Volcanic Ash Lesson 1: The Threat
to Aviation
Introduction to Lesson 1
Pilots and aviation meteorologists are aware of the potential for
volcanic ash to affect a flight. However, the most familiar
examples are those of large commercial aircraft which unexpectedly
encounter a Volcanic Ash Cloud on long-distance flights. Pilots
and aviation meteorlogists may not have been given training to estimate
the magnitude of the threat to their particular area of interest and
operations. Our goal is to provide you with a better understanding of
that problem in this lesson. In particular, Lesson 1 of the NWA
Volcanic Ash Training Module will provide you a basic understanding of:
- The locations of volcanoes
- The hazards associated with volcanoes
- The types of volcanic eruptions and typical altitudes of volcanic
clouds associated with these eruiptions
- Tthe volcano montitoring system currently in place.
Volcanoes Around the World
From 1973 through 2000, about 100 encounters of aircraft with airborne
volcanic ash have been documented. The following image shows the
locations of volcanoes that have affective aviation operations (From
Guffanti and Miller (Figure 2))
Western US Map
Alaska Map
Volcano Hazards
There are a variety
of hazards associated with volcanoes. These include lava flows,
land slides, pyroclastic flows, gas, acid rain, and volcanic ash in the
eruption column and eruption cloud as shown in the image at the
right. Some effects such as the lava flow and tephra may only be
significant locally or regionally. Other hazards such as the
eruption cloud and volcanic ash can have large, and sometimes global,
effects.
A brief description of each hazard follows. Clicking on the link
will take you to an expanded explanation from the USGS.
Gas: Magma contains
dissolved gases that are released into the atmosphere during eruptions.
Gases are also released from magma that either remains below ground
(for example, as an intrusion) or is rising toward the surface.
Lahar: An Indonesian
term that describes a hot or cold mixture of water and rock fragments
flowing down the slopes of a volcano and (or) river valleys.
Landslides: Large
masses of rock and soil that fall, slide, or flow very rapidly under
the force of gravity.
Lava flows: Streams of
molten rock that pour or ooze from an erupting vent.
Pyroclastic flows:
High-density mixtures of hot, dry rock fragments and hot gases that
move away from the vent that erupted them at high speeds.
Tephra: A general term
for fragments of volcanic rock and lava regardless of size that are
blasted into the air by explosions or carried upward by hot gases in
eruption columns or lava fountains. The smallest tephra
fragments form the volcanic ash cloud that can travel large
distances from the volcano.
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Types of Eruptions
Volcanologists have developed a classification system for
eruptions. This system is based on .....
The following represent the current classification of eruptions
| Hawaiian/Strombolian |
Global Frequency: 5-10 per
year
Ash Content: Low - moderate
Other Features: "Dry", volatile-bearing cloumn; few fine
particles; may be difficult to detect visually;
Examples: Kilauea, 1983-1986; Pacay, 1990; Oshima, 1986;
Stromboli, continuous
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| Surtseyan (phreatomagmatic) -
underwater or glaciar volcano |
Global
Frequency: 1-2 per year
Ash Content: High
Other Features: Wet, steam laden cloumns; ash rich; much
aggregation of particles
Examples: White Island, 1976--1982; Ukinrek, 1977; Surtsey,
1963-1965
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|
| Vulcanian |
Global Frequency:
>10 per year
Ash Content: Moderate - high
Other Features: Very common; widely variable style of activity;
associated with small ash flow
Examples: Redoubt, 1989-1990; Sakurajima, continuous; Fuego,
1974-1979
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Sub-Plinian/Plinian (including
phreatomagmatic)
|
Global Frequency:
0.1-1 per year
Ash Content: Moderate to High
Other Features: Coarse and fine particles in column; often
short-lived but maintained
Examples: Hekla, 1991; Mount St. Helens, 1980
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|
| Ash flow / co-ignimbrite
(associated with pyroclastic flows) |
Global Frequency:
<0.1 per year
Ash Content: High
Other Features: Very high column rise rates; large magnitude;
infrequent
Examples: Small - Mount St. Helens, 1980; Pinatubo, 1991.
Large - Katmai, 1912.
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The type of eruption is related to three characteristics of interest to
aviation:
- Altitude of the eruption and resulting volcanic ash cloud
- The composition of the particulate in the ash cloud
- The area impacted by the eruption
Table (from Heiken, 1994) summarizes these characteristics
Height
of
Eruption Column (km)
|
Area
Affected by Ash Fallout (km2)
|
Ash Composition (percent)
|
Types
of
Glass Particles in Ash
|
Potential
Hazard to Aviation
|
|
|
Glass
Shards
and Pumice
|
Mineral
Grains
|
Rock
Fragments
|
|
|
Plinian Eruption
|
10-40
|
100 - 10,000
|
60-100
|
0-35
|
0-30
|
cm to
micrometer shards and pumice
|
High; over
large areas
|
Plinian-hydrovolcanic
Eruption
|
20-50
|
10,000 -
100,000
|
60-100
|
0-35
|
0-40
|
mm- to
micrometer size shards and pumice
|
High; over
large areas
|
Vulcanian Eruption
|
0.3-3.0
|
10-100
|
10-30
|
10-30
|
70-90
|
mm-size
fragments and rare droplets
|
High
locally; medium regionally
|
Strombolian Eruption
|
0.1-2.0
|
0.5-5.0
|
60-80
|
1-5
|
20-40
|
cm- to
mm-size angular, blocky fragments
|
Low
|
Hawaiian Eruption
|
<0.1-0.5
|
<0.05-0.05
|
60-80
|
1-5
|
20-40
|
cm- to
mm-size droplets and bombs
|
Low
|
Surtseyan (hydrovolcanic) Eruption
|
0.3-2
|
1.-200
|
70-100
|
0-10
|
5-30
|
mm- to
micrometer-size angular, blocky fragments
|
Medium
locally
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Altitudes
 |
Volcanic ash may reach into the
stratosphere for large eruption.
However, there is great variability in the height of the eruption
column. The frequency with which volcanic ash clouds reach
particular altitudes provides some idea of the likelihood with which
volcanic eruptions may impact aviation operations.
The image at the left shows the numbers of eruptions that reached each
altitude between the years of 1975 to 1985.
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Composition of A Volcanic Ash Cloud
Volcanic ash consists of mineral fragments, rock, glass shards, and may
also have acid droplets coating ash-grain surfaces. The following
definitions from Wikipedia may help explain the differences between
these constituents of the ash cloud.
Mineral:
A naturally occurring, homogeneous, usually inorganic,
solid substance with a definite chemical composition and characteristic
crystal structure, hardness, and color. Some examples of minerals are
mica, feldspar, quartz, and salt.
Rock
is a naturally occurring aggregate of
minerals
and/or
mineraloids.
Glass is a uniform
amorphous solid material, usually produced
when a suitably viscous molten material cools very rapidly to below its
glass transition temperature,
thereby not giving enough time for a regular
crystal
lattice to form.
To summarize, minerals generally have a crystalline structure and
consist of a homogeneous material while rocks consist of a
mixture of minerals. For example, granite (a rock) is
composed of feldspar, quartz, hornblende,
biotite,
muscovite
and other minerals. Glass is a solid with no crystalline
structure formed by the rapid cooling of materials.
The minerals and rock in the volcanic ash cloud are the solid
constituents of an eruption while the glass is formed from the rapid
cooling of the molten magma . Study of the mineral composition of
the volcanic ash (Bayhurst et al) shows that the following minerals are
present in the cloud: Quartz, Calcite, Magnetite,
Gypsum/anhydite, Mica/clay, Feldspar,
Amphibole. The glass is formed from the molten magma that is
ejected and rapidly cooled.
The mineral and glass composition of the ash clous is important to
understanding the abrasiveness of the ash and the melting properties of
the cloud.
Hardness
Sizes of particles
Amount
Melting temperature
Volcano Monitoring
Their is a network to monitor volcanic activity.
Volcanic Monitoring:World Organization of Volcano Observatories
http://www.avo.alaska.edu/
http://volcanoes.usgs.gov/About/Where/WhereWeWork.html
The Alaska Volcano Observatory (AVO) has established a color code
system as a means of communicating the likelihood of an eruption.
http://www.avo.alaska.edu/avo4/updates/color_code.html
GREEN: Volcano is in its normal "dormant" state. Normal
seismicity and fumarolic activity are occurring.
YELLOW: Volcano is restless.
Seismic activity is elevated. Potential for eruptive activity is
increased. A plume of gas and steam may rise several thousand
feet above the volcano which may contain minor amounts of ash.
ORANGE: Small ash eruption expected or confirmed.
Plume(s) not likely to rise above 7,620 meters (25,000 feet) above
sea level. Seismic disturbance recorded on local seismic
stations, but not recorded at more distant locations.
RED: Large ash eruptions expected or confirmed.
Plume(s) likely to rise above 7,620 meters (25,000 feet) above sea
level. Strong seismic signal recorded on all local and
commonly on more distant stations.
References
