A volcano is a hole or crack in the ground through which gases, lava ,and pyroclastic material are erupted. Gases Water Vapour Sulfer Dioxide Hydrogen Sulfide Nitrogin Hydrogen Carbon Dioxide Some are poisonous • • • • • • Gases can become trapped in the viscous lava and cause pressure to build up. Causes frothing of the magma and explosion. Liquids • Magma – below the Earth’s surface • Lava – the flow of molten rock material which has erupted from the volcano Solids • Pyroclastic Material ▫ Ash – smallest particles 4 mm Some blocks of course material can be much larger ▫ Smallest particles held in the hair for months / years ▫ Particles that are the furthest from the eruption are the finest Weight and size causes the largest material to be dropped nearest the vent More material is found close to the volcano than further away Shield Strata Shield Volcanoes • Formed in oceans • Constructive plate boundaries • Hot spots Shield Volcanoes • Rise from the ocean floor • Have gentle upper slopes and steeper lower slopes • Usually have a roughly circular or oval shape in map view, and cover a wide area • Are composed almost entirely of long thin lava flows, built up over a central vent • Have very little pyroclastic material • Mostly formed by runny lava and flows easily down the slop away from the summit vent Shield Volcanoes • Low viscosity of magma allows the lave to flow quickly down a gentle slope • As it cools it the thickness builds creating the steeper slope Stratovolcanoes • Steeper slopes and narrower bases • Steep slopes result from short, wide and very viscous lava flows that don’t travel very far from the vent • Alternative layers of lava and pyroclastic material • Due to higher viscosity of magmas erupted they are usually more explosive than shield ▫ Lava is paler in color and has a higher silica content – intermediate lava Stratovolcanoes • Can be dormant for long periods • Particularly dangerous due to unpredictability • Sometimes dual explosion ▫ First unblocks the vent and produces pyroclastic material ▫ Second produces the lava Craters • Circular depressions that are usually less that 1 km in diameter • Both types of volcanoes sometimes have them at the summit • Formed when explosive ejection of materials from a central vent Calderas • Huge crater caused when a volcanic cone collapses into a partly empty magma chamber after a powerful eruption. Parasitic cones • Smaller cones which develop on the sides of a bigger volcano • Form when the main vent becomes blocked and the magma finds another outlet Lava Domes • Grow on the sides of stratovolcanoes • Form from very viscous lava that is pale in color and has a high silica content • Cannot flow very far before solidifying • Cones produced have steep convex sides • Often collapse – leading to explosive eruptions and pyroclastic flows The Dangers of Volcanic Eruptions Danger Description Ash Falls Damages property by burying buildings, people not usually harmed directly. Hazard to aircraft. Block out sunlight causing weather to be cooler. Pyroclastic Flow Impossible for people to escape – can be responsible for many deaths. Lateral Blasts Sideways eruption of volcano. Destructive up to 40km away – destroys housing and property. Mudflows (lahars) Form when ash and water mix and travel down river valleys. Very destructive – washes away buildings, roads, bridges and people. Volcanic gases CO2 – causes suffocation. Other gases are poisonous and can burn cause lung disease. The Dangers of Volcanic Eruptions Danger Description Acid Rain Sulpher Dioxide and Hydrogen Sulfide released. Combine with water to make Acid Rain which can damage buildings and plant and animal species. Posteruption famine and disease Disruption to homes, roads and services caused by effects of volcano can result in famine and disease (LEDCs) Tsunami Collapse of volcanoes at sea can result in tsunami Lava Flows Can destroy property, but rarely result in loss of life. They travel slow enough to walk away from. Mitigating Risk Mitigation Description Lava Flow Diversion Mechanical evacuators can be used to channel lava flows away from buildings. Lava flows can also be sprayed with water to cool them down and make them solidify and stop flowing Mudflow Barriers These are walls built across valleys to trap mudflows and protect settlements further down the valley. Building Design Little to be done to stop violent eruptions. Can build roof to withstand ash. Volcano monitoring Measuring features before an eruption, small earthquakes, ground deformation and gas emissions. Remote sensing Monitoring the location of ash clouds from satellites. Hazard Looking at the pattern of past eruptions. Lead to ban on mapping and building in high-risk areas or simply the preparation of planning emergency response. Advantages brought by volcanoes Advantage Description Geothermal Power Use the heat from the Earth to generate electricity. Either directly from steam produced or by pumping water down where it can be heated. Also used for heating systems directly. Fertile Soils Some types of lava and ash weather rapidly and form a rich, thick soil layer, abundant in trace elements Volcanoes Creating Landmass New islands and enlarging existing landmasses. Tourism When safe, volcanoes attract tourists. Helps the economy of some places. Minerals and Mining Sulfur is minded from around active volcanoes. Mineral deposits found around extinct volcanoes. Ancient History Volcanoes supplied large volumes of gases which created the Earth’s atmosphere.
