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EXTRUSIVE VOLCANIC LANDFORMS 1. 2. 2 forms of lava: BASALTIC – low silica, more fluid, so gas bubbles escape easily and so NOT so explosive ANDESITIC AND RHYOLITIC – high silica(acidic) less fluid (high viscosity), gas does not escape easily,pressure builds up and leads to violent explosions. Types of extrusive volcanic landforms. • Lava plateaux – fissure eruptions eg Iceland. Basaltic in nature, lava flows great distances and creates flat, featureless landscapes. Laki, Iceland. Shield / Basic volcanoes Free flowing lava, volcanoes have gentle sides and cover a large area eg Hekla Acid/dome volcanoes Steep sided, convex cones. Viscous lava(rhyolite)eg Puy region, France Ash and cinder cones Sides are steep and symmetrical eg Paricutin, Mexico Composite cones, consist of layers of ash and lava, usually andesitic.eg Mt Etna. Calderas Massive build up of gas causes explosion and a crater to form eg Krakatoa, Indonesia. Location of Krakatoa, Indonesia. The nature of volcanic eruptions • Vulcanologists classify volcanoes according to the nature of their eruptions. • Classification is based upon the degree of violence of explosion, which is a consequence of the pressure and amount of gas in the magma. • Volcanic Explosivity Index (VEI) Used since 1982 to describe the relative size and magnitude of explosivr eruptions, 0-8 index of increasing explosivity. Each increase in number represents an increase of 10. Forms of volcanic eruption Minor volcanic forms • GEYSERS- water heated by geothermal activity reaches such a high temperature that it becomes a gas and explodes out of a fissure in the ground. Due to increased pressure this may not occur until temperatures reach over 120*c(superheated water). STROKKUR GEYSER,ICELAND. HOT SPRINGS/BOILING MUD • If the heated water does not explode onto the surface, but mixes with surface deposits, boiling mud is formed. Iceland. THE BLUE LAGOON, ICELAND • Geothermal heating of groundwater created lagoons, rich in silica, in a lava field. Positive impact of tectonic activity! • TOURISM: AGRICULTURE: SOLFATARA • Small volcanic areas without cones, produced by gas (initially sulphurous) escaping to the surface. Eg Bay of Naples in Italy. FUMAROLES • Areas where superheated water turns to steam as it condenses at the surface. • Very similar to Solfataras, but without the high sulphur content. THE IMPACT OF VOLCANIC ACTIVITY • Volcanic activity can affect the local, national and global area.(spatial context). • Volcanic activity can be classified as having primary and secondary effects. (temporal context). PRIMARY EFFECTS. • 1.VOLCANIC GASES-carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorine. CO2 from Lake Nyos in Cameroon in 1986 suffocated 1,700 people. Laki in Iceland in 1783 poisoned 40% of the population. Affected harvests throughout Europe, caused starvation and the French Revolution! TEPHRA • Solid material of varying grain size from volcanic bombs to ash particles, ejected into the atmosphere. • DUST-flour size • ASH-sand size • LAPPILLI-little stones • CINDERS-rough clinker • BOMBS-up to 1m in diametre • Light materials such as dust and ash become airborne and travel great distances around the world. Larger materials accumulate around the crater and increade cone height. Layers of tephra Definition of Tephra • Tephra, the Greek word for ash, is used to describe any material that is ejected by a volcano into the atmosphere. Tephra includes dense blocks and bombs, and lighter materials such as scoria, pumice and ash. As one moves away from a volcano, the tephra deposits become finer grained (the particles are smaller) and thinner. This is because small particles stay aloft longer and stay within the eruption cloud for a greater distance from the volcano. PYROCLASTIC FLOWS • Very hot(800c),gas charged, high velocity flows made up of a mixture of gases and tephra. • Montserrat.1995-97. • Located in the Soufriere Hills on the island of Montserrat in the Caribbean. • Began with ash emissions, steam and many earthquakes. • Then in 1996 pyroclastic flows began! On 25th June 1997 4-5 million cubic metres of pyroclastic material was emitted. See case study sheet. Montserrat Montserrat and Pyroclastic flows • http://dsc.discovery.com/videos/ultimateguide-to-volcanoes-montserrat.html Montserrat 1995-97 • http://www.google.co.uk/imgres?imgurl=http://w ww.juicygeography.co.uk/images/PFJuly27.jpg&i mgrefurl=http://www.juicygeography.co.uk/monts errat.htm&usg=__xwgN6p0NEKkvntC7KOoCR1 zcA9s=&h=336&w=448&sz=22&hl=en&start=2& zoom=1&itbs=1&tbnid=BYs2goh0ZGreFM:&tbnh =95&tbnw=127&prev=/images%3Fq%3Dvideo% 2Bclip%2Bof%2BMontserrat%2Berupting%26hl %3Den%26sa%3DG%26gbv%3D2%26tbs%3Di sch:1 • Monitoring Volcanoes: • Case study Montserrat. http://www.mvo.ms/en/science/monitoring/science/monitoring Definition of Pyroclasts • Pyroclastic flows can form either from the collapse of an energetic eruption cloud or by the collapse of unstable lava domes that have oozed out of stratovolcanoes. The hot rock tumbles downhill as an avalanche then rapidly transforms into an expanding cloud of hot rock, "ash" and gas as it entrains ambient air and heats it, and as the hot volatiles within the lava violently decompress. The result is a self-supporting dense cloud of hot debris and gas that flows downhill as a fluid body. A cloud of "ash" rises above the flow, and it variously scours its path and/or deposits distinctive linear build ups of rock and debris, depending on the flow conditions. LAVA FLOWS • • • • • BASALT ANDESITIC RHYOLITIC Depending upon silica content. Differing levels of viscosity. • Lava flow during a rift eruption at Krafla, Iceland in 1984 More detailed classification of lava. • Igneous rocks, which form lava flows when erupted, can be classified into three chemical types; felsic, intermediate, and mafic (four if one includes the superheated ultramafic). These classes are primarily chemical; however, the chemistry of lava also tends to correlate with the magma temperature, its viscosity and its mode of eruption. MAFIC LAVA IS ALSO CALLED BASALTIC! • Mafic or basaltic lavas are typified by their high ferromagnesian content, and generally erupt at temperatures in excess of 950 °C. Basaltic magma is high in iron and magnesium, and has relatively lower aluminium and silica, which taken together reduces the degree of polymerization within the melt. Owing to the higher temperatures, viscosities can be relatively low, although still thousands of times more viscous than water. The low degree of polymerization and high temperature favours chemical diffusion, so it is common to see large, well-formed phenocrysts within mafic lavas. Basalt lavas tend to produce low-profile shield volcanoes or "flood basalt fields", because the fluid lava flows for long distances from the vent. The thickness of a basalt lava, particularly on a low slope, may be much greater than the thickness of the moving lava flow at any one time, because basalt lavas may "inflate" by supply of lava beneath a solidified crust. Most basalt lavas are of ʻAʻā or pāhoehoe types, rather than block lavas. Underwater they can form "pillow lavas", which are rather similar to entrail-type pahoehoe lavas on land. This can now be seen on the surface on Iceland. 2 types of basaltic lava Pāhoehoe and Aa flows • Aa is rough lava, whilst Pahoehoe is smooth. Both are Basaltic and are found in Iceland and Hawaii. • http://en.wikipedia.org/wiki/Lava SECONDARY EFFECTS • LAHARS: volcanic mud flows eg eruption of the Nevado del Ruiz in Nov 1985 caused the Colombian town of Armero to be devastated. Video of lahars • http://www.bbc.co.uk/learningzone/clips/vo lcanic-hazards-lahars-inindonesia/3069.html • Complete ppq on primary and secondary hazards/lahars. SECONDARY EFFECTS.cont. • Flooding- melting of icecaps such as Eyjafjallajokull causes a lot of flooding. Bridges had to be rebuilt. • Tsunamis-From the eruption of Krakatoa in 1883 drowned 36,000 people. • Volcanic landslides • Climate change-volcanic debris reduces global temperatures. Human responses to natural hazards. • 1.This involves the assessment of RISK: the exposure of people to a hazardous event. • Why do people expose themselves to risk? • Unpredictability- when or where and with what magnitude/explosivity, • Lack of alternatives- due to economic reasons or lack of knowledge(LIC), • Turn a blind eye!! 2. Vulnerability • This relates to how capable the people are at predicting,preparing and modifying the loss! • Poverty increases vulnerability, • Wealth and higher levels of technical ability, as well as education make people less vulnerable to all kinds of natural hazard. • See “Is California worth the risk?” Human Responses • 1. Prevention! Impossible with volcanoes and Earthquakes, • 2. Modification of vulnerability a. prediction and warning, how much can be given? (see later) b.preparedness, public education,emergency services provision, land use planning/zoning. • 3. Modification of loss: a. aid, b. insurance (see California case study). Volcano hazard management. • http://dsc.discovery.com/videos/volcano-video/ • Eruptions can not be prevented, but they can often be predicted and protection offered to the population. • PREDICTION: • Hazard mapping (previous lava flows and pyroclasts by studying geology), • Analysing seismic shockwave patterns, • Sampling gas and lava emissions, • Remote sensing of changes in topography, • Heat and gas emissions by satellite. • Seismic shock waves alerted vulcanologists to the likely eruption of Popocatepetl, Mexico in 2000 and erupted 24 hours after the local population was evacuated. Protection • This involves reducing the risk of damage by preparing for an eruption. Warnings issued by the USGS in the US are supported by FEMA who instruct how to react before, during and after an eruption. • http://volcanoes.usgs.gov/publications/2010/icel and.php • In the longer term hazard mapping and land use planning may be used to avoid development in areas at risk. http://volcanoes.usgs.gov/activity/ • http://www.fema.gov/hazard/volcano/index.shtm • Check out the above web-sites to take notes on how man can predict, prepare and protect against volcanoes. • Link this to your case studies from high-income and low-income countries. • Take specific notes on the volcano warning systems. • PPQ on prep/prediction of volcanoes.