the last volcano!

In the last few posts I have looked at the volcano exclusion zone and tried to relate it with the maximum height to which a volcano spews ash. In the last one I looked at Mount Mayon.
Unfortunately for my explanation, the exclusion zone in Mount Mayon is related to pyroclastic flows. Ash, it seems, is generally not a danger. Only in rare cases does it cause trouble - to aeroplanes, for instance. The above conclusions are based on downloads from the net:

http://www.vulkaner.no/v/vulkinfo/tomhaz/manyhaz.html

When a person thinks of a volcanic eruption, they always think of a huge cloud of ash sweeping high into the air. In reality, this ashfall is a relatively minor hazard. It rarely claims lives.

The most explosive eruptions send out great clouds of ash in enormous so-called 'plumes'.
In fact, scientists use the height of the ash plume to calculate the explosivity of an eruption.
A minor plume, less than 100m in height, is common for a Hawai'ian volcano, while when a
plume exceeds 25 km, the eruption is far more explosive. Examples of these more explosive
eruptions include Mount St. Helens in 1981, Krakatau in 1883, Tambora in 1815, and an
ancient eruption of Yellowstone caldera 2 million years ago. The following brief table is known
as the volcanic explosivity index. It is used by volcanologists to calculate an eruption's force,
as well as its type. These are placed in order of increasing explosivity.

DESCRIPTION	PLUME HEIGH	VOLUME OF ASH	EXAMPLE
1) Non-explosive	<100m	1,000m3	Kilauea, Hawaii
2) Gentle	100 - 1,000m	10,000m3	Stromboli
3) Explosive	1 - 5km	1,000,000m3	Galeras, 1992
4) Severe	3 - 15km	10,000,000m3	Nevado del Ruiz
5) Cataclysmic	10 - 25km	100,000,000m3	Galunggung, '82
6) Paroxysma	l >25km	1km3	St Helens, '81
7) Colossal	>25km	10km3	Krakatau, 1883
8) Super-colossal	>25km	100km3	Tambora, 1815
9) Mega-colossal	>25km	1,000km3	Yellowstone

Thus the height of an ash plume is an excellent indication of the power of an eruption. But there are individual hazards based around the ash also. The Mount St. Helens' plume extended over 20,000m into the air, while the eruption of 1956 Bezymianny, in Russia's Kamchatka peninsula, generated a plume 45,000m in height.

The hazard first became evident at Galunggung in 1985. It was found that the ash interfered with the functioning of aircraft, and chaos was barely prevented. The hard and angular particles of ash abraded windshields; fine particles, deposited inside the plane's engines, reacted with water to produce a corrosive acid. It has since been found that it is possible for ash to actually melt inside the engines, creating a sticky fluid that stalls them.

During a 1989-1990 series of eruptions, Redoubt Volcano, Alaska, spewed enormous clouds of ash into the air. On December 15, 1989, a 747 (KLM flight 867) flew into the ash cloud. An accident was barely prevented; had a crash occurred, all 231 passengers would have been lost. The eruption of Mount Spurr in 1992 posed similar problems.

The hazard became most notable in 1991 with the catastrophic eruption of Mount Pinatubo. Ash from this volcano travelled more than 5,000 miles to the east coast of Africa, interfering with some 20 aircraft as it did so. In total, the ash from this eruption damaged over 40 planes, causing damage of over $22 million.