1.3b volcano Flashcards
Measuring Volcanoes
Volcanic Explosivity Index (VEI)
Measures size of eruptions based upon volume of ejecta (gas, ash, tephra, lava) erupted and how high this is thrown into atmosphere (plume height)
logarithmic scale
Primary Hazards of a Volcano
Lava Flow Pyroclastic Flows Tephra Gas Release Flank Collapse
Primary Hazards of a Volcano: Lava Flow
Fast moving lava poses a big threat to human life. Viscosity is determined by silica dioxide content. On steep slopes flows can reach 15m/sec.
Greatest lava-related disaster was in 1873 where molten material spilled from Lakagigar fissure, Iceland. An estimated 22% of total population died in the resulting famine.
Primary Hazards of a Volcano: Pyroclastic Flows
Responsible for most volcanic related deaths. Result from frothing of molten magma in the vent. Bubbles burst explosively, ejecting hot gases and pyroclastic material (glass shards, pumice, crystals, ash).
Clouds can be up to 1000 degrees Celsius. Most hazardous when they come out sideways and close to the ground.
Mt Sinabung, Sumatra, Indonesia 2015
Primary Hazards of a Volcano: Tephra
An eruption will sometimes eject rock fragments into the atmosphere (tephra).Size can vary from ‘bombs’ (>32mm in diameter) to fine dust (<4mm).
Ash and larger materials can cause roofs to collapse as well as start fires. Dust can reduce visibility and affect air travel.
Mt Asama, Japan, Volcanic bombs (6m)
Primary Hazards of a Volcano: Gas release
Associated with explosive eruptions and lava flows. The mix normally includes water vapour, sulphur dioxide, hydrogen and carbon monoxide.
Most deaths are associated with carbon dioxide; the colourless and odourless gas can accumulate in valleys undetected.
In 1986, emissions from Lake Nyos, Cameroon, killed 1700 people.
Primary Hazards of a Volcano: Flank Collapse
Rhyolitic magma can form immovable ‘plug’ within a volcano. Pressure from magma below can cause side of volcano to bulge and eventually collapse, leading to a sideways eruption collapse, leading to sideways eruption or lateral blast.
St Helen eruption 1980
Secondary Hazards of a Volcano
Lahars
Jokulhlaup
Secondary Hazards of a Volcano: Lahars
Mudflows composed of fine sand and silt material. Degree of hazard depends on steepness of slopes, volume of material and particle size
Associated with heavy rainfall as a trigger as old tephra deposits on steep slopes can be re-mobilised into mudflows.
Secondary Hazards of a Volcano: Jokulhlaup
A glacial outburst flood. Hazard to people and infrastructure. Cause widespread landform modification through erosion and deposition.
Occur very suddenly with rapid discharge of large volumes of water, ice and debris from a glacial source. Can happen anywhere where water accumulates in a subglacial lake beneath a glacier.
Floods are initiated following the failure of an ice or moraine dam.
