V o l c a n o es. Part I Composition. Types of deposits. Types of volcanoes Distribution

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1 V o l c a n o es Part I Composition Types of deposits Types of volcanoes Distribution Volcano: A mound of material that is extruded to the Earth s surface from a vent that is connected to a magma chamber via a feeder conduit. Volcanoes are classified according to their form. The form of a volcanoes depends on the type of material that it is made up of. The nature of the extruded material (and the volcano itself) depends on the properties of the magma. Magma: Molten rock within the Earth. Magma is called lava when it reaches the surface. The composition of magma determines the type of rock that forms when it cools and its behavior during an eruption. Main controls on behavior: chemical composition (largely silica dioxide - SiO 2 - content) and gas content (largely water vapor and CO 2 ). SiO 2 content controls the viscosity of a magma. Viscosity: a measure of how easily a fluid flows. Water has a low viscosity, molasses has a much higher viscosity. Viscosity, in turn, controls the amount of gas that can be trapped in the magma. Magma Type Chemical Composition Temperature (degrees C) Viscosity Gas Content The greater the viscosity the more gas in the magma. There are three basic types of magma: Basaltic Magma Andesitic Magma Rhyolitic Magma The names are based on the rock type that forms when the magma crystallizes. Basaltic Andesitic Rhyolitic 45-55% SiO 2 ; High in Fe, Mg, Ca; Low in K, Na % SiO 2 ; Intermediate Fe, Mg, Ca, Na, K 65-75% SiO 2 ; Low in Fe, Mg, Ca; High in K, Na Low Intermediate High Low Intermediate High 1

2 Overall, the behaviour of the magma determines the type of volcano that develops: Low SiO 2 magmas, with little gas and low viscosity, flows readily through their vents and across the land surface when the lava escapes the vents. High SiO 2 magmas, gaseous and with high viscosity, tend to plug their vents until the force of escaping magma blows the vent clear; such magmas cause explosive volcanoes. Types of volcanic deposits (photos from USGS) Volcanoes also vary in terms of the types of deposits that they produce. Lava: Hot (up to 1200 degrees C), fluid,molten rock that flows along the land surface. Lava can flow like viscous water, including forming lava falls. Pahoehoe: Lava with a ropelike surface texture due to partial cooling as the lava flowed. Relatively hot, low viscosity lava. Pahoehoe A thick deposit of pahoehoe lava 2

3 Aa: Blocky, rough lava flow. Due to high viscosity lava that flowed pushing chunks of solid and semi-solid blocks. Lava tube: A tube formed by cooling and solidifying of the lava walls while fluid lava continued to flow inside. Pillows: A form of closed lava tube (with a bulbous end) that forms when a lava flows into water (e.g., a lake or ocean) and cools very rapidly. Ash: tephra that is finer than 2 mm in diameter. Pyroclastic material: Debris formed by a volcanic explosion. Results when magma is very viscous. Tephra: The general term for all pyroclastic material that is ejected from a volcano. Different terms apply according to the size of the tephra. (syn. Ejecta) Lapilli: from 2 mm to 64 mm in diameter. Bombs: soft, partially melted fragments greater than 64 mm in diameter. Blocks: hard fragments greater than 64 mm in diameter. 3

4 Tuff: A deposit made up of ash. Welded tuff: A deposit of pyroclastic material that was laid down while still very hot and particles become fused together. Ash flow: Pyroclastic debris that flows downslope. Lahar: A water saturated slurry of ash and other volcanic debris that flows downslope. Ash fall: Fallout of very fine ash from the air. Volcanic ash fall during mid-day with the eruption of Mount Pinatubo in the Philippines. Nuée Ardente (glowing cloud): A hot, gaseous cloud of ash that flows down slope. Flow speeds can reach 160 km/hr and temperatures can exceed 600 degrees C. Classification of volcanoes Volcanoes are classified according to their morphology. The processes and deposits dictate the morphology of volcanoes. Three types of volcano: Shield volcanoes: dominated by lava flows. Cinder cones: dominated by pyroclastics. Forms an isolated conical mound of tephra. Muana Loa Volcano the world s largest volcano. Photograph by J.P. Lockwood on 1 December Photograph by J.D. Griggs on January 10,

5 Stratovolcanoes: mixture of lavas and pyroclastics. Syn. Composite volcanoes Mount Mageik volcano, Alaska Shield Volcanoes Dominated by fluid, high temperature, low viscosity basaltic magma. Low, dome-shaped profile, like an inverted shield. Photograph by R. McGimsey on 15 July Typical slopes approximately 15 degrees. Lava flows downslope, away from a central vent or a series of vents. Many shield volcanoes have a central caldera: Calderas form after an eruption when the surface collapses. Each caldera is located at the site of a former eruption. Hawaiian Isands and Iceland are built from shield volcanoes. Mauna Loa is the largest volcano on Earth. It makes up most of the island of Hawaii. The volcano rises 4,170 m above sea level. It covers an area of 5,271 km 2. Total volume of rock: 80,000 km 3 USGS Began to form 700,000 to 1,000,000 years ago when lava began to flow to the sea floor. Eruptions reached the surface 400,000 years ago. Its great weight depresses the underlying crust by 8 km. 5

6 Low viscosity lava forms fountains of lava flowing from vents near the volcano summit. The lava flows easily down the gentle slopes.reaching the ocean during some eruptions. Where the lava is relatively cool eruptions form small cinder cones on the volcanoes surface. Cinder Cones Dominated by viscous, gaseous magmas Relatively cool basaltic magmas or andesitic magmas predominate. Mount Edziza, British Columbia Internally constructed entirely of layers of pyroclastic deposits (blocks, bombs, lapilli). Slopes are steep, at angle of repose. Angle of repose: the natural maximum angle that a pile of loose, unconsolidated material will form. Typical angles: 30 to 40 degrees. Range from several metres to over 300 m in height. Commonly associated with old shield volcanoes with a relatively cool, basaltic magma. 6

7 Paricutin volcano began to erupt in a corn field in Mexico in 1943 and continued until The farmer had noticed a fissure (vent) had opened in the field one morning and from it was pouring black ash. In the first year the volcano grew to 336 m (almost 1 metre per day). Rate of growth decreased steadily; by 1952 the volcano was 424 m in height. Volcanoes that alternate between periods of lava flows (constructive phase) and periods of explosive eruptions (destructive phase). Commonly called composite volcanoes because they are made up of both lava and pyroclastic deposits. Steep slopes, at angle of repose or greater. Stratovolcanoes Noemi Emmelheinz 2001 May lay dormant for thousands of years. Mt. St. Helens Before On average, andesitic magmas with a high gas content. Actually, a mix of basaltic and rhyolitic magmas in many cases. Gases add great pressure when the feeder conduit becomes plugged, contributing to the explosive power. Can grow to thousands of metres high during constructive lava flow phases. The constructive phase often ends with a destructive phase an explosive eruption. Mt. St. Helens After Extensive ash falls and ash flows are commonly produced during explosive phases. After an eruption a large caldera remains. Crater Lake is a caldera that remains following an explosive eruption 7,700 years ago. The eruption was 42 times more powerful than Mt. St. Helens. 7

8 Styles of Volcanic Eruption Eruption style is determined by the explosiveness and the height of the column of tephra. Hawiian Eruptions Eruptions dominated by lava flows. No significant ash column. Any explosions are of a small scale. USGS Strombolian eruptions Intermittent explosion or fountaining of basaltic lava from a single vent or crater. Caused by the release of volcanic gases, and they normally occur every few minutes. Vulcanian Eruptions Periodic eruptions (decades apart) that are moderate explosions. Ejecta are hard (not melted). Andesitic, gaseous magmas. Tephra ranges from ash to bombs. Pelean Eruptions Similar to Vulcanian Eruptions but include hot gas clouds (Nuees Ardentes) Plinian Eruptions Very explosive volcanic eruptions; long periods between eruptions. Involve a very viscous, rhyolitic magma. Form a plume of tephra that extends upwards to the stratosphere (>11 km high). Dust can remain in the stratosphere for years and significantly cool the Earth. 8

9 Ultraplinian Eruptions The Distribution of volcanoes Very explosive eruptions that eject approximately 100 km 3 or more of tephra and produce ash columns exceeding 25 km in height. Phreatic Eruptions Eruptions of tephra and large volumes of steam produced when water makes contact with the magma. Water flashes to steam and causes a very violent eruption. The vast majority of volcanoes are located: Parallel to oceanic trenches. Along the oceanic ridge. Over hot spots originating from the mantle. Volcanoes along trenches Examples: Japan, most Pacific Islands, Caribbean Islands, west coast of North and South America. 2/3 of all volcanoes are along the Ring of Fire that surrounds the Pacific Ocean. Volcanoes result from magma rising off the melting subducted plate. The composition of the magma is andesitic (melted basaltic crust plus sediment carried on the crust). Magma is very gaseous, particularly enriched with water vapor. Stratovoclanoes are constructed from feeder conduits extending to the surface. 9

10 Granitic (rhyolitic) intrusions are also formed, becoming trapped within the volcanic pile overlying the region of subduction. Potential for very explosive eruptions. Mt. Fuji, Japan A stratovolcano that has erupted 16 times since 781 AD. The most recent eruption was in cubic km of ash, blocks, and bombs were ejected during that eruption. (Greater than Mt. St. Helens and there were no fatalities). Similar situation on the west coast of North and South America. Volcanoes formed by intrusion into the mountain chains that result from compressive forces between oceanic and continental crust. Ojos del Salado, Chile The world s highest volcano. Perched at 6,887 metres above sea level. A stratovolcano that has not erupted in historic time. Photo by Peter Francis Volcanoes in Canada? There are many inactive volcanoes in the Canadian Rocky Mountains None are erupting at the present time. At least three have erupted over the past several hundred years. For a catalogue of Canadian Volcanoes go to

11 Oceanic Ridge Volcanoes Most volcanic activity is under water. Intrusion of material from the magma chamber creates new oceanic crust as the sea floor spreads. Shield volcanoes occur where volcanic activity extends to the surface (e.g., Iceland). Iceland is growing by volcanic expansion of the ridge. Basaltic pillow lavas dominate the submerged volcanoes. Hekla Volcano in the distant background. Unlike the Hawaiian Island shield volcanoes most of Iceland s lava flows issue from linear fissures: fissure eruptions rather than vent eruptions. Hekla erupted four times in the 20th century, the last time in Hekla covers about 80% of Iceland and its volume is approx. 12 cubic km. Vatnaolder Volcano forms a single peak with the classic shield form. Cinder cones also form on the older portions of the island, away from the most active area that runs along the middle of the oceanic ridge. 11

12 Very small cinder cones, called spatter cones form where small fountains of very fluid basaltic lava extrude material to the surface for relatively short periods of time. Some Icelandic volcanoes are buried beneath glaciers. In 1973 the Eldfell volcano erupted on the island of Heimaey Photos from the USGS. Over three months the island grew substantially and the harbor was greatly improved. 12

13 Volcanoes and Hot Spots Volcanism associated with rifting Volcanism Associated with subduction Hot Spot: a point on the crust immediately above a hot plume within the mantle. Heat from the mantle (and some magma) rises to the hot spot. Rising mantle material termed a mantle plume. Hot spots can occur beneath oceanic or continental crust. The Hawaiian Islands consist of eastern active volcanic islands and inactive volcanic islands to the northwest. Mechanism first proposed by J. Tuzo Wilson (a Canadian geophysicist) to illustrate that plates actually move. Further northwest of the islands are seamounts (underwater mountains that are submerged islands). Just southeast of Hawaii is an undersea volcano known as Loihi. 13

14 The modern active island rests close to the hot spot and its shield volcanoes are fed from the magma that the hot spot generates. Until 1996 Loihi was thought to be an inactive seamount. It began erupting in 1996 and the eruptions were preceded by a cluster of small earthquakes indicating the movement of magma. The Pacific plate is moving towards the northwest. As the crust moves it ages, becomes cooler and more dense, causing it to subside. The volcanic islands have been successively pushed off the hot spot by plate movement. The seamounts are old islands that have subsided to below sea level. The seamounts represent even older islands that have been pushed further from the hot spot. Recent studies suggest that the Hawaiian Hot Spot has moved over time. 14