103 Lab1 Volcanoes

Name ________________________

Lab 1. Volcanic Hazards
VOLCANOES
INTRODUCTION
Few things excite the imagination as much as a volcanic eruption, one of nature’s magnificent spectacles, which can result in devastating consequences. The very word “volcano” brings to mind thunderous explosions, rivers of fiery orange lava, and images of smoking destruction. The actual capacity for destruction is not overrated -- volcanic eruptions are one of the most devastating of all geologic hazards. There are about 700 potentially dangerous volcanoes in the world, and about 50 eruptions occur each year worldwide. The United
States is the third most volcanically active country in the world. Here, 58 volcanoes have erupted over 470 times just since 1700 A.D. Only Japan and Indonesia have had more volcanic eruptions.
DISTRIBUTION OF VOLCANOES
Much of the present-day volcanic activity is clearly related to plate tectonic movements and most volcanoes are located at or close to lithospheric plate boundaries (Fig. I-1). About 80% of the volcanoes occur above subduction zones at convergent boundaries, either as volcanic belts on continental margins or as island-arcs comprised of a chain of volcanoes formed in an oceanic setting. Typical examples of continental volcanic belts are the volcanoes of the Cascade Range in the northwestern United States and of the Andes in the western
South America. The Japan and the Philippine islands are good examples of island-arc volcanic chains. The socalled Ring of Fire, defined by the volcanic chains that rim the Pacific Ocean, is actually a ring of subduction zones. About another 15% of the world’s recent volcanoes occur at divergent boundaries such as the Mid-Atlantic
Ridge. By far the largest amount of volcanic rocks in the earth occur along the mid-oceanic ridge system, but the volcanism along this spreading center network is predominantly of the quiet, fissure eruption type rather than through volcanic vents.
Finally, a few areas of volcanic activity lie within lithospheric plates and are not related to plate boundaries.
These are attributed to hot spots (or mantle plumes) -- areas below the crust that have enough heat to generate magma which eventually reaches the Earth’s surface as volcanic eruptions. The Hawaiian Islands, near the center of the Pacific Plate, and Yellowstone Park, within the North American Plate, are examples of volcanism associated with hot spots.

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Table 1. Selected major volcanic eruptions.

Year

Volcanic Eruption

VEI*

Comments

7

1390 B.C. ±
79

Carter Lake (Mt. Mazama),
Oregon
Santorini, (Thera), Greece
Vesuvius, Italy

130 +
1631
1783
1792
1815

Taupo, New Zealand
Vesuvius, Italy
Laki, Iceland
Unzen, Japan
Tambora, Indonesia

7
4
4
2
7

1883

Krakatoa, Indonesia

6

1902

4

1912

Mount Pelée, Island of
Martinique, West Indies
Katmai, Alaska

1914-1917

Lassen Peak, California

3

1919
1959
1963
1968
1980

Kelut (Java)
Kilauea, Hawaii
Agung, Bali
Fernandina, Galapagos
Mount St. Helens,
Washington

4
2
4
4
5

1982
1985
1991

4
3
5

1991

El Chichon, Mexico
Nevado del Ruiz,.Columbia
Mount Pinatubo,
Phillippine Islands
Unzen, Japan

1991-1993

Etna, Italy

?

1983-1993

Kilauea, Hawaii

?

Post-eruption collapse formed caldera; 42 billion cubic meters of new material ejected.
Late Minoan civilization devastated; explosion and tsunami.
Pumpeii and Herculaneum buried; 2.6 billion cubic meters of new material; 3,000 to 16,000 people killed.
16,000 square km area devastated.
Modern Vesuvius eruptive cycle begins, pyroclastic flows.
Largest historic lava flows; 9,350 and most livestock killed.
Debris avalanche and tsunami killed 14,500.
Most explosive eruption in history; 25 billion cubic meters of new material; 92,000 killed; global cooling ("year without summer") due to encircling volcanic ash.
18 billion cubic meters of new material; caldera collapse;
36,000 killed, mostly by tsunami; worldwide temperature o drop by about 0.5 C that persisted for almost 10 years.
Saint Pierre destroyed; 30,000 to 40,000 killed by ash flow in a matter of minutes; spine extruded from lava dome.
May be the largest 20th century eruption; 21 cu. km of tephra ejected; volcanic ash carried for more than 160 km.
California's last historic eruption; 1.0 billion cubic meters of new material; pyroclastic flows, debris flows, and lava flows covered over 16 sq. km area.
Mudflows; 5,110 deaths.
Lava lake formed which is still cooling.
1,100 killed; climatic effects.
Caldera floor dropped 350 meters.
Ash flow; about 2.0 billion cubic meters of new material; 57 killed; 600 sq. km area devastated and timber valued at several hundred million dollars destroyed.
Ash flows killed 1,877; climatic effects.
Mud flows killed more than 23,000 people.
Probably the second largest eruption of the 20th century; huge SO2 emission; 300 killed; thousands evacuated.
Pyroclastic flow killed 41, including 3 volcanologists; over
10,000 people evacuated; lava dome.
Longest activity (473 days) in 300 years; 300 cubic meters of lava extruded.
Longest continuing eruption, with over 50 eruption vents; about 78 sq. km covered by lava and 120 hectometers of new land added to the island of Hawaii.

4895 B.C. ±

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* VEI = Volcanic Explosivity Index
Sources of data: Global Volcanism Program, Smithsonian Institution, Washington, D.C., U.S.A.; U.S. Geological
Survey Circular 1073 (1992); Geology and the Environment by B.W. Pipkin (1994).

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Study Figure I-1 and answer the following questions (it may be helpful to mark the divergent and convergent boundaries with colored pencils for easy distinction):
(a) With which type of plate boundary are most of the modern volcanoes associated?

(b) Identify the following volcanoes and indicate the plate-tectonic environment of each (all of these are listed in Table 1):
Volcano

Plate Tectonic Environment

1.

Mount St. Helens, WA

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2.

Kilauea, Hawaii

3.

Krakatoa, Indonesia

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4.

Mount Pinatubo, Phillipines

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5.

Mount Vesuvius, Italy

6.

Mount Pelee, Martinique

7.

Nevado del Ruiz, Columbia

8.

Katmai, Alaski

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9.

Laki, Iceland

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(c) Would you expect an overlap between the volcanic belts and earthquake zones? If so, why?

Magma Types, Eruptive Styles, and Volcanic Landforms
A volcano is a channel for molten rock, gases, water and heat that rise up from the asthenosphere (and sometimes from even deeper in the mantle). The basic model of the plumbing system for volcanic vents is simple: Magma rises along a thin pipe and fills a reservoir, or a magma chamber, in the crust. As the chamber fills, it swells and bulges the Earth’s surface upward. After an eruption, the magma chamber is partially or completely emptied and the crust above the chamber collapses, only to swell again as the chamber beings to refill. When magma reaches the surface, it is called lava. Depending on the type of magma, the eruptive style of different volcanoes can vary widely. Whether a volcanic eruption will be passive or explosive depends on the viscosity of the magma. The viscosity of a fluid is a measure of its resistant to flow, which increases with increasing silica content and decreases with increasing temperature. For example, honey has a higher viscosity than water.

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In general, as magma rises toward the surface, its pressure and temperature decreases, and the gases dissolved in the magma exsolve (escape). The exsolved gas phase (mostly superheated steam) escapes relatively easily in a fluid, low viscosity magma, but a viscous magma tends to trap the gas and this leads to a building up of pressure in the magma chamber. Eventually, the pressure becomes high enough for the magma to break through to the surface with explosive violence, shattering the rocks all around its passage and carrying with it the broken fragments (pyroclastic ejecta). The more explosive an eruption, the greater the amount of pyroclastic ejecta and, hence, the higher the risk of volcanic hazards.
Basaltic (mafic) magmas, which have a relatively high iron and magnesium and low silica content (~ 50% SiO2), have a low viscosity, so they are very fluid, they move upward through the magma pipes very easily, and gases in the lava easily escape into the atmosphere. This type of magma is often associated with divergent plate boundaries (i.e. mid-ocean ridges), where the magmas are generated by partial melting of the upper mantle, and hot spots. Basaltic lavas tend to spread far and rapidly from the vent, building thin sheets of gently dipping flows, one on top of the other. With time, a broad dome forms that resembles the convex surface of a shield and are thus referred to as shield volcanoes (Ex. Hawaii)
Rhyolitic (felsic) magmas can contain as much as 70% SiO2, but have low iron and magnesium contents, and are thus very viscous and flow slowly. Eruptions of felsic lava tend to be highly explosive because of the magma’s high viscosity and trapped gases. This type of magma is often associated with subduction zones (convergent plate boundaries), where the magmas are generated by melting of a variety of materials – oceanic crust and sediments, continental crust, and upper mantle – but all less mafic than the upper mantle. A highly explosive eruption can form a caldera, a broad, steep-sided basin that forms when so much magma is ejected from the magma chamber that the roof above it collapses (Ex. Yellowstone Caldera in Wyoming).
Andesitic magma’s are also known as intermediate magmas because they contain about 60% SiO2, which is intermediate between basaltic and felsic magmas in their eruptive styles and explosivity. Andesitic volcanoes emit both large amounts of pyroclastic material and thick lava flows. With time, the steep flanks of an andesitic volcano are built up by interbedded layers of lava and pyroclastics. The resultant cone-shaped landform is called a stratovolcano (Ex. Mt. St. Helens, Mt. Rainier, Mt. Fuji).
For a given magma type, the violence of a volcanic eruption is largely a function of the amount of water vapor contained in the lava. Commonly, volcanoes located on islands have a great deal of seawater that seeps into the system. By coming into contact with the hot magma, the water turns to steam and blows up the volcano like an overheated steam boiler, resulting in a phreatic eruption. This type of eruption resulted in the explosion of the
Krakatoa volacano in 1883.

Magma
Basalt (mafic)
Andesitic
Rhyolite (felsic)

SiO2
Low
Int.
High

Fe, Mg
High
Int.
Low

Viscosity
Low
Int.
High

Geologic Setting
Divergent, Hot Spots
Convergent (subduction)
Convergent (subduction)

Volcano Type
Shield
Strato
Caldera

In the absence of any rigorous measurement device or computational procedure for describing the “bigness” of a volcanic eruption, geologists commonly use the Volcanic Explosive Index (VEI), originally devised by C.G.
Newhall of the U.S. Geological Survey, to compare the energies released during different historic explosions.
The VEI scale goes from 1 (lowest) to 8 (highest), and is based on the volume of ejecta, the height of the cloud column, and other observations pointing to the ferocity of an eruption. No eruption in the last 10,000 years has been assigned a VEI of 8. The Tambora eruption, believed to be the most explosive eruption ever recorded, has been assigned a VEI of 7 (Table-1).
VOLCANIC HAZARDS
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All the hazards related to volcanism are caused directly or indirectly by the material expelled during volcanic eruptions: (a) lava flows, the molten material that erupts from volcanic cones and fissures and flows like a fluid on the surface; (b) pyroclastics, the rock debris produced during explosive volcanism; and
(c) gases (or volatiles).
Particularly dangerous are the pyroclastic ejecta produced by explosive volcanism, because such material, when mixed with water or volcanic gases, can move very rapidly down the flanks of a volcano (Table I-2).
Volcanic gases are composed predominantly of water vapor and carbon dioxide, but may also contain significant concentrations of gases such as carbon monoxide, sulfur dioxide (which may combine with wat er to form sulfuric acid) and hydrochloric acid, which are toxic. Small amounts of carbon dioxide in the volcanic gas is not toxic, but large amounts of it may prove fatal. For example, the death toll of more than 1,700 resulting from the
1986 eruption in Cameroon is believed to have been caused by a plume of cool carbon dioxide gas emitted from the bottom of Lake Nyos, a crater lake within the volcanic cone.
(a) Mudflows/Lahars (pyroclastics + water)
Lahars may be hot or cold and are generated when pyroclastic ejecta around a volcano is mobilized because of mixing with water. The water may be supplied by heavy rain caused by the condensation of the large volumes of water expelled during a volcanic eruption, or by the rapid melting of massive glacial ice or snow on a volcanic peak due to the heat released during an eruption. As this water moves downslope, it initially incorporates large amounts of volcanic ash and soil, and then, eventually, large boulders and trees. Most mudflows follow the established drainage network in the area. Depending on the topography, mudflows can reach distances of several tens of kilometers beyond the actual volcano, choking the drainage system on their way and increasing the risk of floods in the area. A lahar may rush down the slopes without much warning, because the volcano itself may not be very active when such a flow is generated.
(b) Pyroclastic flows/Nuee ardente (pyroclastics + hot gas)
When explosive eruptions occur, pyroclastic debris mix with volcanic gases, air, and water to form hot o (temperatures can be over 1,000 C in the interior) gas-and-ash-charged “clouds” that, because of their density, move as a dust storm along the ground. Such flows or clouds, termed variously as ash flows and nuée ardente (the French term for “glowing cloud”), may travel downslope at velocities of more than 100 kilometers per hour and cover areas of a few hundred square kilometers near the volcano, burning or suffocating everything in their path. Tuff is the general name given to any rock composed mainly or exclusively of pyroclastic material. The deposits that result from hot ash flows are called welded tuffs, because the mineral grains are flattened and welded together by the intense heat; other tuffs are unwelded.
(c) Tephra deposits.
Tephra refers to pyroclastic debris which is blasted into the atmosphere during eruptions. The tephra ejected from explosive volcanoes ranges from fine-grained dust (32 mm in size). Most of the tephra falls back close to the volcano on its leeward side and accumulates as ash-fall deposits, which may be several tens of feet in thickness. A large volume of ash-fall in areas proximal to the volcanic eruption may cause roof collapse and disruption of normal life for days (because inhaling of the ash-filled air is neither comfortable nor healthy). When violent eruptions discharge tephra high into the air, ash may fall on areas
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hundreds of miles downwind, requiring a massive clean-up operation. Fine dust may even circle the planet and remain for years in the atmosphere (as was the case with the 1815 Tambora eruption in Indonesia).
(d) Lava flows.
Lava flows pose little hazard to life because they follow pathways dictated by topography. Because their likely downslope courses can be predicted before an actual eruption, there is usually plenty of time for evacuation. Douching with water has proven effective in checking advance of lava flows in Iceland.
Construction of obstruction structures to divert the lava flow to less harmful channels have been attempted in some areas, but have not been as successful.

What Happened At Lake Nyos?
On the evening of August 26, 1986, without warning, a deadly cloud of carbon dioxide (CO2) gas was released from Lake Nyos in Cameroon, West Africa, killing over 1,700 people and 6,000 head of livestock in the valley below. Lake Nyos is a “volcanic lake” - one of over 30 in Cameroon. These lakes formed following volcanic episodes in which plumes of magma, rising from the Earth’s depths, encountered subsurface water. The resulting steam explosions formed craters that became lakes. The magma contained large amounts of CO2, which was released into the groundwater; this CO2-enriched groundwater was ultimately deposited, via natural springs, into the bottom of the lakes. The gas remained dissolved in this deep lake water due to the pressure of the overlying water. Lake Nyos is also strongly stratified – the bottom waters and the surface waters generally do not mix.
This condition led to the further buildup and entrapment of CO2 in the bottom waters.
But what caused the release of the gas in 1986? One hypothesis suggested that a volcanic erupt ion caused the release of the gas. Additional testing and analysis of the dissolved gases in the lake supported that theory that the CO2 was produced by magma. However, because of the undisturbed sediment, clear deeper waters, and the absence of other chemicals, it appeared that no major volcanic eruption had occurred in the lake. Scientists concluded that much, if not all, of the CO2 released was stored in the lake prior to the event, dissolved in the cold, deeper lake water. As this water rose to the surface, the gases rapidly came out of solution; decreasing pressure allowed bubbles to form, just as when a bottle of soda is opened. The eruption spewed a column of water and gas over 80 meters into the air. Because CO2 is more dense than air, it flowed down the mountainside into a populated valley below, causing the residents of the area to lose consciousness and quickly die of asphyxiation.
Although it is not clear what finally triggered these events, possible causes include heavy rain, strong winds, an earth tremor, a landslide, or a seasonal change in the layers of the lake water – any disturbance that many have moved the deeper, gas-rich waters closer to the surface.

Materials Needed
1 Birthday candle
1 Candle holder
1 Measuring cup
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1 Plastic cup
1 Glass jar or beaker
1 Forceps
1 Teaspoon
Matches
Vinegar
Baking soda
Volcanic model and plastic box

Procedure
1. Add one level teaspoon of baking soda to the reservoir in the crater of the volcano.
2. Place the candle in the holder and set it at the foot of the volcano. Light the candle.
3. Pour 40 mL of vinegar to the baking soda in the mouth of the volcano. The reaction will flow out and down the sides of the volcano.
What happens to the candle? Why?

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Exercise 1
Name ___________________ Lab Instructor _________________

Section ___________

1. The chemical reaction that took place when you mixed the baking soda and vinegar together is shown below.
What is the “product” that snuffed out the candle?
NaHCO3 + CH3COOH ? NaC2H3O2 + H2O + CO2

2. Why did the gas flow down the volcano instead of mixing with the air in the room?

3. How did the invisible cloud of CO2 gas kill the people in the valley below Lake Nyos? Relate your answer to the candle at the foot of the volcano experiment.

4. How did volcanism and groundwater act together to form the depression that became Lake Nyos?

5. What was the source of CO2 that was stored in the depths of Lake Nyos?

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Exercise 2
Name ___________________ Lab Instructor _________________

Section ___________

1. What parts of the world have the greatest number of active volcanoes? Why?

2. How may volcanic eruptions influence global climate? Can you cite any examples?

3. What types of phenomena commonly precede volcanic eruptions and can be used to predict their occurrences? 4. What factors influence the viscosity of a magma?

5. Which type of magma is associated with the most explosive type of volcanic eruption? Why?

6. What is a phreatic eruption? Give an example where the phreatic eruption was a major cause of the devastation associated with volcanism.

7. What are the differences between a lahar and a pyroclastic flow?

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