of Royal Gardens and Kalapana. Eruptions of Pu’u O’o crater, Kilauea volcano, Hawaii, on the East Rift Zone send basaltic lava flows downslope. Fig. 6-13, p. 147

Shield Volcanoes Mount Etna, Sicily
Largest continental volcano on Earth Most active volcano in Europe, except Stromboli Flank eruptions have built three radial ridges (like Hawaii’s) Cinder cones on ridges erupt cinders and lava flows Continuous eruptions include some violent, sub-Plinian episodes

Figure 6-16: Mount Etna This is all that remains of a house that was in the path of a 1983 Mount Etna lava flow north of Nicolosi, Sicily. Mount Etna erupted a prominent plume of dark ash on October 30, 2002, while fires were ignited by lava pouring down its north flank. Light-colored plumes are gas emissions from a line of vents along a rift extending out from the summit. Roads and towns on the flanks of the volcano are faintly visible in the upper right and across the bottom of the photo. Fig. 6-16, p. 149

Cinder Cones Also from basalt
Characterized by small size, low viscosity, steep sides, moderate volatile content Lava and cinders erupt from vent, fall around in loose, steep-sided pile Usually erupt only over one short period, few months to few years Build pile of cinders m high, followed by basalt flow from base Basalt material degrades to form fertile soil

Figure 6-17: Cinder Cone Volcano
Sunset Crater, near Flagstaff, Arizona, a typical cinder cone, shows a smooth-sided cone capped by a central crater. Fig. 6-17, p. 150

Stratovolcanoes Large, steep-sided cone
Characterized by moderate volume and size, moderate viscosity and slope, moderate to high volatile content Moderate-viscosity magma: lava flows solidify Large eruptions of ash and blocks build cone close to vent Mostly in coastal chains above subduction zones Eruption behavior, intervals vary dramatically

Figure 6-20: Stratovolcano
Mount Griggs in the Aleutian Range of Alaska is a stratovolcano. Fig. 6-20, p. 151

Lava Domes Rhyolitic volcanoes characterized by small to moderate size, high magma viscosity, steep flanks, low to moderate volatile content Rhyolite magmas emerge slowly, with little steam, to solidify in large dome Extremely viscous magma within may continue to rise, forcing pieces of dome to break off Broken pieces may tumble downhill as pyroclastic flow Over time, form loose, rubbly, steep slopes

Figure 6-21: Sources of Searing Ash
These four sketches show common mechanisms that generate pyroclastic flows. Fig. 6-21, p. 152

Magma rises into vent with resulting collapse
Continuous eruption with continuous or intermittent column collapse (e.g., Mount St. Helens, 1980, after initial blast) Magma rises into vent with resulting collapse Landslide Magma Landslide of bulge releases pressure on magma, initiates eruption (e.g., Mount St. Helens, 1980) Ash flow Collapse of dome with or without gas explosion (e.g., Mt. Pelee, 1902; Unzen volcano, Japan 1993) Figure 6-21: Sources of Searing Ash These four sketches show common mechanisms that generate pyroclastic flows. Stepped Art Fig. 6-21, p. 152

Giant Continental Calderas
Rhyolitic volcanoes characterized by high magma viscosity, high volatile content, gently sloping flanks Yellowstone National Park: typical giant rhyolite caldera Erupt rhyolite in enormous volume, mostly explosively, until magma chamber has emptied enough for ground surface to collapse in, creating depression in landscape

Giant Continental Calderas
New magma filling magma chamber may create resurgent dome Eruptions are infrequent – at intervals of hundreds of thousands of years Can inject enough ash, volcanic gas into upper atmosphere to change climate

Figure 6-22: Collapse Into a Magma Chamber
A. The ground above an erupting rhyolite magma chamber subsides to make a caldera during the eruption of a giant pyroclastic flow; it then domes, or resurges, again as new magma refills the magma chamber. B. A small (2.5 km diameter) caldera in Kaguyak volcano on Katmai National Park, Alaska. A prominent lava dome rose in the caldera after collapse. C. The 32-kilometer-diameter Long Valley Caldera of southeastern California, as seen from rim to rim. Its tree-covered resurgent dome is behind the tree on the right. Fig. 6-22, p. 153

Magma Eruption of rhyolitic ash flows from ring fracture: partial evacuation of magma chamber Magma Caldera collapse along ring fracture zone Pyroclastic flow deposits partly fill the caldera Magma Resurgent doming Figure 6-22a: Collapse Into a Magma Chamber A. The ground above an erupting rhyolite magma chamber subsides to make a caldera during the eruption of a giant pyroclastic flow; it then domes, or resurges, again as new magma refills the magma chamber. B. A small (2.5 km diameter) caldera in Kaguyak volcano on Katmai National Park, Alaska. A prominent lava dome rose in the caldera after collapse. C. The 32-kilometer-diameter Long Valley Caldera of southeastern California, as seen from rim to rim. Its tree-covered resurgent dome is behind the tree on the right. Stepped Art Fig. 6-22, p. 153

Deadly Lahar: Mount Pinatubo, Philippines, 1991
Andesitic volcano, 90 km from Manila Previous 400 years: no eruptions Philippine volcanologists and U.S. Geologic Survey began monitoring volcano Geologic mapping showed 600-year-old pyroclastic flows across densely populated areas and Clark Air Force Base

Frequency of earthquakes increased, moved higher in volcano Volume of sulfur dioxide emissions increased Occasional small pyroclastic flows swept down from volcano Major eruption seemed imminent within two weeks Evacuation recommended for area within 10 km of summit

Lava dome began growing, harmonic tremor increased Evacuation recommended for area within 30 km of summit Plinian eruption began June 12 with huge plume of steam and ash, towering km Ash layers 30 cm thick blanketed region Typhoon Yunya’s intense rain mixed with ash to collapse roofs, form lahars

p. 154

Figure 6-CIP01c: The satellite view, below, of Mount Pinatubo shows the distribution of mudflow deposits a few months after the eruption. The extent of one-centimeter-thick ash is shown in yellow dashes. The map of Mount Pinatubo shows the centimeter-depths of ash laid down between June 12 and 15, 1991, and the mudflow deposits two months after the eruption. p. 154

Twenty million tons of SO2 combined with water in atmosphere to make droplets of sulfuric acid, reflecting incoming ultraviolet radiation Mean global temperatures dropped up to 1oC Spectacular sunsets for few years 58,000 people evacuated 350 people killed, mostly from ash-collapsed buildings Later 932 people killed by disease Timely warning and broad evacuations saved tens of thousands of lives

Figure 6-CIP01f: Dust and SO2 generated by the Pinatubo eruption encircled the Earth, scattering incoming sunlight and reducing global temperature. Pinatubo is at the red dot in left view. p. 155

<10–3 10–2 >10–1 April 10-May 13, 1991 June 15-July 15, 1991
August 23-September 30, 1991 Figure 6-CIP01f: Dust and SO2 generated by the Pinatubo eruption encircled the Earth, scattering incoming sunlight and reducing global temperature. Pinatubo is at the red dot in left view. <10–3 10–2 >10–1 Stepped Art p. 155

Long Periods between Collapse-Caldera Eruptions: Santorini, Greece
Now appears as ring of islands along rim of submerged caldera Twelve major explosive eruptions in last 360,000 years, about one every 30,000 years Caldera collapse, followed by growth of new andesite volcano, up to next eruption and collapse

Long Periods between Collapse-Caldera Eruptions: Santorini, Greece
Approximately 1620 B.C.: Series of catastrophic Plinian eruptions of rhyolite ash and pumice, culminating in caldera collapse Eruptions may have lasted weeks Area buried under several tens of meters of pumice and ash Town of Akrotiri destroyed, possible source of Atlantis legend Modern town of Thera built into caldera wall

Figure 6-CIP02b: A. Santorini’s main islands surround the caldera that collapsed during the great eruption of 1620 B.C. The eruption ended the Minoan civilization. B. The inside wall of Santorini caldera exposes white Minoan pumice at the top right. White houses along the lower caldera rim are built mostly into the same pumice erupted in 1620 B.C. p. 157

Future Eruptions of a Giant Caldera Volcano
Yellowstone Volcano, Wyoming: typical giant rhyolite caldera Eruptions occurred 2 million, 1.3 million and 640,000 years ago Resurgent caldera eruptions: by far largest, most destructive of all volcanic eruptions – nothing comparable in historic time (1000 times volume of 1980 Mount St. Helens eruption) Ash from most recent eruption covered much of North America

Future Eruptions of a Giant Caldera Volcano
If such an eruption occurred today: Destruction or disruption of transportation, communication, energy systems throughout U.S. Huge volumes of ash in atmosphere would block sunlight, lower temperatures  agricultural disaster and famine Since 1870, thermal areas getting hotter Resurgent bulges in caldera rise and fall over time Will erupt again, but no one knows when

Figure 6-CIP03b: The Lava Creek ash erupted 642,000 years ago during collapse of the immense Yellowstone Caldera. The northwestern rim of Yellowstone Caldera formed during that collapse. Yellowstone Canyon has eroded down through 1,200 feet of the rhyolite ash that erupted and filled the Yellowstone Caldera. p. 158

Figure 6-CIP03c: he Lava Creek ash from the Yellowstone Caldera, at the northeastern end of the Snake River Plain, covered most of the central plains of the United States. The hotspot track is marked by the white dotted line. p. 159

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