Types of Hazards Posed by an Active Volcano
These are the Types of hazard that can posed by an active volcano, according to PHIVOLCS.
LAVA FLOW
Lava flow is a higly elongated mass of molten rock materials cascading downslope from an erupting vent. The lava flow being extruded has low silica and low water contents.
Rate of flow: 3 km/day (slightly high viscosity) or 45 km/hour (low viscosity). Speed and geometry of lava flows depend on local topography. Steep slopes encourage faster and longer flows than gentle slopes or terrain
DOME GROWTH
Lava dome is a pile or mound of lava that grew on the floor of an active crater, on the side slopes via a feeder vent that breached through the surface of the edifice, or inside the volcanic edifice.
Types: Exodomes - lava domes that were formed on the surface of the volcanic edifice)
PYROCLASTIC FLOW
Pyriclastic flow refers to hot dry masses of fragmented volcanic materials that move along the slope and in contact with ground surface. This includes: pumice flow, ash flow, block-and-ash flow, nuee ardente and glowing avalanche.
Mechanism...
Pyroclastic flow mechanism:
Nuee ardente is a glowing eruption cloud characterized by: extreme heat (about 500 °C or higher)
1. high gas contentrapid flow down the slope of an erupting volcano enormous amounts of ash and other fragmental volcanic materials
A nuee ardente may originate directly from an active crater or from a collapse of a growing lava dome.
PYROCLASTIC SURGE
Pyroclastic surges are turbulent low-concentration density currents of gases, rock debris and in some cases, water, that move above the ground surface at high velocities.
Types: Ground surge, Ash-cloud surge, Base surge
HOT BLASTS
* Hot blasts arise when pent-up gases facilitate their way out through the impermeable overlying materials and cause a very rapid escape into the atmosphere. Blasts that are directed obliquely often do much damage and could exact a high toll in human lives. * Lateral blasts are combination of pyroclastic flows and pyroclastic surges with an especially strong initial laterally-directed thrust. They have an initial velocity of 600 kph and slow down to about 100 kph near its margin 25 km from the volcano.
TEPHRA FALLS
Tephra falls may consist of pumice, scoria, dense lithic materials or crystals or combination of the four.
Particle size: less than 2 mm diameter (ash) , 2-64 mm diameter (lapilli) , more than 64 mm diameter (blocks and bombs)
VOLCANIC GAS
Volcanic gas is one of the basic components of a magma or lava. Active and inactive volcanoes may release to the atmosphere gases in the form of: water vapor, hydrogen sulfide, sulfur dioxide , carbon monoxide, hydrogen chloride and hydrogen fluorid.
Aside from the major constituents, minor amounts of nitrogen, methane, argon and helium may be also present in volcanic gases. The proportion of these components changes with changing temperature.
LAHAR
Lahar (an Indonesian term), sometimes called mudflows or volcanic debris flows, are flowing mixtures of volcanic debris and water. Lahars are classfied into: Primary or hot lahar - associated directly with volcanic eruption and Secondary or cold lahar - caused by heavy rainfall.
Lahar distribute and redistribute volcanic ash and debris deposited around the volcano after the materials has cooled and has become water logged.
Lahar in tropical areas can be produced by:
* sudden draining of a crater lake, caused by either an explosive eruption or collapse of a crater fall (e.g. Agua, Kelut, Ruapehu) * movement of a pyroclastic flow into a river or lake, displacing and mixing with water * avalanche of water-sustained rock debris, where water can be from heavy rain, hydrothermal activity or other sources * torrential rainfall on unconsolidated deposits on slope of a volcano (e.g. Pinatubo) * collapse of a temporary dam, where recent volcanic deposits have blocked a steam channel (e.g. Asama, Pinatubo
TSUNAMI
Tsunami are long-period sea waves or wave trains that are generated by the under-the-sea earthquake. Most tsunamis are caused by fault displacements on the sea floor and of volcanic sudden displacement of water. They travel at high speed water as low broad waves and build to great heights as they approach shores. Origin including volcanic or volcano-tectonic earthquakes, explosions collapse or subsidence, landslides, lahars, pyroclastic flows or debris avalanches entering bodies of water, and atmospheric waves that couple with the sea.