GEOL 110 1st EditionLecture 10Outline of Last Lecture II. FracknationOutline of Current Lecture III. Global Patterns of VolcanismIV. Shield VolcanoesA. BasaltV. Cinder ConesVI. StratovolcanoesVII. Volcanic HazardsA. AshB. Pyroclastic FlowC. LaharD. Volcanic GasesCurrent LectureI. Global Patterns of VolcanismA. Convergent v. Divergent1. Convergent boundaries are where plates come together2. Divergent boundaries are at ocean ridges, and produce more ashB. Hawaii1. Clearly Hawaii does not occur at a plate boundary. Instead it is located entirely within, and very near to the center of, the Pacific Plate. For this reason, volcanism of the type observed at Hawaii is termed intraplate or within-plate volcanismC. Types of VolcanoesII. Shield VolcanoesA. Largest type of common volcanoesThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.B. They have a low profile because they are composed almost entirely of basalt, which is ahot magma (1200-1300°C) with a low viscosity (i.e., it is hot and fluid and flows easily). C. The big island of Hawaii is a group of over-lapping shield volcanoes. The most active one is Kilauea, located on the southeast side of the island. Virtually all of the volcanism in Hawaii is basaltic.D. Basalt: a fine-grained igneous rock – a type of lava - that has a mafic composition. When it erupts it is generally quiet and non-explosive. 1. The important things to remember about basaltic lava are: it erupts at a relatively high temperature (1200-1300°C), it is a relatively fluid type of melt (it has a low viscosity), it has a relatively low volatile content (low H2O and CO2). 2. Can have 2 texturesI. A’A’: thick, rubbly and sharpII. Pahoehoe: smooth and ropey 3. Columnar jointing: Basaltic lava can contract and sometimes fracture during the final stages of cooling. E. Mafic composition: relatively low in silicon (Si) and high in iron and magnesium (Fe, Mg).III. Cinder ConesA. Have relatively steep sides and a large crater at the summitB. Composed of lava, volcanic ash, and scoriaC. They are the smallest type of volcanoesD. They can be dangerous, and are found in the USAIV. StratovolcanoesA. Intermediate in size, usually triangular in shape B. Formed from a mixture of lava and pyroclastic material (ash, pumice, scoria)1. Pyroclastic materials are hot chunks that are formed when they are exploding out of the volcano. They can be very large.2. Pumice: is known to be light enough to float on water because of the air trapped in the rock3. Scoria: ‘cinder’, is formed as gases escape from melts that have mafic to intermediate compositions4. Volcanic ash: small pieces of pumice and scoria ground up during an eruption. On a microscopic scale, we see that volcanic ash is predominantly small shards of volcanic glass. The glass is formed by very rapid cooling of magma during an eruption. The violent grinding of the magma foam (pumice, scoria) during the eruption produces the fine ash. C. Most dangerous because they commonly erupt explosivelyD. Are generally found at convergent boundaries, especially on continental marginsE. Usually have accessory cinder cones around the base that act as ventsV. Volcanic HazardsA. Ash1. The most famous example of volcanic ash disrupting air traffic was the eruption of Alaska’s Redoubt Volcano in December 1989.2. A Boeing 747 airliner (244 people aboard) encountered the ash on its approach to the Anchorage airport. The ensuing loss of power for the plane and near-disastermarks a milestone in the history of volcano monitoring and its interaction with the airline industry.3. Airline flight paths over volcanoes are now monitored carefully B. Pyroclastic Flow1. It is a hot cloud of ash flowing along the ground surface, can move up to 450mph and reach temperature of 1000 degrees Celsius. 2. The collapse of a volcanic dome producing a pyroclastic flow was captured on film during the 1991 eruption of Mt. Unzen Volcano in Japan.3. Small and intermediate size pyroclastic flows are often triggered by the collapse of a volcanic dome. I. The domes are small plugs of very thick (viscous) lava at the caldera of a volcano. Lava does not flow like basaltic lavaII. Composed of intermediate and felsic lava (andesite-rhyolite). This magma is relatively low temperatures (800-1000°C) with high Si contents.C. Lahar1. Mixture of volcanic ash and water (the water is generally snow melt, mizxed with ash, tephra, etc)2. Nevadodel Ruiz is the tallest volcano in Colombia, with a summit elevation of 5389 m (over 17500 ft). Because of this, and because the volcano lies at low, tropical latitudes (4.9°N), there is a continuous cap of snow and ice on the volcano summit. The 1985 disaster, when it erupted, proved that even a small eruption at Ruiz can be enough to melt a volume of snow and ice that is large enough to produce dangerous and destructive lahars.3. When the water from the lahars drains, a thick compact layer of ash and mud ia leftbehind that can completely bury towns.4. Lahars are a serious hazard at Mt. Rainier because the volcano is covered with a large volume of snow and ice. Even a small volcanic eruption at Mt. Rainier is capable of very quickly melting that snow and ice, producing water that will mix with the large volume of pyroclastic debris on the volcano’s surface, producing muddy torrents or lahars.D. Volcanic Gases &Limnic Eruption: Lake Nyos, Cameroon1. Trees and wildlife may be killed by voluminous emissions of CO2 through the soil.2. Remember that gases come out of magmas for the same reasons that they come out of carbonated beverages: When the pressure is reduced (magma rises or the bottle is opened) the gases which were dissolved in the liquid (in the soda or magma) come out of solution to form bubbles, which have a low density and rise tothe surface.3. Lake Nyos is a small but relatively deep lake (up to 200 ft deep) which has formed inthe crater of a small volcano. The problem at Lake Nyos is that there is still some magma that is sitting at some depth beneath the volcano, cooling and solidifying and giving off significant volumes of CO2 gas which rise through the fractured rock and soil and enter the bottom of the lake. Over time, this results in strongly stratified lake water which at the bottom, contained extremely large volumes of dissolved CO2. In August, 1986 the lake underwent a rapid degassing event, whichover-turned the