Volcanic Hazards

Lava flows rarely threaten human life because lava usually moves slowly a few centimeters per hour for silicic flows to several km/hour for basaltic flows.
Major hazards of lava flows: burying, crushing, covering, burning everything in their path.
Lava flows can dam rivers to form lakes that might overflow and break their dams causing floods.
Methods for controlling paths of lava flows: (1) construct barriers and diversion channels, (2) cool advancing front with water, (3) disruption of source or advancing front of lava flow by explosives.

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They are mixtures of pulverized rock, ash, and hot gases, and can move at speeds of hundreds of miles per hour.
These currents can be dilute, as in pyroclastic surges, or concentrated, as in pyroclastic flows. They are gravity-driven, which means that they flow down slopes.
High concentration density flows are called "pyroclastic flows" and are essentially nonturbulent and confined to valleys. Low concentration density flows are called "pyroclastic surges" which can expand over hill and valley like hurricanes. Temperatures may be as hot as 900 degrees Celsius, or as cold as steam.

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Pyroclastic flows contain a high-density mix of hot lava blocks, pumice, ash and volcanic gas. They move at very high speed down volcanic slopes, typically following valleys.
Most pyroclastic flows consist of two parts: a lower (basal) flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow.
Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.

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Pyroclastic falls, also known as volcanic fallout, occur when tephra fragmented rock ranging in size from mm to tens of cm (fractions of inches to feet) is ejected from a volcanic vent during an eruption and falls to the ground some distance away from the vent.
Tephra in pyroclastic fall deposits may have been transported only a short distance from the vent or, if it is injected into the upper atmosphere, may circle the globe.
Tephra falls are usually not directly dangerous unless a person is close enough to an eruption to be struck by larger fragments.
The effects of falls can be, however. Ash can smother vegetation, destroy moving parts in motors and engines (especially in aircraft), and scratch surfaces.

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Lahars are part of the family of debris flows that are fluids composed of mixtures of water and particles of all sizes from clay-size to gigantic boulders. The abundance of solid matter carries the water, unlike watery floods where water carries the fragments.
Debris flows have the viscous consistency of wet concrete, and there is a complete transition to watery floods. Lahars are composed of volcanic particles and originate directly or indirectly from volcanic action.
Lahars are not as fast or hot as other volcanic hazards, but they are extremely destructive. They will either bulldoze or bury anything in their path, sometimes in deposits dozens of feet thick.

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A tsunami is a long-period sea wave or wave train generated by a sudden displacement of water. Tsunamis travel at very high speeds through deep water as low broad waves and build to great heights as they approach the shallow bottom of shores.
Most are caused by fault displacements on the sea floor, but many have been caused by volcanic action. The eruption of Krakatau in 1883 produced tsunamis that killed 36,000 people.
The pyroclastic flow generated by this eruption displaced the water that initiated the tsunamis.

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References:

http://volcanology.geol.ucsb.edu/hazards.htm

http://geology.com/volcanoes/volcanic-hazards/lava-stream.jpg

http://geology.com/volcanoes/volcanic-hazards/

https://volcanoes.usgs.gov/vhp/pyroclastic_flows.html

http://geology.com/volcanoes/volcanic-hazards/pyroclastic-flow.jpg

http://geology.com/volcanoes/volcanic-hazards/volcanic-ash-weight.jpg

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