EXTRUSIVE VOLCANIC LANDFORMS 2 forms of lava: 1. BASALTIC – low silica, more fluid, so gas bubbles escape easily and so NOT so explosive 2. ANDESITIC AND RHYOLITIC – high silica(acidic) less fluid (high viscosity), gas does not escape easily,pressure builds up and leads to violent explosions.
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EXTRUSIVE VOLCANIC
LANDFORMS
2 forms of lava:
1. BASALTIC – low silica, more fluid, so gas bubbles escape easily and so NOT so explosive
2. ANDESITIC AND RHYOLITIC – high silica(acidic) less fluid (high viscosity), gas does not escape
easily,pressure builds up and leads to violent explosions.
• 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.
• 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
• 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.
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.
• 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.
• 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.