Hazards from Volcanic Activity

Question 1

Characteristics of explosive eruptions

Answer 1

• Convergent plate boundaries
• Rhyolite (more acid) and andesite (less acid) lavas
• Lava has high % silica (acid), high viscosity, lower temperature at eruption
• Violent bursting of gas bubbles when magma reaches surface; highly explosive; vent and top of cone often shattered
• Gas, dust, ash, lava bombs, tephra all released
• Tend to have long periods with no activity
• Steep-sided strato-volcanoes; caldera

Question 2

Characteristics of effusive eruptions

Answer 2

• Divergent plate boundaries
• Basaltic lava
• Lava has low silica % (basic), low viscosity, higher temperature at eruption
• Gas bubbles expand freely; limited explosive force
• Gas, lava flows both released
• Tend to be more frequent; an eruption can continue for many months
• Gently sloping sides, shield volcanoes; lava plateaux when eruption from multiple fissures

Question 3

Products of explosive eruptions

Answer 3

Composite cone volcanoes are made up of layers of ash and acidic lava. Internal lava flows form sills and dykes. The acidic magma does not flow easily and solidified magma plugs the vents. Calderas (deep craters) form when the cone is destroyed by an explosive eruption

Question 4

Products of effusive eruptions

Answer 4

Basic magma flow freely and covers large areas: flood basalts

Question 5

Icelandic lava eruptions

Answer 5

Characterised by persistent fissure eruption. Large quantities of basaltic lava build up vast horizontal plains. On a large scale they have formed the Deccan Plateau and the Columbia Plateau

Question 6

Hawaiian eruptions

Answer 6

Involve more noticeable central activity than the Icelandic type. Runny basaltic lava travels down the sides of the volcano in lava flows. Gases escape easily. Occasional pyroclastic activity occurs but this is less important than the lava eruption

Question 7

Strombolian eruptions

Answer 7

Characterised by frequent gas explosions which blast fragments of runny lava into the air to form cones. They are very explosive eruptions with large quantities of pyroclastic rock thrown out. Eruptions are commonly marked by a white cloud of steam emitted from the crater

Question 8

Vulcanian eruptions

Answer 8

Violent gas explosions blast out plugs of sticky or cooled lava. Fragments build up into cones of ash and pumice. Vulcanian eruptions occur when there is very viscous lava solidifies rapidly after an explosion. Often the eruption clears a blocked vent and spews large quantities of volcanic ash into the atmosphere

Question 9

Vesuvian eruptions

Answer 9

Characterised by very powerful blasts of gas pushing ash clouds high into the sky. They are more violent than Vulcanian eruptions. Lava flows also occur. Ash falls to cover surrounding areas

Question 10

Plinian eruptions

Answer 10

Gas rushes up through sticky lava and blasts ash and fragments into the sky in a huge explosion. The violent eruptions create immense clouds of gas and volcanic debris several kilometres thick. Gas clouds and lava can also rush down the slopes. Part of the volcano may be blasted away during the eruption

Question 11

Super-volcanoes

Answer 11

Volcanoes that erupt more than 1,000km^3 of material in a single event. An example is the Yellowstone super-volcano in Wyoming, which has a caldera measuring 75km in diameter

Question 12

Volcanic Explosivity Index (VEI)

Answer 12

Magnitude (the amount of material erupted) and intensity (the speed at which the material is erupted) can be used to compare different eruptions. The VEI combines these two factors into a single figure on a scale of 0-8 (8 being most explosive). Each integer increase represents a ten-fold increase in explosivity

Question 13

Lava flows

Answer 13

Flows or streams of molten rock that pour from an erupting vent. The speed at which lava moves depends on the type of lava (basaltic lava is free-flowing and runs considerable distances, while acidic lava is thick and does not flow easily), its viscosity, the steepness of the ground and whether the lava flows as a broad sheet, through a confined channel, or down a lava tube. Lava of any type is extremely destructive and will burn, bury or bulldoze infrastructure, property, natural vegetation and agricultural land

Question 14

Pyroclastic flows - nuées and ardente

Answer 14

Flows of gas and tephra which are extremely hot (over 500C) and flow down the side of the volcano at speeds of over 100km/h

Question 15

Gas emissions

Answer 15

Carbon dioxide, carbon monoxide, sulphur dioxide and chlorine escape through fumaroles (openings in or near a volcano, through which hot sulphurous gases escape). When sulphur dioxide combines with water, acid rain is produced, leading to weathering and pollution

Question 16

Tephra

Answer 16

Volcanic bombs and ash ejected into the atmosphere. Size ranges from ash to larger bombs of more than 6cm in diameter

Question 17

Lahars and flooding associated with melting ice

Answer 17

Lahars are a type of mud flow. Snow and ice on a volcano summit melt during the eruption. Rock, ash and soil mix together and destroy and bury anything in the path of the rapid flow of material as they follow valleys. The melting of snow and ice associated with volcanic eruptions can also lead to flooding as large volumes of water are released. In Iceland, these are known as jökulhlaups

Question 18

Tsunamis

Answer 18

Violent eruption of an island volcano can displace oceanic water and lead to a tsunami - a large wave travelling at speeds up to 600km/h