Types of Volcanoes

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Printed: January 26, 2015

www.ck12.org Chapter 1. Types of Volcanoes

CHAPTER 1 Types of Volcanoes

Lesson Objectives

• Describe the basic shapes of volcanoes.• Compare the features of volcanoes.• Describe the stages in the formation of volcanoes.

Vocabulary

• caldera• cinder cone• composite volcano• shield volcano• supervolcano

Introduction

A volcano is a vent through which molten rock and gas escape from a magma chamber. Volcanoes differ in manyfeatures such as height, shape, and slope steepness. Some volcanoes are tall cones and others are just cracks in theground ( Figure 1.1). As you might expect, the shape of a volcano is related to the composition of its magma.

FIGURE 1.1Mount St. Helens was a beautiful, clas-sic, cone-shaped volcano. The volcano’s1980 eruption blew more than 400 meters(1,300 feet) off the top of the mountain.

Composite Volcanoes

Composite volcanoes are made of felsic to intermediate rock. The viscosity of the lava means that eruptions at thesevolcanoes are often explosive ( Figure 1.2).

The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopesof a composite volcano. Viscosity also causes some eruptions to explode as ash and small rocks. The volcano is

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FIGURE 1.2Mt. Fuji, the highest mountain in Japan, isa dormant composite volcano.

constructed layer by layer, as ash and lava solidify, one upon the other ( Figure 1.3). The result is the classic coneshape of composite volcanoes.

FIGURE 1.3A cross section of a composite volcanoreveals alternating layers of rock and ash:(1) magma chamber, (2) bedrock, (3)pipe, (4) ash layers, (5) lava layers, (6)lava flow, (7) vent, (8) lava, (9) ash cloud.Frequently there is a large crater at thetop from the last eruption.

Shield Volcanoes

Shield volcanoes get their name from their shape. Although shield volcanoes are not steep, they may be very large.Shield volcanoes are common at spreading centers or intraplate hot spots ( Figure 1.4).

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FIGURE 1.4Mauna Loa Volcano in Hawaii (in thebackground) is the largest shield volcanoon Earth with a diameter of more than 112kilometers (70 miles). The volcano formsa significant part of the island of Hawaii.

The lava that creates shield volcanoes is fluid and flows easily. The spreading lava creates the shield shape. Shieldvolcanoes are built by many layers over time and the layers are usually of very similar composition. The lowviscosity also means that shield eruptions are non-explosive.

This Volcanoes 101 video from National Geographic discusses where volcanoes are found and what their propertiescome from (3e): http://www.youtube.com/watch?v=uZp1dNybgfc (3:05).

MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/1405

Cinder Cones

Cinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than acomposite volcano. Cinder cones rarely reach 300 meters in height but they have steep sides. Cinder cones growrapidly, usually from a single eruption cycle ( Figure 1.5). Cinder cones are composed of small fragments of rock,such as pumice, piled on top of one another. The rock shoots up in the air and doesn’t fall far from the vent. Theexact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder conesusually have a crater at the summit.

Cinder cones are often found near larger volcanoes ( Figure 1.6).

Supervolcanoes

Supervolcano eruptions are extremely rare in Earth history. It’s a good thing because they are unimaginably large.A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km3 forMount St. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly,supervolcanoes are the most dangerous type of volcano.

Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated.However, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. Thiscreates a huge hole or caldera into which the surface collapses ( Figure 1.7).

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FIGURE 1.5In 1943, a Mexican farmer first witnessed a cinder cone erupting in hisfield. In a year, Paricutín was 336 meters high. By 1952, it reached 424meters and then stopped erupting.

FIGURE 1.6This Landsat image shows the topogra-phy of San Francisco Mountain, an extinctvolcano, with many cinder cones nearit in northern Arizona. Sunset crater isa cinder cone that erupted about 1,000years ago.

The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sitsabove a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago.Yellowstone has produced many smaller (but still enormous) eruptions more recently ( Figure 1.8). Fortunately,current activity at Yellowstone is limited to the region’s famous geysers.

Long Valley Caldera, south of Mono Lake in California, is the second largest supervolcano in North America (Figure 1.9). Long Valley had an extremely hot and explosive rhyolite about 700,000 years ago. An earthquakeswarm in 1980 alerted geologists to the possibility of a future eruption, but the quakes have since calmed down.

• An interactive image of the geological features of Long Valley Caldera is available here:

http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/interactive/B&R_LongValleyCaldera.swf

A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesiswould be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some ofthe mass extinctions in our planet’s history. No one knows when the next super eruption will be.

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FIGURE 1.7The caldera at Santorini in Greece is solarge that it can only be seen by satellite.

FIGURE 1.8The Yellowstone hotspot has producedenormous felsic eruptions. The Yellow-stone caldera collapsed in the most re-cent super eruption.

Interesting volcano videos are seen on National Geographic Videos, Environment Video, Natural Disasters, Earth-quakes: http://video.nationalgeographic.com/video/player/environment/ . One interesting one is “Mammoth Moun-tain,” which explores Hot Creek and the volcanic area it is a part of in California.

Lesson Summary

• Composite, shield, cinder cones, and supervolcanoes are the main types of volcanoes.• Composite volcanoes are tall, steep cones that produce explosive eruptions.

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FIGURE 1.9The hot water that gives Hot Creek, Cal-ifornia, its name is heated by hot rockbelow Long Valley Caldera.

• Shield volcanoes form very large, gently sloped mounds from effusive eruptions.• Cinder cones are the smallest volcanoes and result from accumulation of many small fragments of ejected

material.• An explosive eruption may create a caldera, a large hole into which the mountain collapses.• Supervolcano eruptions are devastating but extremely rare in Earth history.

Review Questions

1. Rank, in order, the four types of volcanoes from smallest to largest in diameter.2. What factor best determines what type of volcano will form in a given area?3. Which type of volcano is most common?4. Why do pahoehoe and a’a lava erupt from shield volcanoes? Why don’t they erupt from composite volcanoes?5. Why are cinder cones short-lived?6. If supervolcanoes are so big, why did it take so long for scientists to discover them?

Points to Consider

• Composite volcanoes and volcanic cones usually have craters on the top. Why are the craters sometimes U-or horseshoe-shaped?

• Think about plate boundaries again. What type of volcanoes do you think are found at convergent, divergent,and transform boundaries? How about at intraplate sites?

• Some people have theorized that if a huge asteroid hits the Earth, the results would be catastrophic. Howmight an asteroid impact and a supervolcano eruption be similar?

References

1. Left: Courtesy of DR Mullineaux/US Geological Survey; Right: Courtesy of Lyn Topinka/US GeologicalSurvey. Left: http://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_gallery_29.html; Right: http://commons.wikimedia.org/wiki/File:MSH82_st_helens_plume_from_harrys_ridge_05-19-82.jpg . Public Domain

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2. Chris Moore. http://commons.wikimedia.org/wiki/File:Fujifrommarubun.jpg . The copyright holder of thiswork allows anyone to use it for any purpose including unrestricted redistribution, commercial use, andmodification

3. Zachary Wilson. CK-12 Foundation . CC BY-NC 3.04. Nathan Forget. http://www.flickr.com/photos/nathanf/5999830480/ . CC BY 2.05. Courtesy of K Segerstrom/US Geological Survey. http://commons.wikimedia.org/wiki/File:Paricutin_30_-

612.jpg . Public Domain6. Courtesy of NASA’s Earth Observatory. http://earthobservatory.nasa.gov/IOTD/view.php?id=6585 . Public

Domain7. Courtesy of NASA. http://commons.wikimedia.org/wiki/File:Santorini_Landsat.jpg . Public Domain8. User:Kbh3rd/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Yellowstone_Caldera.svg . Pub-

lic Domain9. Image copyright Doug James, 2014. http://www.shutterstock.com . Used under license from Shutter-

stock.com

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