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Eruption column

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Eruption column over Mount Pinatubo in the Philippines
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   Eruption column over Mount Pinatubo in the Philippines

   An eruption column consists of hot volcanic ash emitted during an
   explosive volcanic eruption. The ash forms a column rising many
   kilometres into the air above the peak of the volcano. In the most
   explosive eruptions, the eruption column may rise over 40km,
   penetrating the stratosphere. Stratospheric injection of aerosols by
   volcanoes is a major cause of short-term climate change.

   A common occurrence in explosive eruptions is for column collapse to
   occur. In this case, the eruption column is too dense to be lifted high
   into the air by air convection, and instead falls down the flanks of
   the volcano to form a pyroclastic flow or surge.

Formation

   Eruption columns form in explosive volcanic activity, when the high
   concentration of volatile materials in the rising magma caused it to be
   disrupted into fine volcanic ash and coarser tephra. The ash and tephra
   are ejected at speeds of several hundred metres per second, and can
   rise rapidly to heights of several kilometres, lifted by enormous
   convection currents.

   Eruption columns may be transient, if formed by a discrete explosion,
   or sustained, if produced by a continuous eruption or closely spaced
   discrete explosions.

Structure

   The solid or liquid material in an eruption column is lifted by
   processes which vary as the material ascends:
     * At the base of the plume, material is forced upwards out of the
       vent by the pressure of expanding gas, mainly steam. The gas
       expands because the pressure of rock above it rapidly reduces as it
       approaches the surface. This region is called the gas thrust region
       and typically reaches to only one or two kilometres above the vent.

     * The convective thrust region covers most of the height of the
       plume. The gas thrust region is very turbulant and surrounding air
       becomes mixed into it and heated. The air expands, reducing its
       density and rising. The rising air carries the solid and liquid
       material from the eruption entrained in it upwards.

     * As the plume rises into less dense surrounding air, it will
       eventually reach an altitude where the hot, rising air is of the
       same density as the surrounding cooler air. In this neutral
       buoyancy region, the erupted material will then no longer rise
       through convection, but solely through any upward momentum which it
       has. This is called the umbrella region, and is usually marked by
       the column spreading out sideways. The eruptive material and the
       surrounding cool air has the same density at the base of the
       umbrella region, and the top is marked by the maximum height which
       momentum carries the material erupted. Because the speeds are very
       low, or negligible in this region it is often distorted by
       stratospheric winds.

Column heights

   Eruption column rising over Redoubt volcano, Alaska
   Enlarge
   Eruption column rising over Redoubt volcano, Alaska

   The column will stop rising once it reaches an altitude where it is no
   longer less dense than the surrounding air. Several factors control the
   height that an eruption column can reach.

   Intrinsic factors include the diameter of the erupting vent, the gas
   content of the magma, and the velocity at which it is ejected.
   Extrinsic factors can be important, with winds sometimes limiting the
   height of the column, and the local thermal temperature gradient also
   playing a role. The atmospheric temperature in the troposphere normally
   decreases by about 10  K/km, but small changes in this gradient can
   have a large effect on the final column height. Theoretically, the
   maximum achievable column height is thought to be about 55km. In
   practice, column heights ranging from about 2-45 km are seen.

   Eruption columns over 10-15 km high break through the tropopause and
   inject ash and aerosols into the stratosphere. Ash and aerosols in the
   troposphere are quickly removed by rain and other precipitation, but
   material injected into the stratosphere is much more slowly dispersed,
   in the absence of weather systems. Substantial amounts of stratospheric
   injection can have global effects: after Mount Pinatubo erupted in
   1991, global temperatures dropped by about 0.5°C. The largest eruptions
   are thought to cause drops of up to several degrees, and are
   potentially the cause of some of the known mass extinctions.

   Eruption column heights are a useful way of measuring eruption
   intensity since for a given atmospheric temperature, the column height
   is proportional to the fourth power of the mass eruption rate.
   Consequently, given similar conditions, to double the column height
   requires an eruption ejecting 16 times as much material per second. The
   column height of eruptions which have not been observed can be
   estimated by mapping the maximum distance that pyroclasts of different
   sizes are carried from the vent – the higher the column the further
   ejected material of a particular mass (and therefore size) can be
   carried.

Hazards

Column collapse

   Eruption columns may become so laden with dense material that they are
   too heavy to be supported by convection currents. This can suddenly
   happen if, for example, the rate at which magma is erupted increases to
   a point where insufficient air is entrained to support it, or if the
   magma density suddenly increases as denser magma from lower down in a
   stratified magma chamber is tapped.

   If it does happen, then material reaching the bottom of the convective
   thrust region can no longer be adequately supported by convection and
   will fall under gravity, forming a pyroclastic flow or surge which can
   travel down the flanks of a volcano at speeds of over 100 km/hour.
   Column collapse is one of the most common and dangerous volcanic
   hazards in a plinian eruption.

Aircraft

   Several eruptions have seriously endangered aircraft which have
   encountered the eruption column. In two separate incidents in 1982, an
   airliner flew into the upper reaches of an eruption column generated by
   Mount Galunggung, and the ash severely damaged both aircraft.
   Particular hazards were the ingestion of ash stopping the engines, the
   sandblasting of the cockpit windows rendering them largely opaque and
   the contamination of fuel through the ingestion of ash through
   pressurisation ducts. The damage to engines is a particular problem
   since temperatures inside a gas turbine are sufficiently high that
   volcanic ash is melted in the combustion chamber, and forms a glass
   coating on components further downstream of it, for example on turbine
   blades.

   In one case, the aircraft lost power on all four engines, and in the
   other, three of the four engines failed. In both cases, engines were
   successfully restarted but the aircraft were forced make emergency
   landings in Jakarta. See British Airways Flight 9

   Similar damage to aircraft occurred due to an eruption column over
   Redoubt volcano in Alaska in 1989. Following the eruption of Mount
   Pinatubo in 1991, aircraft were diverted to avoid the eruption column,
   but nonetheless, ash dispersing over a wide area caused damage to 16
   aircraft, some as far as 1000 km from the volcano.

   Eruption plumes are not usually visible on weather radar and may be
   obscured by cloud or night. Because of the risks posed to aviation by
   eruption plumes, there is a network of nine ash advisory centers around
   the World which continuously monitor for eruption plumes using data
   from satellites, ground reports, pilot reports and meteorological
   models.
   Retrieved from " http://en.wikipedia.org/wiki/Eruption_column"
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