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# Activity 1 Where Are the Volcanoes?

CS_Ch14_Volcanoes 3/2/05 3:02 PM Page 888

Volcanoes

Activity 1 Where are the Volcanoes?

Goals Think about It In this activity you will: Volcanoes are one of ’s most feared, yet spectacular • Find the and of volcanoes activities. nearest your community when given a map of historically • Can volcanoes form anywhere on ? Why or active volcanoes. why not? • Search for and describe What do you think? Record your ideas about this question in patterns in the global your EarthComm notebook. Be prepared to discuss your distribution of volcanoes. responses with your small group and the class. • Make inferences about possible locations of future volcanic activity.

• Understand that most volcanism occurs beneath the .

• Understand that map projections distort near the poles and eliminate some data.

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Activity 1 Where are the Volcanoes?

Investigate 1. “Thought experiments” are b) If you drew a short east–west line experiments that scientists dream up with a certain length near the and then run in their imagination, southern end of your , rather than in a real laboratory. They and another east–west line with are a useful way to develop new the same length near the northern ideas and insights into scientific end of your continent, how would problems. Here’s a thought the lengths of the lines compare experiment for you to do, to help when they are projected onto the you understand the Mercator map cylinder map? projection. It wouldn’t even be too c) If you drew a short difficult to do this in real life, if you line with a certain length near the could obtain the right materials. southern end of your continent, Visualize a large, see-through plastic and another north–south line with ball. Poke holes on opposite sides, and the same length near the northern stick a wooden dowel or a chopstick end of your continent, how would through to make the and the lengths of the lines compare the . Install a bright light when they are projected onto the directly at the center of the ball, cylinder map? somehow. With a felt-tipped pen, draw d) How would the image of a a fake continent on the ball. Make the continent that is centered on the continent extend from near the North Pole project onto the to near the North Pole. cylinder map? Now wrap a clear sheet of stiff plastic around the globe, to make a tight- e) Which part of your map shows fitting cylinder that’s parallel to the the least distortion? Earth’s axis. See the figure on the right for how to arrange this. Turn the light on, and observe how the border of wooden your continent projects onto the plastic dowel cylinder. Trace that image on the cylinder with the felt-tipped pen. Unwrap the cylinder from the globe, and lay it flat on the table. You now have a map with what’s called a

Mercator projection of your continent! light a) Describe how the image of your continent is changed in shape transparent (distorted) when it is projected plastic ball onto the cylinder.

transparent plastic sleeve

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Volcanoes

200 400 600 800 1000

70˚

50˚ 30˚ 10˚ EQUATOR 0 200 400600 800 1000 1200 KILOMETERS Scale 1:30 000 000 at the Equator One centimeter equals approximately 300 kilometers (186 miles) at the Equator One inch equals approximately 473 miles (762 kilometers) at the Equator

VOLCANOES_(from the Global Volcanism Program, Smithsonian Institution, Washington, D.C.)

Erupted A.D. 1900 through 1993

Erupted A.D. 0 to A.D. 1900

Holocene eruptions (within past 10,000 ), B.C. and undated A.D. eruptions Uncertain activity and fumarolic activity

2. Obtain the USGS map called This 4. Obtain a copy of a world map. Dynamic Planet. Look at the map Use this map to summarize any key, also shown above, to learn the patterns in the global distribution meaning of the various symbols of volcanoes. and how to use the map scale. a) When volcanoes follow a linear a) What do each of the four kinds pattern, draw a thick line on of triangles represent? the world map. Use the string of volcanoes within the b) What do the solid red lines and southern represent? as an example. c) Describe how the scale of the b) For the red lines that appear on map changes with latitude. the USGS map, draw thin lines d) Does the map cover the entire on your copy of the map. See Earth? Why or why not? the examples in the near Oregon and 3. For each time interval of volcanic Washington. activity shown on the map, find the latitude and longitude of three c) Where volcanoes are less volcanoes closest to your community. concentrated, outline (circle) the area that they cover. Try to be a) Make a data table to record as accurate as possible. See the your results. When complete, group of volcanoes in the the data table should list Cascades as an example. 12 volcanoes. 5. When your map is complete, b) Compare your data with that of answer the following questions in other groups in your class. Did your notebook: your class agree on the locations of the nearest historically active a) Are most volcanoes found in volcanoes? How did you resolve random places or do they show any differences? a trend or pattern? Explain.

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b) Does the USGS map show in or near your state? Support volcanoes that have not erupted your answer with evidence from during the last 10,000 years? this activity. c) Does the USGS map show g) What are some limitations of eruptions after 1993, or new the evidence you used? volcanoes? d) Does the USGS map show any volcanoes associated with the red lines in the ocean basins? e) What information does the map give about the size or hazard of the volcanoes? f) Suppose that tomorrow a volcano forms somewhere in the . Could it form

Reflecting on the Activity and the Challenge By looking at a world map of recent at are incomplete. This limit volcanic activity you found patterns the conclusions you can draw. in the data. This helped you to make However, you now have some inferences about the possible location knowledge that will help you decide of the next volcanic eruption in the where in the U.S. you might “stage” United States. The data you looked a volcanic eruption.

Digging Deeper THE GLOBAL DISTRIBUTION OF VOLCANOES Volcanoes beneath the The USGS map This Dynamic Planet shows historical volcanic activity throughout the world. It tells a story about how our dynamic planet releases its internal storehouse of energy. No single source of data tells the whole story, but a map is a great place to begin. On average, about 60 of Earth’s 550 historically active volcanoes erupt each . Geologists have long known that volcanoes are abundant along the edges of certain .The presence of volcanic rocks on the floors of all ocean basins indicates that volcanoes are far more abundant under water than on land.

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All of the Earth’s ocean basins have a continuous mountain range, called a mid-ocean ridge extending through them.These ridges, over 80,000 km long Geo Words in total, are broad rises in the ocean floor.They are usually in water depths of mid-ocean ridge: a 1000 to 2000 m. Figure 1 shows a vertical section through a mid-ocean continuous mountain range extending through the North ridge.At the crest of the ridge there is a steep-sided rift valley. Magma and South Atlantic , (molten rock) from deep in the Earth rises up into the rift valley to form the , and the submarine volcanoes.These volcanoes have even been observed by scientists South Pacific Ocean. in deep-diving submersibles.All of the floors of the oceans, beneath a thin layer rift valley: the deep central of sediments, consist of volcanic rock, so we know that volcanoes form all cleft in the crest of the mid- oceanic ridge. along the mid-ocean ridges, at different times at different places.At a few magma: naturally occurring places along the mid-ocean ridges, as in , volcanic activity is especially molten rock material. strong, and volcanoes build up high enough to form islands. generated within the Earth

ridge sea surface crest

irregular sea floor rift valley

crust

plate # 1 motion motion flowing rising magma rock plate # 2 flowing lithosphere rock

10 km asthenosphere

Figure 1 Cross section through a mid-ocean ridge. Volcanoes on Land Volcanoes that erupt on land are much more dangerous than volcanoes beneath the ocean. Eruptions along the western edge of the United States have formed the Cascades volcanic mountain range.They also form island chains, like the Aleutians in Alaska.Volcanoes like these are common in a narrow belt all around the Pacific Ocean. Geologists call this the “.” A famous example of an eruption along the Ring of Fire was the dramatic eruption of Mt. St. Helens in Washington in 1980.

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Activity 1 Where are the Volcanoes?

Ring of Fire Around the edges of the Pacific Ocean, the plates of the Pacific Ocean slide down beneath the continents. Look at Figure 2 to see an example.The Nazca Plate, moving eastward from the East Pacific Ridge, slides down beneath the west coast of .The plate is heated as it sinks into the much hotter rocks of the deep Earth.The heat causes fluids, especially water, to leave the plate and rise into overlying hot rocks.The added water lowers the melting point of the solid rock. If enough water is added the rock melts and magma is formed.The magma rises upward, because it is less dense than the rocks. It feeds volcanoes on the overlying plate. Nearly four-fifths of volcanoes on land form where one plate slides beneath another plate.

active volcanoes, plate tectonics, and the "ring of fire"

North American Eurasian Plate Eurasian Plate Plate

"Ring of Fire" Juan de Fuca Plate Plate Cocos Plate Nazca Indo-Australian Plate South Plate American African Pacific Plate Plate Plate

Scotia Plate Plate Antarctic Plate

divergent plate boundaries convergent plate boundaries volcanoes transform boundaries diffuse poorly defined boundaries

Figure 2 The plates of the Earth, and the “Ring of Fire” around the Pacific.The circles show active volcanoes.

Volcanoes Formed by Rifting on the Continents Volcanoes in the valley form where two parts of the African continent are moving apart from each other.The process is very similar to what happens at mid-ocean ridges.The continental plate is stretched and broken. One of the breaks becomes the main one, and opens up to form the rift valley, as shown in Figure 3.

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upwarping In the United States, continental rifting long Geo Words ago formed the rocks lava: molten rock that issues lithosphere that make up the tall cliffs from a volcano or fissure. on the western bank of hot spot: a fixed source of asthenosphere the Hudson River.These abundant rising magma that A. rocks formed when forms a volcanic center that magma intruded the crust has persisted for tens of rift valley millions of years. during this rifting.The map projections: the process rocks are seen for more of systematically transforming than 80 km along the positions on the Earth’s bank of the Hudson River spherical surface to a flat map and can be up to 300 m while maintaining spatial B. thick! Other evidence of relationships. magma formed during Mercator projection: a map this rifting is found in projection in which the Equator many states along the is represented by a straight line Figure 3 Formation of a rift valley on a continent. true to scale, the meridians East Coast. by parallel straight lines perpendicular to the Equator and equally spaced according to their distance apart at the Equator, and the parallels by straight lines perpendicular to the meridians and the same length as the Equator. There is a great distortion of distances, areas, and shapes at the polar regions.

Figure 4 Mount Kilimanjaro is a famous example of volcanism at a continental rift. Many other volcanoes in the East African rift valley have erupted in historic times.

Volcanoes at Hot Spots Volcanoes discussed so far occur near the edges of plates. However, a small percentage of volcanoes occur in the interior of a plate.The Hawaiian Islands, shown in Figure 5, are an example. Studies of volcanic rock show that

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Activity 1 Where are the Volcanoes?

the islands get 160˚ 180˚ 160˚ 140˚ 120˚ Geo Words older to the 60 ˚ seamount: a peaked or flat- northwest. Only North topped underwater mountain s the youngest island, nd America rising from the ocean floor. Isla the “big island” of E an m Aleuti 50˚ p Hawaii, has active e

r

o o o

volcanoes. r S e PA C I F I 40˚ a C O Here’s how m C E o A N geologists explain u n 30 t ˚ s the pattern of the Ha wa Hawaiian Islands. iian Islan 20 Deep beneath ds ˚ Hawaii, there is a fixed source of 10˚ abundant rising Figure 5 The Hawaiian island chain and the Emperor magma, called a seamount chain. hot spot.As the Pacific Plate moves to the northwest, away from the East Pacific Ridge, it passes over the fixed hot spot. Magma from the hot spot punches its way Check Your through the moving plate to form a chain of islands.The sharp bend in the Understanding chain was formed when the direction of movement of the plate changed abruptly at a certain time in the past. Far to the northwest the chain consists 1. What evidence do of seamounts. geologists have that volcanoes occur on the Map Projections ocean floor? There is always a big problem in drawing a map of the world, because you 2. What is the Ring of Fire, and where is it have to try to show the curved surface of the Earth’s globe on a flat sheet of located? paper. Many different ways of doing this, called map projections, have been 3. Where do most developed, but they all have some kind of distortion.The USGS map uses a volcanoes on land Mercator projection.As you move away from the Equator, the map form? becomes more and more distorted. For example, it makes all lines of latitude 4. How are rift valleys look like they are equal in length.This makes it difficult to measure distances formed? on the map.Another problem is that the USGS map stops at 70° north and 5. What are hot spots? south latitude, because of the Mercator projection.This keeps you from Provide an example of seeing all of the data. For example, the USGS map cuts off the mid-ocean a hot spot on Earth. ridge north of Iceland.The scale of the map also presents a problem.The 6. Why does the larger the area covered by the map, the less detail the map can show. In this horizontal scale of a case, the triangular symbols that represent volcanoes often overlap in areas Mercator projection increase with latitude? with many volcanoes.This makes them difficult to count.

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Understanding and Applying What You Have Learned 1. What difficulties did you have 6. Of the average of 60 volcanoes finding the latitude and longitude that erupt in any given year, how of volcanoes? many are likely to erupt along the Ring of Fire? 2. Where on Earth do most volcanoes occur? Explain your answer. 7. Why did the Mercator projection not show volcanoes near the 3. Are most volcanoes on land caused Earth’s poles? by the Earth’s plates moving away from each other or moving toward 8. Do most volcanoes on land occur each other? Explain your answer. in the or the ? Explain 4. In your own words, describe the why you think this is so. likely cause of historically active volcanoes in: a) The continental United States b) The Aleutian Islands and southern Alaska c) The Hawaiian Islands. 5. Based on your results from this investigation, list the five states that you think are most likely to experience the next volcanic eruption. Explain each choice.

Preparing for the Chapter Challenge Think about how you can help the audience understand why you chose the probable location of the volcanic eruption for your story. Explain the map that you made for this activity. Note the volcanic eruptions that are closest to your area. Explain where most volcanoes occur in the United States. You should also note where they have not happened recently.

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Activity 1 Where are the Volcanoes?

Inquiring Further 1. Eruptions near your community • How do volcanoes at mid-ocean ridges affect the temperature Find out more about the historical of seawater? eruptions at the volcanoes nearest your community. The Volcano • How do volcanoes change the World web site lists hundreds of chemistry of seawater? historically active volcanoes. • How does seawater affect the (Consult the AGI EarthComm web composition of the volcanic site for current addresses.) Your rock that is formed at the mid- data table of ocean ridge? and will help you to identify them. • Would volcanoes affect a small body of seawater, such as the , the same way as a large 2. Volcanoes and the water on Earth ocean like the Atlantic? (the hydrosphere) • Can a change in the volume of Research to find answers to the volcanic rock formed at mid- following questions, and any other ocean ridges change sea level? questions which you have formed:

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