Volcanoes and plate tectonics

Transcription

1 Volcanoes and plate tectonics 1. divergent plate margins: rift volcanoes 2. convergent plate margins: 84% of eruptions ocean-ocean margins: island-arc volcanoes ocean-continent margins: andesitic volcanoes continent-continent margins: few volcanoes 3. hot spots: upward flowing convection currents in the mantle; independent of spreading centres hot spot remains nearly stationary; volcano develops over hot spot if a plate drifts slowly over the hot spot, volcanoes drifting away from the spot become extinct, and new volcanoes develop in their place Hawaiian Islands: drifting of the Pacific plate NW over a hot spot

2 Volcanoes and magma 1. volcano: conduit in crust through which magma reaches the surface 2. magma contains dissolved gases under pressure 3. viscosity: ability of a lava to flow chemical composition varies inversely with temperature 4. low viscosity - explosive eruption 5. types of lava flows: pahoehoe: smooth, ropy surface aa: blocky, rough irregular surface

3 Volcanic cones 1. type of eruption determines shape of cone; depends on type of lava 2. shield volcano: broad flat shape gentle outpourings of fluid lavas Iceland, Hawaii, Galapagos 3. stratovolcano (composite cone) large, highly symmetrical cones layers of ash, cinders, lava explosive activity; greatest hazard Mt Fuji, Mt Shasta, Mt Hood, Mt Rainier, Mt Vesuvius 4. cinder cone small, symmetrical, steep-sided usually short-lived many cinder cones found in a volcanic area Mt Mayon, Paracutín 5. caldera: very large crater, usually more than 1.5 km in diameter

4 6. fissure: crack in crust

5 Types of volcanic eruptions 1. Icelandic: fissure eruptions, quietly release large volumes of fluid lava 2. Hawaiian: thin, fluid flows; quiet to moderate eruptions 3. Strombolian: moderate explosions, bombs, cinders, moderately fluid lava, steam clouds 4. Vulcanian: short, thick flows of lava, bombs, pumice and ash; crusts over between eruptions 5. Surtseyan: violently explosive, hot fluid lava comes in contact with sea water 6. Vesuvian: extremely violent expulsion of gascharged lava 7. Plinian: more violent; caldera collapse; wide dispersal of tephra 8. Peléean: gas, ash and blocks move downslope in one or more blasts

6 Primary volcanic hazards 1. associated directly with the volcanic eruption 2. pyroclastic flows (nuées ardentes) mixture of hot gases, crystals, ash, pumice, glass shards flows downhill in a blast 3. tephra: fragmented material which is ejected by the volcano bombs: > 32 mm in diameter blocks scoriae: cinders lapillae: 2-60 mm in diameter sand ash: < 0.1 mm 4. lava flows

7 Secondary volcanic hazards 1. volcanic gases 2. lahars: volcanic mudflows occur whenever large quantities of water are present on the steep sides of a volcano heated, gas-rich flow converted to heated mudflow 3. landslides 4. earthquakes and tsunami 5. glacier bursts (Jökulhlaups) melting of glaciers or snowfields by hot lava or gases 6. climatic effects suspended particles can remain in atmosphere for months or years reduces amount of solar radiation reaching the ground average temperature decreases of C

8 Prediction of volcanic eruptions 1. seismic activity increase in local earthquake activity earthquake swarms audible rumblings 2. ground deformation swelling or uplifting of the volcanic cone changes in ground slope near the volcano 3. hydrothermal phenomenon increased discharge from hot springs increased discharge of steam from fumaroles rise in temperature of hot springs, steam, crater lakes melting of snow or ice on the volcano withering of vegetation on the slopes of the volcano 4. chemical changes 5. geomagnetic and geo-electric activity

9 Protection against volcanoes 1. protection against lava flows bombing artificial barriers water sprays 2. protection against lahars artificial mounds reduce the amount of water available barriers to deflect lahars warning systems

10 Risk assessment 1. problems: ashfall can disrupt communities hundreds of kilometres away occur on long timescale eruptions may not come singly, can continue for months 2. restricting development in hazardous areas depends on long-range forecasts of probability of volcanic activity, identification of areas of potential risk 3. volcanic-risk maps show possible extent of volcanic hazards in future inferred from geologic evidence of past events 4. lack of knowledge: size of future eruption environmental conditions at time of eruption

11 Living in volcanic hazard zones 1. 10% of world's population lives on or near potentially active volcanoes 2. volcanoes also provide resources fertile soils energy resources building materials other products: dental pumice, powdered pumice tourism