Volcanic Activity and Volcanic Hazards Information
Volcano - The name for the plumbing system that reaches from several tens of kilometers deep in the earth all the way to the surface, and including the deposits that emerge from that plumbing system. Mostly we know about what's above the surface---and very little about the rest.
Names for the extruded material: depends on size and physical properties -
A) Lava - a liquid flow of melted rock on the earth's surface or solid rock that forms from this.
Can form lava lakes, fountains, or lava falls.
Mobilized lava flows will burn and bury everything in their path (up to 1,100 °C).
Can sometimes be redirected (constructed trenches, explosions, or water-chilled).
Cooled lava can form tubes (underground pathways)
Two main descriptive names for cooled basaltic lava flows (Hawaiian names) related to the composition:
aa - rough, blocky surface, sharp and jagged pieces (higher silica content)
pahoehoe - smooth, ropy surface (lower silica content)
B) Pyroclastic material - "fire broken" - chunks of airborne ejected magma (tephra) that cool quickly.
|
Name |
Size |
|---|---|
|
Blocks |
> 32 mm (cold ejecta) |
|
Bombs |
> 32 mm (hot ejecta) |
|
Lapilli |
4 - 32 mm |
|
Ash |
<4 mm |
|
|
|
Pyroclastic hazards:
Ashfalls: ash carried by wind can be deposited hundreds of miles away from the volcano
Ash can also enter the upper atmosphere and affect climate.
Buried the ancient city of Pompeii, 79 A.D. (Mount Vesuvius).
Ashflows: turbulent mixtures of gases and pyroclastic material
often the result of the collapse of a vertical column ejected upward from the volcano.
Can move very quickly downhill (up to 100 mph) and are extremely deadly (very hot and powerful)
One such flow killed over 30,000 people in 1902 (Martinique, West Indies, Mount Pelée)
Debris flows: Water from heavy precipitation, melted snow or streams may help mobilize pyroclastic material
(rocks, ash, and cinders) into a fast-moving debris flow (called a lahar - Indonesian word).
Sometimes, only a minor eruption is necessary, just need to mobilize a lot of material.
Buried the village of Armero in Colombia, 1985, with a wall of mud 130 feet high.
Predicted hazards from lahars in the region surrounding Mount Rainier (visible from Seattle).
Composition and physical properties
Igneous rocks - classification of rocks is based on chemical composition.
Composition is directly related to the source for the magma and the tectonic setting, and different lava compositions have different
physical properties and types of volcanic eruptions.
|
Composition |
Volcanic rock name |
|
Silicic (rich in SiO2) |
Rhyolite |
|
Intermediate |
Andesite |
|
Mafic (rich in Mg, Fe, <SiO2) |
Basalt |
Where do you find volcanoes?
Most of the world's ~1500 active volcanoes are located around the margin of the Pacific Ocean ("The Ring of Fire")
directly linked to activity on plate boundaries (usually limited to divergent and convergent boundaries, not transform).
A fourth type of boundary can now be introduced - intraplate
An intraplate volcano is linked to a hot spot - a region of thin crust where hot material from deep in the earth (lower mantle or near the core) pokes through the crust.
Examples include the Hawaiian Islands (plate movement over a stationary hot spot) and Yellowstone National Park.
Nature of volcanic eruptions
Volcanoes that erupt explosively can be extremely hazardous to society.
Lava flows, like in Hawaii, are much less hazardous - still destructive, but not explosive.
Scale used to describe the explosiveness of a particular eruption - Volcanic Explosivity Index (0-8)
based upon amount of material (volume), height of eruptive column, and duration.
Mt. St. Helens was a VEI 4, last VEI 7 was 1815 in Indonesia (92,000 killed and changed the global climate).
Largest recent eruption was Mt. Pinatubo in the Philippines, 1991 (VEI 5-6).
Climate Hazards from eruptions are closely related to the mass of sulfur gases erupted.
SO2 is the main gas
it slowly converts to aerosol (H2SO4)--it takes several weeks to react
If the eruption is explosive, the gas is put in the upper atmosphere, above clouds
The aerosol stays there for several years, affecting the solar radiation
So, eruptions with high SO2 releases affect global climate much more.
What causes explosive vs. non-explosive eruptions?
An explosive eruption requires a very viscous magma (viscosity describes the resistance to flow) and a lot of gas (mainly water vapor and CO2).
A highly viscous magma is very sticky and holds in all of the gases, non-viscous lava is runny and low in gas
Once the pressure is relieved as the magma reaches the earth's surface, the gas is released violently (like champagne).
The viscosity of a magma is related to the silica content.
Si-tetrahedra in a magma link together to make it sticky.
Low silica lavas (mafic/basalt) are very fluid and not linked to explosive eruptions.
High silica lavas (silicic/rhyolite) and magmas are linked to explosive eruptions.
Temperature is also important - higher T = less viscosity, less bonds form.
Links: General Volcano Information
Volcanoes of the world: Smithsonian Volcanoes of the World
K-12 Volcano Site: Volcano World UND
Different plate boundaries are linked to different types of magma, eruptions, and ultimately the shape a volcano will take.
|
Eruptive Style |
Erupted Material |
*Volcano Type* |
Tectonic Setting |
|
Non-Explosive |
Basalt lava flows |
Shield Volcano |
Hot Spot or Divergent |
|
Explosive |
Andesite and Rhyolite lava. Pumice, ash, dust, and other pyroclastic material |
Stratovolcano (composite) Also cinder cone, lava dome or caldera |
Convergent (subduction zones beneath islands and continents) |
Volcano type:
Shield volcano - broad, gently sloping volcano - looks like a warrior's shield laying down.
Examples include Mauna Loa and Kilauea (big island of Hawaii).
Not violent, but lava flows can damage property
Stratovolcano (Composite) - combined explosive eruptions of lava and ash (viscous) form a steep-sided volcano.
Examples include Mount Saint Helens and Mount Fuji.
Other explosively erupting volcanoes:
Cinder cone - all pyroclastic material (no lava), small cones that quickly erode. Example: Parícutin, Mexico
Lava Dome - very viscous lava piles up around a volcanic vent and forms a bulbous plug.
Has the potential for a very violent eruption.
Often form inside the crater of a larger stratovolcano.
Example - Mt. Pelée, Martinique, Caribbean.
Caldera - extremely violent eruptions that produce a huge crater (10's of km across).
eruptions are so violent (VEI ~6) that the top of the volcano is blown off
when the magma chamber empties, the large underground chamber collapses.
Example - Yellowstone (multiple calderas, still geothermally active),
Example - Crater Lake (Oregon) - Formerly Mt. Mazama (VEI 7 eruption 4895 B.C.)
Selected Case Studies for Volcanic Eruptions
Hawaii (Big Island): 2 active volcanoes - Mauna Loa and Kilauea. Kilauea is more active and visible. Terms: Intraplate hot spot, shield volcano, basaltic magma (flows smooth and black).
Are volcanic eruptions on Hawaii explosive? Why or why not?
What kind of hazard is associated with these types of eruptions?
Why do people choose to live so close to an active volcano?
Mount Vesuvius, Pompeii, Italy. Erupted 79 A.D. and buried 20,000 people. In 1738, researchers began to excavate the site and found molds the people who had perished in the eruption. This excavation was the first ever of its type and laid the groundwork for the scientific discipline of archeology.
What major modern Italian city is within range of (inactive since 1944) Mt. Vesuvius?
Mount Saint Helens, May 18, 1980. Near Seattle in a group of stratovolcanoes. Note the difference between smooth and fluidHawaiian eruptions and sticky, explosive stratovolcano eruptions.
This was a very well documented explosion. Most people were evacuated, but ~70 people still died. Harry Truman, a lodge owneron Spirit Lake, refused to leave. A few of the fatalities were geologists observing the volcano who were caught by surprise in the near-horizontal blast from the mountain's north face (following an earthquake-triggered landslide). Lahars killed about 57 people, tens of miles away.
What happened to the forests surrounding the volcano? Imagine the amount of power the ashflow possessed in order to blow over and incinerate thousands of acres of forests.
Why did the eruption blast in a horizontal direction rather than vertical?
Mexico, Parícutin - started as a small crack in a farmer's field, erupted almost continuously between 1942-1953. Allowed people to watch the birth of a volcano.
Montserrat Island, West Indies, Caribbean Islands. Soufriere Hills, a stratovolcano on the island, began to erupt in 1995. Therewere concerns about a nuée ardent that might occur. Very similar in nature to the neighboring island of Martinique. In 1902, when the volcano Mount Pelée erupted, a nuée ardent engulfed the village of Saint-Pierre, killing 30,000 people (they had been encouraged to stay there by the government, citing no danger).
Mount Pinatubo, Philippines, 1991 eruption. Second largest eruption of this century (after Katmai, Alaska). Huge volumes of SO2 gas emitted, global drop in temperature. Scientists were the only people to stay behind and observe the eruption. The military base was evacuated and is now abandoned.
Links: What is going on right now?
Best overall eruption information site: Smithsonian Institution Global Volcanism Network
More information on a volcano: USGS CVO Useful Links about Volcanoes
Worldwide Volcano Observatories: Glyn Williams-Jones, Open Univ
Newspapers: Press Reports from all Over
Kilauea, active since 1983: USGS HVO Kilauea report
More Hazards associated with volcanic eruptions
Tsunamis (discussed with earthquakes) can also be generated by volcanic eruptions, for example - Krakatoa, Indonesia, 1883. Major eruption that extended 300 meters below sea level and produced a giant tsunami that killed >30,000 people. Once again, most tsunamis occur in the Pacific Ocean.
Effect on climate - large ashfall eruptions can cause global cooling of up to several degrees for 1-2 years after the eruption --crop failure and famine, colder winters, milder summers. Ash in the atmosphere reflects sunlight and so less sunlight gets through to warm the surface (sort of the opposite effect of the greenhouse effect).
Gases - Water is a major gas released during volcanic eruptions, but other more harmful gases are also released, including toxic gases and acids: CO2, CO, SO2, H2S, HCl, HF. Acids can mix with precipitation and fall as acid rain - changes water chemistry (kills plants and animals) and damages buildings. Toxic gases can be catastrophically released in large quantites.
Lake Nyos, Cameroon, 1986 - Volcanic vents bubbled CO2 into the lower water layers of the lake, like soda water. For some reason, the lake waters turned over, releasing much of the CO2 at once and killing 1700 people, animals, etc. The relatively heavy gas mass rolled downhill, asphyxiating everyone in its path.
But, volcanism is thought to be a major source for our present atmosphere - the early earth had almost no atmosphere, but overtime, much of the water and oxygen came from volcanic eruptions.
Benefits
A) Volcanic material from the deep earth enriches soils. In the tropics especially important nutrients in soils are continually leached, and where volcanoes are located soils benefit from continual replenishment of elements from the deep earth.
B) Geothermal energy - heat from shallow magma is harnessed to generate electricity - creates steam that drives generators. Iceland, Indonesia, the Philippines, and California are some examples of places where geothermal power plants are used.
C) Metallic ore deposits - many kinds of metallic ore (economically useful) deposits are found in the roots of old volcanoes. Warm water generated by volcanoes mobilizes metals and concentrates them.
D) Tourism - Many volcanoes are part of National Parks and Monuments, both old (Devil's Tower, Wyoming) and young (Hawaii, Crater Lake, OR).
E) Creation of new land - This is the main method of creating and extending continents and islands. There is a new Hawaiianisland forming off the southeast coast of Hawaii called Loihi. It is fed by the same magma chamber and HOT SPOT that feedsKilauea and Mauna Loa on Hawaii. Loihi is still under water (called a sea mount) but in the future may break the surface as an island.
Predicting volcanic eruptions
Monitoring precursor events - ground swelling, underground temperature increases, earthquake swarms, composition ofvolcanic gases.
Most major eruptions come with some warning and some success has been achieved (e.g., Mt. Pinatubo) by scientists inproviding enough advance warning to evacuate people. Minor eruptions are still difficult to predict for active volcanoes and the results can be deadly (lahars - Armero, Colombia).
Satellite Monitoring is a very bright new area.
Mitigation
For explosive eruptions, there is only one alternative - evacuate.
The problem is knowing what areas to evacuate and when to do it. It is too difficult to predict the path of erupted material or even the trajectory (Mt. St. Helens was a lateral blast).
Generally volcanic eruptions have been much more successfully forecast and mitigated than earthquakes in spite of much lower amounts of money committed.
Advanced warning systems for lahars.
Volcanic clouds and aircraft
For non-explosive eruptions - possible mitigation includes diversion (using piles of material to redirect lava flows), chilling with water (fire hoses, Iceland), and bombing.
Links: Volcano and Volcanic Hazards Awareness
Toba Bottleneck: Stanley Ambrose--Univ Illinois
Volcanoes of the USA: USGS CVO--Volcanoes of US
Volcano Monitoring Strategy: USGS--Volcano Monitoring
Eruption Warning and Real-time Notifications: USGS--Warnings about hazards
Volcanic Hazards Primer: Colleen Riley, Michigan Tech
Volcanic Cloud Hazards: Volcanic Ash/Aircraft CEOS