Topic 4 - Volcanism and Volcanoes Outline Introduction Volcanic Products: Gases, Lava and Pyroclastics Non-Explosive vs. Explosive Eruptions Types of Volcanoes Volcano Distribution Prediction of Volcanic Eruptions Index of Volcanic Explosivity
Volcanoes produce a variety of hazards that kill or injure people and destroy property. Some hazards, such as landslides or lahars, may occur when a volcano is not erupting.
Mudflow / lahars.
Death e.g. Mount Pelée - May 8, 1902 St. Pierre, Martinique, West Indies, Caribbean 28,000 casualties
Only 2 out of the city’s 28,000 residents survived. Nu ée ardente also burned ships in the harbor.
Introduction Volcanic activity on a world wide scale is a relatively rare process. Usually occurs in sparsely populated areas. Therefore these eruptions do not cause significant damage. However, if and when volcanic eruptions occur in densely populated areas, great catastrophes result. volcanism Designates the aggregate of processes associated with the transfer of material from the Earth’s interior to the surface and the construction of various structures at the surface.
volcano A vent or fissure through which molten and solid material and hot gases pass upward to the Earth’s surface. Forms a conical mountain. Many shapes and sizes. A volcano is a relatively high topographic feature. Built as a result of the addition of volcanic material. A volcano can have a small crater in the center. Or a larger feature called a caldera. crater A circular depression at the summit of a volcano. Generally less than 1 km in diameter.
caldera A very large volcanic crater. > 1 km in diameter. May form via the coalescence of several smaller craters, repeated explosions, collapse or the stoping of surface rocks by a large underground magma chamber. e.g. Crater Lake, Oregon (mis-named…should be Caldera Lake). Mount Mazama erupted 6600 years ago. Great volume of material collapsed and was lost. Depression that remains is filled with water and is now called Crater Lake. Wizard Island is the new volcanic cone. e.g. Yellowstone is a caldera. Which has swelled up again as new magma is introduced to the chamber underneath. Supervolcano. Volcanoes and extrusive igneous rocks can build various kinds of landforms. More on this following the next section.
Events leading to the origin of Crater Lake, Oregon.
Caldera Formation: Crater Lake
Yellowstone caldera. Formed approximately 600,000 years ago. Supervolcano .
Tephra or pyroclastics
usually first stage of volcanic activity is the release of gases
major constituent of volcanic gas is superheated steam
constitutes 50-95% of the volume
some of this steam is:
recycled ground water
from reaction of the hydrogen from the magma with the oxygen of the atmosphere
some is juvenile water
water formerly dissolved in molten rock (magma) deep beneath the Earth’s surface
Other examples of volcanic gases: CO 2 , N 2 , SO 2 , SO 3 , CO, HCl, H 2 S, CH 4
Early Earth’s atmospheric composition.
Hazardous gases rarely reach populated areas in toxic concentrations, however:
SO 2 can react in the atmosphere: acid rain downwind.
Some gases may be absorbed by volcanic ash: which then falls on land. May be incorporated into plants and animals and ultimately humans. e.g. fluorine (as HF).
Acid gases may produce detrimental effects on vegetation or kill the vegetation. e.g. Kilauea in Hawaii damaged plum and other fruit trees at distances up to 30 miles way.
Trees killed by carbon dioxide rising from beneath Mammoth Mtn volcano, CA.
Dormant volcanoes may emit gases for long periods of time. e.g. Lake Nyos in Cameroon, 1986. Gas dissolved and kept in the bottom sediments by the hydrostatic pressure. Shaking of ground from a small earthquake, led to a subaqueous land slide and release of gases from the bottom sediments. Release of gas, believed to be CO 2 (colourless, odorless), killed approximately 2000 people and numerous animals. CO 2 denser than air, therefore flowed downhill along the surface.
molten rock material originating deep beneath the Earth’s surface
magma rises because it is lighter than the surrounding rock (dissolved gases)
works its way to the surface along fractures or dissolving rocks in its path
eventually this molten rock material may escape through a central vent or elongate fissure at the Earth’s surface… lava
Lava tube : hollow beneath a lava flow.
Collapsed roof of a lava tube. Can observe the active flow underneath.
2 main types of subaerial basaltic lava flow
flow has a ropy texture and smooth surface
lower viscosity than aa flows
flow is rough and jagged
angular blocks and fragments are common
depending on their composition and temperature, magmas differ greatly in their viscosity
Aa flow Pahoehoe flow
Pahoehoe lava flow in Hawaii.
Aa lava flow advances over an older pahoehoe lava flow in Hawaii.
Pillowed Lava Flow
lava flow that formed under water
interaction of water with the molten lava forms a thick glassy crust around the flowing lava
form tube structure: pillowed lava flow
Pillow lava: modern versus ancient examples.
Thick massive lava flow cooling.
heat contraction cracks
perpendicular to cooling surface
Tephra or Pyroclastic Debris Solid material. Solid rock material ejected from volcanoes consists of particles torn from the walls of the volcanic vent (both volcanic and non-volcanic rocks). Lava clots sprayed into the air cool rapidly and are solid by the time they reach the ground. tephra A collective term designating all particles ejected from volcanoes. Irrespective of size, shape or composition. pyroclast An individual particle ejected during a volcanic eruption. Classified according to size.
Pyroclast classification was presented in the previous lecture on igneous rocks. Given below, one more time. May range in size from: dust < 1/16 mm ash 1/16 to 2 mm lapilli 2 to 64 mm blocks or bombs > 64 mm Blocks are angular (originally were pieces of rock). Bombs are rounded or streamlined (originally globs of fluid). Dust ash emitted during a volcanic eruption may be blown high into the air. Can be carried great distances by the prevailing winds.
Lapilli Volcanic Bomb Ash
e.g. Mount Pinatubo, Philippines (June 15, 1991). Ash was sent into the upper atmosphere. Resulted in cooler temperatures. Ash was visible from airplanes at cruising altitude (33,000 feet): haze. Lahar : a mudflow composed of volcanic material such as ash (has consistency of wet concrete). Marella River valley, Philippines. Tropical storm followed the volcanic eruption.
Homes partly buried by a lahar . Mount Pinatubo, Philippines. June 1991.
Lahars inundated Armero, Columbia on November 13, 1985. 23,000 were killed. Second most tragic volcanic event of the 20 th century. (Mount Pelée - May 8, 1902: 28,000 casualties)
Mt. St. Helens May 18, 1980 (a) Location map. (b) Mudflows. (c) Tree blowdown. (d) Ash cloud.
Nu é e ardente – defined as a swiftly flowing, turbulent gaseous cloud, sometimes incandescent, erupted from a volcano, and containing ash and other pyroclastic materials. Density current of pyroclastic flow. “Glowing cloud”. Mt. St. Helens 8:45 am May 18, 1980
Car incinerated by hot pyroclastic flows and buried in ash. Note sun-visor remains (i.e. metal frame).
Day turned to night in nearby towns: May 18, 1980. Yakima, Washington. 85 miles downwind.
Ash clogged air filters – vehicles abandoned.
Shoveling heavy ash (not snow!) – density at least 2.5 x that of snow.
Dust and ash from Mt. St. Helens eruption. Erupted Sunday May 18, 1980. May long-weekend in Canada (Monday May 19 th - Victoria Day). Ash reached Brandon by Monday May 19 th . Street lights came on in the day time on Tuesday May 20 th .
View from Brodie Building - looking north.
View from Brodie Building - looking north. Monday May 19, 1980.
Non-Explosive vs. Explosive Eruptions Type of volcanic eruption depends on whether gases escape easily or are confined . Depends on their composition. Magmas differ greatly in their fluidity (inverse of viscosity). Magma viscosity is determined mainly by the silica content. As discussed earlier there are three categories of magmas based on SiO 2 content. This relationship is also applicable to viscosity: 45-52 % low viscosity Hawaii, shield volcanoes, basaltic 53-65 % composite volcanoes e.g. Mt. St. Helens > 65 % high viscosity e.g. Mt. Lassen, California Two types of eruptions are the result: “ non-explosive ” and explosive .
“ Non-Explosive” Eruptions If a magma flows easily (low viscosity), gases that come out of solution have little difficulty rising through the melt and escaping. Thus reducing the possibility of a pressure build up and explosion. Steady escape of gases. Low silica, viscosity. Molten rock flows “quietly” as fluid lava. Flows may travel up to 100 km/hr down steep slopes. e.g. Hawaiian lava. “Hot-spot” volcanism. Mafic lava. Basalt. e.g. Iceland lava. Mid-oceanic ridge volcanism. Mafic lava. Basalt.
e.g. “Non-Explosive” Eruptions Heimaey, Iceland .
First day of the Heimaey, Iceland volcanic eruption: January 23, 1973.
Eruption on Heimaey, Westman-Islands began on January 23, 1973.
Explosive Eruptions In contrast, the escape of gases from high viscosity magma is hindered. Gas pressure may build up to a critical level. Finally released as an explosion. e.g. Mt. St. Helens . Andesite to rhyolite (i.e. dacite). Moderate to high silica content. Moderate to high viscosity. At the other extreme, explosive release of trapped gases when the magma reaches the surface may disintegrate the magma into blebs and clots of molten magma, partly solidified magma and fragments of the walls of the volcanic vents are blown into the atmosphere.
Lateral blast. Mt. St. Helens viewed from the northeast: May 18, 1980.
Mt. St. Helens viewed from the south: May 18, 1980.
Mt. St. Helens Before After
Types of Volcanoes Lava and pyroclastic debris. Accumulate in various proportions around volcanic vents. Builds up various landforms. Five key landforms: Cinder Cones, Shield Volcanoes, Composite Cones or Strato-volcanoes, Lava Dome, Fissure Eruptions.
Cinder Cones Mostly pyroclastics. Cones made as material is blasted out and then settles to the ground. Composed of ash and cinders. Small to moderate size. e.g. Springerville, Arizona.
Cinder cone (230 m high) in Lassen Volcanic National Park, CA.
View from the top of the cinder cone in Lassen Volcanic National Park, CA.
Cinder cones in Iceland: Eldfel and Helgafel. Active in 1973.
Shield Volcanoes Mostly lava. Low relief. Slope of 3.5º near the top of the volcano. Lava may flow downslope in lava tubes. “ Non-explosive”. Basaltic. 45 - 52 % SiO 2 . e.g. Mauna Loa, Hawaii. Total relief is nearly 34,000 feet. 160 km in diameter. Equal height above and below seawater. e.g. 5 km below sea level and 4.1 km above sea level.
Shield volcano . Each layer consists of numerous thin basalt lava flows.
Mauna Loa, active shield volcano on Hawaii, with its upper 1.5 km covered by snow.
Crater Mountain is an extinct shield volcano in Lassen County, CA. Approximately 10 km across and 460 m high.
Composite Cones or Stratovolcanoes Interbedded lava and pyroclastics. Formed from intermittent eruptions. Steep-sided volcanic cone (30º). This equals the angle of repose. Therefore can not get any steeper for pyroclastic deposits. Produced by alternating layers of pyroclastic debris and lava flows. Interbedded pyroclastics and intermediate silica lavas (» 60 % silica). Produce majestic peaks in the Andes, Cascades, Alaska, B.C. and Japan. e.g. Mt Fuji, Mt. St. Helens, Mt. Rainier. Composite volcanoes are responsible for most volcanic hazards. Why are composite volcanoes responsible for most volcanic hazards? Explosive eruptions. Common along convergent plate boundaries. Usually near coastal areas: densely populated.
Mayon volcano, Philippines Mount Shasta, California Mount Shasta, California Composite Volcanoes or Stratovolcanoes
Nyiragongo, Democratic Republic of Congo, Africa. Composite volcano.
Mount Pinatubo, Philippines Composite volcano . June 1991
Lava Dome Dome shaped. Steep sided. Made up of very viscous lava. High silica content (> 65 %). Rhyolitic composition. Very explosive and dangerous: plug the volcanic vent. e.g. Mt. Lassen, California. Chaos Crags, Lassen Volcanic National Park, California. Four lava domes .
Steep-sided lava dome , Alaska.
Active Fissure Fissure Eruptions Lava erupted through long fissures are termed “fissure eruptions”. More common in the geological past: Miocene and Pliocene (5-17 mya). Form “basalt plateaus”. e.g. Columbia River Basalts in Oregon and Idaho. Up to 1000 m thick. Most recent example in Iceland. 930 A.D. and 1783 A.D. Extensive basalt flows.
Fissure eruptions . Columbia River basalts (> 20 lava flows) exposed in the canyon of the Grand Ronde River, Washington. Pyroclastic flows from Mount Pinatubo (June 1991) filled this valley to a depth of up to 200 m.
Volcano Distribution Principle active volcanoes of the Earth are located at the boundaries of lithospheric plates. Formation of new crustal material. Takes place between separating crustal plates by volcanic processes. This helps to explain the distribution of some active volcanoes. Much volcanic activity in zones of plate separation occur beneath the sea. But in places, such as Iceland, it is above sea level. Approximately 80% of active volcanoes occur in the Circum Pacific Belt (“Ring of Fire ”). Extending along western S.A., western N.A., Japan, Indonesia, etc. Note: no volcanoes along the collision zone between India and Asia. Continent-to-continent convergent boundary. Note: Africa to Europe collision zone does have some volcanoes. Ocean crust is being subducted.
As part of the “ Ring of Fire ” belt in the U.S., a number of volcanoes form part of the Cascade Range, California, Oregon and Washington. Many of these volcanoes have erupted in historic times, examples include: Lassen Peak, California 1914, 1915 Mt. Baker, Washington 1843, 1854, 1858, 1870 Mt. St. Helens 1843, 1980 Mt. Rainier 1820, 1843, 1846, 1854, 1858, 1870, 1894 Mt. Gaschaldi 10,000 years BP Mt. Meager 2500 years BP Stikine Belt, B.C. 200, 1340 years BP Yukon, White River Ash 1200, 1500 years BP
Mudflows are a significant hazard for the next eruption of Mt. Rainer: Seattle and Tacoma, Washington. Tacoma Seattle
Mid Ocean Ridges Basaltic magma derived from the mantle. Shield volcanoes produced. e.g. Iceland. Mid Atlantic Ridge. Volcanoes occur along divergent and convergent plate boundaries. Also occur (more rarely) in intraplate areas (e.g. Hawaii). Three main areas of distribution: - Mid Ocean Ridges - Subduction Zones - Intraplate Volcanism
Subduction Zones Andesitic volcanoes. Magma is mixed with both oceanic and continental crust. Intermediate silica composition. Pacific Rim of North America: California, Oregon, Washington, B.C. Related to the subduction of the Juan de Fuca plate. Ocean plate has to be subducted for melting to occur. Note: this can occur at ocean-ocean or ocean-continent boundaries, not continent-continent.
Intraplate Volcanism e.g. Hawaii. “Hot spots”. Basaltic lavas. Shield volcanoes. Hawaiian Islands are anomalous. They lie within, rather than at the margin of the Pacific Plate. Seem to be related to a “hot spot” in the underlying mantle. Forms a string of volcanoes as the Pacific plate migrates northwest.
Tectonic Settings and Volcanic Activity
Prediction of Volcanic Eruptions Generalization: volcanic eruptions are predicted with a much higher success rate than earthquakes. If volcanic eruptions can be predicted then it may be possible to significantly reduce the loss of life or property damage associated with an eruption. People can be evacuated from a threatened area and moveable property taken way from the area. Predictions of the time at which a volcanic eruption is likely to occur are of two types: General vs. Specific. A general prediction consists of a statement that a volcano is likely to erupt in the near future. “Near future” may be from hours to years. Does serve to alert the population to the potential hazard.
However, there are many possible ramifications. Loss of business (e.g. tourism). Volcanologist under extreme pressure to get the right answer. Loss of life, if the evacuation is not successful. Predictions of volcanic eruptions can be based on several lines of evidence: Past History Behaviour of Fumaroles Magnetic Properties of the Rock Temperature Swelling Earthquakes
Past History Where a long enough record of past history for a particular volcano exists it may be found to have a more or less regular period of eruptive cycles. In that case, once a period of average quiescence is completed, another eruption can be expected in the near future. Probably not a very accurate predictive tool since the length of quiet periods for individual volcanoes varies greatly . Historical records are not very good. Also, the magnitude of the volcanic eruption also needs to be considered. Magnitude varies . Might be a longer quiet period after a huge blast because the pressure is greatly relieved. Is this a good method for predicting volcanic eruptions?
Behaviour of Fumaroles Not much detailed information. Lack of consistency. Fumarole is defined as a volcanic vent from which gases and vapours are emitted. Occur along fissures. Some poisonous gases. But for some volcanoes, the following have been noted prior to an eruption: Temperature of fumarole gas is found to increase ( T). Volume of gas increases ( gas volume). Composition of the gas changes prior to a volcanic eruption. A change in volcanic gas chemistry preceded a later eruptive phase for Mt. St. Helens: composition, volume, and temperature changes.
Magnetic Properties of the Rocks As magma accumulates beneath a volcano and begins to rise towards the surface preceding an eruption, the heating of the surrounding rock reduces the magnetic properties of the rocks. In fact the rocks may be heated above the Curie temperature ( above which thermal agitation prevents spontaneous magnetic ordering) and they could lose their magnetism. Temperature A possible method of detecting a forth coming volcanic eruption is to detect increases in the temperature by aerial infrared photography repeated at regular intervals. Heavy forest cover can effect the results. Valuable in areas which do not have too heavy a forest cover. Russians have used this technique in the Kurile Islands and Kamchatka. They have suggested that satellites could be used for this purpose. This technique was used on Mt. St. Helens. With some success prior to the main eruption in May 1980.
Swelling When magma pushes its way up beneath a volcano it causes the structure of the volcano to swell. This swelling is detected by measurements of the tilting of the ground surface. Measured using lasers or tiltmeters. Tiltmeters can measure movement as little as 1 mm in 1 km. Two tiltmeters placed at right angles to one another are all that is required to define the direction and amount of tilt. e.g. Kilauea, Hawaii between 1964-1966. North-south and east-west tiltmeter data predicted eruptions prior to their occurrence. Eruption of the Kilauea volcano has occurred at various stages of tilting. Long periods of increased tilting do not always lead to an eruption. This indicates that the magma can recede or it begins to intrude into the flanks of the volcano.
Problem is that at the present time, tilt measurements are only made at a few volcanoes around the world. Distance measurements can augment tiltmeter information. Use lasers to measure distances and combine this information with tiltmeter data in order to get an accurate representation of the magnitude of swelling. Swelling occurred in Mt. St. Helens prior to the March 1980 eruption. On the north side of the mountain. Bulge moved at 1.5 m per day!
Led to the lateral blast . Landslide triggered by the swelling reduced the strength of the mountain wall and directly led to the lateral blast. Could predict with confidence that an eruption was going to occur at some point in time in the future, however were unable to predict the exact date or type. Most geologists felt the eruption would be vertical based on a previous analysis of Mt. St. Helens eruption history. Nobody predicted a lateral blast despite the prominent bulge on the northern margin. Lateral blast was not predicted . Tragic that a geological post was set up and “manned” on the north side of Mt. St. Helens. Post was several km north of the mountain. Same side as the swelling. USGS geologist was killed (David Johnson).
Mount St. Helens, Washington: Debris Avalanche and Eruption
Earthquakes Volcanic eruptions are commonly preceded by earthquakes. Earthquake foci will move upwards towards the surface. As the magma makes its way to the surface. At Kilauea, shallow earthquakes numbering in the thousands per day may precede volcanic eruptions. Called “Harmonic Tremor”. Harmonic tremor is defined as a more or less continuous oscillating ground motion. With a frequency of 0.5 to 10 Hz. Harmonic tremors may be generated by the drag of the magma against the walls of the conduit. At Kilauea it indicates the eruption will follow in a few hours. These shallow earthquakes occur during the period of swelling and continue for periods of several days to several hours, commonly stopping with the beginning of an eruption.
Similar swarms of earthquakes may also occur however during times of magma recession when the top of the volcano is sinking. So shallow earthquakes can only be used to indicate impending eruptions when the character of ground tilting is also known. Major earthquakes preceded the main Mt. St. Helens eruption and increased earthquake activity occurred just hours before a later (August) eruption. Similar pattern before the Mt Pinatubo eruption. Evacuated a US air force base in the Philippines. Pattern of Earthquake Behaviour : (i) earthquakes increased in frequency (ii) earthquakes increased in magnitude and intensity (iii) earthquakes (foci) became shallower
Mt. St. Helens Chronology of events. Predictions and hazard analysis.
Hazard zone designations. April 30, 1980. Approximately 2 ½ weeks before the eruption. Note the prediction of a vertical, symmetrical blast, and the position of USGS geologists .
Actual hazard zones. May 18, 1980. Tree-blowdown area. Pyroclastic flows. Mudflows.
Downed trees near headwaters, south fork, Toutle River: May 19, 1980.
Ash fall and downed trees (parallel orientation of trees).
National Guard identifying victims of Mt. St. Helens.