SJSU Geol 4L Planet Earth Lab

Lab 13

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Chapter 13: Volcanic Hazards In this lab, we’ll briefly examine volcanic hazards, and evaluate the level of risk that such hazards pose to humans and their property. _____________________________________________________________________________ Volcanoes generate many phenomena that can affect Earth’s surface and atmosphere. If humans and their property are in the way of these volcanic hazards, casualties and damages may result. The vulnerability of humans and their property to a particular hazard is called its risk, which may be high, low, or somewhere in between. Volcanic hazards are listed and described on the Fact Sheet on p. 7-8 of this lab. Consult this fact sheet often: it serves as a glossary and has a couple of illustrations. I. Volcanic Ash Read about ash on the fact sheet, and then answer the following questions. A. The diagram at right shows an estimate of the probability that ≥10 cm of volcanic ash will accumulate in the Pacific Northwest due to eruptions from the Cascade volcanoes, which are shown as triangles. 1. What volcano is expected to be the chief source of ash? _______________________ 2. Why do you think the “bulls-eye” pattern is not exactly concentric? In other words, why are the bands wider on the east than on the west?

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B. About 730,000 years ago, a massive eruption happened in eastern California, near the modern resort town of Mammoth Lakes. So much magma was ejected from underground that the surface of the earth collapsed in an oval-shaped area. The resulting depression, called Long Valley Caldera, has partly filled since then with volcanic and sedimentary deposits. In the map at right, the oval dashed line (“Caldera Boundary”) shows the edge of the original depression.

1. How well does the modern topography show the margins of the original depression?

Along the circular dashed line (“resurgent dome”), find the geothermal plant and Hot Creek, which is a stream with natural hot springs in it (very popular with Mammoth skiers in winter). 2. A geothermal plant produces electricity. What energy source do you think it uses?

3. Based on the nature and locations of the geothermal plant and Hot Creek, what do you think a “resurgent dome” is?

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II. Landslides Plate 8 shows the minimum extent of the deposits left by a huge landslide (a type called a debris avalanche) that affected Mt. Shasta in northern California about 300,000 years ago. Examine the map, read about landslides on the fact sheet, and answer the questions below. 1. The dotted/orange area shows parts of the landslide that are currently exposed at the surface. However, note the dashed-line boundary on much of the east side of the landslide. It’s labeled “western edge of post-avalanche lava flows.” In your own words, describe what this means.

2. Note the dotted line (east of Owls Head, for instance), and read its identifying text. In your own words, what is the significance of this line?

3. Find the town of Montague. Near it are three small areas where landslide deposits are absent. Name two possible reasons for their absence. Which is more likely, and why do you think so?

III. Mudflows Read about mudflows on the fact sheet, and then answer the following questions. A. Both of these photographs show volcanoes that are about 14,000 feet high. 1. Which (upper or lower) is more likely to have a very high risk of mudflows? __________ 2. Why do you think so? [Hint: the photos show two distinctly different characteristics that contribute to the production and severity of mudflows.]

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B. The map at right shows the deposits left behind by two mudflows from Mt. Rainier, an active volcano. Mt. Rainier is in the lower right, Tacoma is in the upper left, and Seattle is just off the map to the north. Puget Sound (upper left) is connected to the Pacific Ocean. The Electron Mudflow, shown in light gray, was deposited about 500 years ago. The Osceola (AH-see-OH-la) Mudflow, shown in dark gray, was deposited about 5000 years ago. The dashed line is the geographic boundary between the Cascade Range and the flatter Puget Sound Lowland. 1. The Electron and Osceola mudflows are geographically coincident with the Puyallup River and White River (including West Fork), respectively. Why do you think this is so?

2. Most of the towns in this region—even ones not shown on the map—were built on one of the two mudflows. Why do you think they were built there?

Geologists estimate that the Osceola Mudflow would have flowed ~40 meters/sec on and near the mountain, and ~10 meters/sec on the lowland. 3. Assume Mt. Rainier produces another mudflow similar to the Osceola. How much time would people have to move to higher ground in Enumclaw? ___________ in Auburn? ____________

SJSU Geol 4L Planet Earth Lab

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IV. Lava flows Plate 9 shows recent lava flows on Kilauea, a volcano on Hawai’i (big island). Examine the maps on Plate 9 and below, read about lava flows on the fact sheet, and answer the questions below. 1. This region has been continuously active since 1983. What are the names of the two vents that have emitted most of the lava? ________________ ________________________ 2. Towards what compass direction did all the flows move? ____________ (north is at the top of the map) 3. Why did they flow in this direction? [The map shows the only hint you need.]

4. Find the former shoreline (label in ocean near Kalapana). You can follow this line intermittently southeast all the way to W. Highcastle. Explain why the modern shoreline is not in the same location as the “former shoreline.”

5. On the map, draw a line around the probable original extent of the May 02–Sept 03 flows. This isn’t as easy as it looks; you should draw a single, continuous line. 6. Identify (name or date) a flow that was confined to a narrow channel. ______________ 7. Identify (name or date) a flow that covered a broad area at least 2 km wide. _______________ 8. Based only on what is shown in the two maps, describe the cumulative effects of these lava flows on humans, their property, and their construction works.

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V. Volcanic Gases Examine all four of the illustrations on Plate 10, read about volcanic gases in the fact sheet, and answer the questions below. The map in the bottom left of Plate 10 shows the location of the town of Mammoth Lakes, a popular recreation destination (skiing in the winter, hiking etc. in the summer). It also shows Mammoth Mountain, an active volcano, and the outline of Long Valley Caldera, which we investigated earlier in this lab. 1. What is the geographic relationship of Mammoth Mountain to Long Valley Caldera?

2. What is the “problem gas” at Mammoth Mountain? _______________ 3. What hazards are identified or depicted on Plate 10? In other words, what negative effects does the problem gas have on life?

4. Mammoth Mountain hasn’t exploded violently in recorded history, yet the problem gas could be a deadly hazard. Where does the gas originate, and how does it become a problem?

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Volcanic Hazards Fact Sheet Modified after USGS Fact Sheet 002-97, July 2004; Bobbie Myers, Steven R. Brantley, Peter Stauffer, and James W. Hendley II; graphics by Susan Mayfield and Sara Boore

The diagram at right shows the major features and hazards of volcanoes. Though it’s eye-catching, it has some drawbacks: (1) It shows all hazards present at the same volcano, but this is almost never the case in the real world. (2) All the hazards are shown at an erupting volcano, but some hazards happen even at quiet volcanoes. (3) The labeled terms don’t fit our needs perfectly. In this lab, we’ll use different terms for some features and hazards, ignore some others, and talk about a couple other terms that aren’t shown at all. **So feel free to refer to this diagram as a general guide, but use the text of this fact sheet when completing the lab.**

Lava Flow Molten rock (magma) that flows onto the Earth's surface is called lava. The more silica (SiO2) within the lava, the higher its viscosity and the less easily it flows. For example, basalt lava has a comparatively low viscosity and can flow 50 kilometers/hr, both in confined channels and in broad sheets. In contrast, andesite and rhyolite lavas have higher viscosity and travel very short distances from a volcanic vent, rarely making it down the flank of the volcano.

Ash (or What Goes Up...) An explosive eruption blasts rock fragments (tephra) and volcanic gases into the air with tremendous force. The largest fragments usually fall

back to the ground within a few kilometers of the vent and thus pose little risk. Small (1 mm or less) fragments, called ash, rise high into the air in a fairly narrow column. At higher elevations, the ash spreads out to form a dense cloud. Ash poses two main hazards. (1) While still in the air, it can cause serious trouble for aircraft. Since 1990, about 80 commercial jets have been damaged by inadvertently flying into ash clouds, and several have nearly crashed because of engine failure. (2) Ash is blown about by winds, but ultimately falls back to the ground. Heavy ash fall can collapse buildings, and even minor ash fall can damage crops, livestock, electronics, and machinery. Ash causes respiratory problems, and can “turn day into night.”

Lab 13

SJSU Geol 4L Planet Earth Lab

Pyroclastic Flows These incandescent mixtures of gas, hot ash, and rock fragments are too dense to rise into the air; instead, they move down the sides of a volcano. You don’t want to be around when this happens: pyroclastic flows can be as hot as 900 °C and as fast as 250 kilometers/hr. The denser types tend to follow valleys and are capable of knocking down and burning everything in their paths. The less dense ones can “climb” ridges that are several hundred feet high.

Mudflows Sometimes known by their Indonesian name lahars, these mixtures of mud, water, and rock rush down valleys and stream channels at 30–60 kilometers/hr and can travel over 80 kilometers. Some mudflows contain so much rock debris (60 to 90% by weight) that they look like rivers of fast-moving wet concrete. Close to their source, they are powerful enough to rip up and carry trees, houses, and huge boulders. Farther downstream, they entomb everything in mud.

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a volcano is actively erupting. The most common water sources are (1) rainfall and (2) melting snow and ice. Mt. Pinatubo in the Philippines has experienced mudflows every year since its eruption in 1991; in typhoon season, heavy rainfall washes loose ash off the mountain’s flanks, producing annual mudflows that threaten the surrounding towns.

Volcanic Gases Volcanoes belch forth gases during explosive eruptions, but even in quiet times, gases leak up along underground cracks. Over 90% of all gas emitted by volcanoes is water vapor (steam); the remainder includes carbon dioxide (CO2), sulfur dioxide (SO2), and fluorine. CO2 is heavier than air, so it can accumulate in low areas and suffocate people and animals. SO2 can react with water droplets in the atmosphere to create acid rain, which corrodes metals and harms vegetation. Fluorine can poison livestock and contaminate domestic water supplies.

Recipe for a mudflow: Add water to volcanic ash; send downhill. Mudflows can form whether or not

Landslides A landslide is a rapid downhill movement of rocky material, snow, or ice. Volcano landslides range in size from small movements of loose surface debris to massive collapses of the entire summit or sides of a volcano. Steep volcanoes are susceptible to landslides because they are built partly of layers of loose rock fragments. Landslides on volcano slopes are triggered when eruptions, heavy rainfall, or large earthquakes dislodge these fragments, which then move downhill. The photo shows many small hills and a lumpy, irregular landscape. In the background is Mt. Shasta, an active volcano. Geologists have determined that the many small hills—plus many others not seen in this view—are remnants of a gigantic landslide that happened about 300,000 years ago when part of ancient Mt. Shasta collapsed. The cone-like shape of modern Mt. Shasta was restored by eruptions that happened after the catastrophic collapse.