Environmental Geology - Geol 406/506

Module 6 - Volcanoes

by Susan Williams, Anni Watkins, Laura De Grey, and Kaplan Yalcin


Reading Assignment: Keller, Chapter 8

Supplemental: ISU lecture in PDF format or as a powerpoint show.

lassen.jpg (64585 bytes)

Mt. Lassen, a plug dome


Volcanoes are proof that the Earth is alive, active, and ever-changing. The word volcano comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan--the blacksmith of the Roman gods. They thought that the hot lava fragments and clouds of dust erupting from Vulcano came from Vulcan's forge as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful. The volcano Furnas in the Azores has entered our language as the word furnace. Today we know that volcanic eruptions are not supernatural but natural phenomena that can be studied and interpreted by scientists.

Your study of volcanoes will be greatly enhanced by a journey through the Internet.   Look for links that will take you to visit some of the greatest volcanoes on Earth. The links are in the text of this page and at the end.

hood_lost_lake.jpg (41755 bytes)
Mt. Hood, a beautifully symmetric stratovolcano

A volcano is simply a vent at the surface of the earth through which lava and other volcanic materials are ejected from the Earth’s interior. Lava is the term used for magma that has reached the surface because of a volcanic eruption. Magma is the molten material below the Earth's surface. Most lava erupted onto the Earth’s surface is basalt. The viscosity of the magma determines both the type of volcano which forms and the activity associated with that type of volcano. Magma viscosity is directly related to silica (SiO2) content (approximately 50-70 %) and temperature.

More than half of the Earth's volcanic activity above sea level takes place in the Ring of Fire (click to see a map), a belt of convergent plate margins (subduction zones) surrounding the Pacific plate.

Popocatepetl composite volcano in Mexico is on the Ring of Fire; USGS photo by John W. Ewart
Popocatepetl composite volcano


Mauna Loa, Hawaii, is an excellent example of a shield volcano.


Shield volcanoes, the largest of the three types, are gently sloping and built almost entirely of low viscosity basaltic lava flows. The eruptions are generally nonexplosive due to the low silica content. Shield volcanoes are typified by those on the Hawaiian and Galapagos Islands and on Iceland, although Iceland also caontains other types of voclanoes.. Numerous small shield volcanoes are typical throughout the eastern Snake River Plain in Idaho, USA. Examples include the Wapi lava field and Hells Half Acre.

Shield volcano in cross section



Mt. Shasta is a stratovolcano or Composite Volcano.


Composite volcanoes are the most beautiful - - - and ! - - - the most deadly of the volcano types, at least in Holocene time.

Their lovely steep-sided, symmetrical cone shapes are built up by eruptions of intermediate viscosity andesitic lava and explosive tephra.

Examples of composite volcanoes, also called stratovolcanoes, are Mount Shasta in California, Mount St. Helens and Mount Rainier in Washington state, and Mount Fuji in Japan.





stratovolcano.gif (9900 bytes)
Cross-section of a composite volcano.


East Butte, on the eastern Snake River Plain in Southern Idaho (USA), is a rhyolitic volcanic dome.

Photo by Scott Hughes
EaButte_reduced.jpg (20504 bytes)


Volcanic Domes comprise the third primary type of volcano. They are formed by highly viscous rhyolitic magma (approximately 70% silica). Volcanic domes are typically small. Some are subject to explosive blowouts during dome building processes. Domes commonly occur adjacent to or within craters of composite volcanoes. Other domes begin as shallow laccolithic intrusions that grow and expand beyond subsurface confinement.


Those of us living in the Pocatello, Idaho, area are able to see examples of such volcanic domes. Big Southern Butte and East Butte on the Snake River Plain are two excellent examples.

dome5.gif (10262 bytes)
Assignment: Part 1

Investigate two recent eruptions of different types of volcanoes (last two decades). The Smithsonian Global Volcanism Program is an excellent archive of volcanic activity. List and briefly describe the primary and secondary environmental effects of each eruption.  What were the long term effects of each of these eruptions on the global environment?

Other Types of Volcanic Activity

There are parts of the world covered by thousands of square kilometers of thick basalt lava flows called Flood Basalts. Individual flows may be more than 50 meters thick and extend for hundreds of kilometers. The largest of the flood basalts in the United States is the Columbia River Basalt Group (CRBG or just CRB). The CRBG covers most of southeastern Washington State, part of western Idaho, areas throughout Oregon, and extends all the way to the Pacific Ocean.


The Grande Ronde Basalt in Oregon, an example of a flood basalt. Photo by Thor Thoradson

Crb97-f9.jpg (36878 bytes)


Closer to Pocatello is the Snake River Plain (SRP), a broad arch across the southern part of Idaho. The SRP extends 600 kilometers eastward from the Oregon border to the Yellowstone Plateau. It is covered by basaltic lava flows as recent as approximately 2,000 years ago (just a minute ago by geologic time standards!).  These were earlier considered to represent a part of the Columbia River flood basalt system, but the basalt lavas of the SRP were erupted from numerous fissures and small shields to form "basaltic lava plains" a term coined by R.Greeley, Arizona State University.

The Deccan Traps in northwest India are even larger than the Columbia River Basalt Province. You can read about this area and see photographs at http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/india/deccan.html

floodbasalts.gif (11898 bytes)

Assignment: Part 2

Use the Internet to learn more about flood basalts.  What extinction events are associated with flood basalts such as the Deccan Traps?  What can be said about the eruption of flood basalts in relationship to extinction? 

yellowstone_caldera.gif (20756 bytes)

Yellowstone National Park is a resurgent caldera.

The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magmal onto the Earth’s surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides, or collapses, into the emptied space to form a huge depression called a caldera. Many cataclysmic caldera-forming eruptions have taken place throughout the world in the past million years; those occuring during historic time, though enromoulsy destructive, pale in size to older events during the Pleistocene.

Recent caldera forming eruptions, their location, and volume of magma erupted:

For comparison, the 1980 eruption of Mount St. Helens erupted only 1 km3 of magma!

The Toba caldera on Sumatra, Indonesia, site of the second largest volcanic eruption ever discovered.  The caldera is partly filled by Lake Toba.  The flat area in the distance are pyroclastic deposits from the eruption.

Three major caldera systems are found in North America: Long Valley, California;  Valles, New Mexico; and Yellowstone, Wyoming.. A major volcanic explosion happened 700,000 years ago, producing the Long Valley Caldera and its deposit, the Bishop Tuff. Harmonic tremors (earthquake swarms, up to a magnitude of 5-6) occurred in the early 1980’s, indicating magma movement is still occurring.

Caldera_eruption.gif (12106 bytes)

The Yellowstone Volcanic Field has had three major eruptive periods. The oldest event produced the Huckleberry Ridge Tuff, approximately 2,100,000 years ago. The second event occurred 1,300,000 years ago resulting in the Mesa Falls Tuff. The most recent cataclysmic explosion occurred a mere 600,000 years ago, producing the Lava Creek Tuff. Yet the Yellowstone system is still active. Old Faithful and other geysers and hot springs indicate magma is still present beneath the caldera floor.

Both the Long Valley Caldera and Yellowstone Caldera are considered resurgent calderas. Changes in the caldera floor indicate magma movement at various depths beneath both calderas. The caldera floors have been slowly doming upward since the cataclysmic explosions occurred eons ago.

Click on the phrase for an excellent discussion, "Principal Types of Volcanoes," which includes pictures and diagrams.


The effects of volcanoes can be divided into primary and secondary effects, which could also be called primary and secondary causes for human concern.

Primary effects are produced directly by the volcanic activity. Examples include lava flows, ash-flows, lateral blasts, ash-falls, and gases. Secondary effects are the result of primary effects. Examples include lahars/mudflows, floods, fires, and tsunamis.  Disruption of normal human activity, such as sanitation and farming leads to famine and disease.

volcano_hazards.gif (35017 bytes)


Lava Flows

Lava flows are streams of molten rock. Lava flows can erupt relatively nonexplosively and move very slowly (a few meters to a few hundred meters per hour) or they can move rapidly (typically down steep slopes). Gas content and eruptive temperature are essential factors in the lava flow’s behavior. The fastest moving lava flows (higher gas content and temperature) produce Pahoehoe, which has a ropy texture and a smooth surface when hardened. Slow moving lava flows (cooler and less gas-enriched) break up while moving. This produces A’a, which has a blocky texture and a jagged surface.

LavaFlow1.jpg (7944 bytes)

LavaFlow2.jpg (15916 bytes)

Most lava flows are slow enough that they are seldom a threat to human life. Such flows generally follow a predictable course. However, lava flows can cause extensive damage or total destruction by burning, crushing, or burying everything in their paths. Whole villages have been known to completely disappear beneath lava flows! Therefore, to avoid such destruction, controlling a lava flow has become important. Several instances of successful deflection have actually occurred. Hydraulic chilling (see- Case History of Mount Helgafell, Iceland, on page 213 of the textbook), constructing walls, and bombing are methods that have been used. However, there have been many unsuccessful attempts to deflect lava flows. Consequently, this area is ripe for further study and evaluation.

Lava flows on Kilauea Volcano move through forests of ohia trees, creating a dense cloud of wood smoke. The lava fountain (photograph on left) erupted from the Pu'u O'o vent on June 26, 1986, and supplied lava to the advancing flow, which had traveled just 3.6 kilometers by the time the eruption ended later in the day. The temperature of basalt lava at Kilauea reaches 1160 degrees Celsius. These photos were obtained from the U.S. Geological Survey volcanic hazards program.  Click HERE for additional U.S.G.S. volcanic hazard photos of lava flows.

Pyroclastic Hazards

Volcanic explosions produce volumes of tephra. Tephra is the material blown out of the volcanic vent when an explosion occurs. Ash-flows, lateral blasts, and ash-falls are the types of pyroclastic activity that produce tephra, with composite volcanoes and large calderas the vent sources.

Pyroclastic flows (also called ash-flows) are high speed avalanches of hot ash, rock fragments, and gas which move down the sides of a volcano during explosive eruptions. These flows occur when the vent area or ash column collapses. Because pyroclastic flows can reach 1500 degrees F and travel at high speeds (160-250 kilometers per hour and up), they are extremely destructive and deadly. Pyroclastic flows are typical of composite volcano eruptions, but are also associated with large caldera systems.

15_small.gif (15694 bytes)

During the May 1980 eruption at least 17 separate pyroclastic flows descended the flanks of Mount Saint Helens.

Pyroclastic flows are sometimes called nuees ardentes, French for "glowing avalanches," because of the hot, often incandescent mixtures of volcanic fragments and gases. During the 1902 eruption of Mont Pelee in Martinique, a nuee ardente demolished the coastal city of St. Pierre, killing nearly 30,000 people.

Lateral Blasts are explosive events in which gas and ash are ejected from the side of a volcano and travel away from the volcano at velocities that sometimes exceed the speed of sound. See Figure 8.28 in your textbook for a photograph of the lateral blast at Mount St. Helens. Base surges form by collapse of steam-saturated eruption columns and travel outward along the ground surface.

10.jpg (83275 bytes)

Effects of lateral blast on forests near Mount St. Helens, 1980.

An explosive eruption blasts molten and solid rock fragments (tephra) into the air with tremendous force. The largest fragments, bombs, fall back to the ground near the vent. The smallest rock fragments, ash, rise into the air, forming a huge and billowing eruption column. Volcanic ash is composed of fragments of rock, minerals, and glass that are less than 2 millimeters in diameter.

Ash raining out of an eruption column causes another hazard called Ash-fall. Large eruption columns form ash clouds that can travel hundreds of kilometers downwind from a volcano, resulting in ashfall over enormous areas. Ash from the May 18, 1980, eruption of Mount St. Helens was deposited over 22,000 square miles of the western United States. In the adjacent figure, ash-fall from Mount St. Helens is compared with a recent eruption from Mt. Pinatubo, as well as an eruption from a Yellowstone-type caldera.

ashfall.gif (5691 bytes)

Even a small ash-fall poses a serious threat to people, crops, machinery, and computers. People can have serious respiratory problems due to fine ash particles. Vegetation covered by layers of volcanic ash is virtually destroyed. Surface water (lakes, streams) can be seriously contaminated. The weight of thick and/or wet layers of ash can cause structural damage (example- collapsing roofs). Windborne ash causes serious problems for aircraft. During the past 15 years, about 80 commercial jets have been damaged when volcanic ash was sucked into jet engines by inadvertently flying into an ash cloud.

volcanic_ash.gif (277742 bytes)

Volcanic Gases

All magmas contain dissolved gases that are released during and between eruptive episodes. These gases are predominately steam, followed in abundance by carbon dioxide, compounds of sulfur and chlorine, and lesser amounts of other gases. While they rarely reach populated areas in lethal concentrations, gases can be injected to great heights in the atmosphere by volcanic eruptions, in some cases spreading throughout the globe.

 These gases are converted to acid droplets in the atmosphere by oxidation on water droplets, inlcuding hydrochloric, hydroflouric, and sulfuric acid.  Sulfuric acid droplets affect climate by reflecting incoming solar radiation, producing a general cooling of climate for one to as much as five years after an eruption. The cooling effect of voclanic eruptions lasts longest when sulfur compounds are injected into the stratosphere, where removal mechanisms are slow.  The last great famine in human history followed the eruption of Tambora in 1815, which resulting in cooling for 2-3 years following the eruption.  In New England, the year following the eruption has come to be known as "The Year Without a Summer" because of widespread snow and killing frosts in June, July, and August of that year.  More recently, the eruption of Pinatubo in 1991 was followed by a global temperature drop of 0.5 to 0.6 degrees C, temporarily counteracting the effects of global warming due to anthropogenic carbon dioxide emissions.

Carbon dioxide is heavier than air and tends to collect in depressions, such as valleys, where it can occur in concentrations lethal enough to cause suffocation of people and animals. Be sure to read about the tragedy of Lake Nios, Cameroon, in your textbook beginning on p. 210.

fumarole_sampling_mageik.jpg (71105 bytes)

USGS scientist collecting samples of volcanic gases at Mt. St. Helens


Lahars (Debris Flows/Mud Flows) are mixtures of water, rock, ash, sand, and mud that originate from the slopes of a volcano. They can travel over 80 kilometers and commonly reach speeds of 35 to 65 kilometers per hour. Lahars containing a high percentage of rock debris look like fast-moving rivers of concrete. Close to a volcano, they have the strength to rip huge boulders, trees, and structures from the ground and carry them for great distances. Farther downstream the coarser debris settles to the bottom of the flow, leaving mud to continue on to cover everything it passes.

Lahars are formed when masses of unconsolidated, wet debris become mobilized, and are commonly initiated by:

Historically, lahars have been one of the most deadly of the volcanic hazards. The 1985 lahars off Nevado del Ruiz in Colombia killed approximately 23,000 people. Nearly 135 miles (220 kilometers) of river channels surrounding Mount St. Helens were affected by the lahars of May 18, 1980.

A mudline left behind on trees shows depths reached by the mud. Scale indicated by scientist- center far right.

13.jpg (61655 bytes)

Click here to see a chart of volcanic hazards.


Keep in mind that hazards such as volcanic eruptions become natural disasters only when humans get in the way. A primary focus of volcanology is to provide scientific and educational information that can lead to hazard mitigation.

Scientists assess volcanic hazards based on a volcano’s past behavior. They carefully monitor volcanoes for signs of restlessness and inform the public about possible dangers. Scientists have helped to save thousands of lives and millions of dollars worth of property throughout the world.

For instance, mapping studies of Mount Rainier indicate a history of large-scale debris flows. The last giant debris flow, the Osceola Mudflow, swept down Mount Rainier’s western slopes about 500 years ago and reached Puget Sound. Since future eruptions would probably follow a similar pattern, monitoring of the volcano is crucial because of the large number of people who would now be at risk.

How then, do scientists study active volcanoes to predict when an eruption will occur and how extensive it will be? The following are several methods used by scientists to monitor volcanic activity.


Earthquakes commonly provide the earliest warning of volcanic unrest, and earthquake swarms immediately precede most volcanic eruptions.

Ground Movements

Upward and outward movement of the ground above a magma storage area commonly occurs prior to an eruption. Several techniques exist to measure the changing shape of a volcano’s surface caused by the pressure of magma moving underground.

Geophysical Properties

Changes in electrical conductivity, magnetic field strength, and the force of gravity also trace magma movement, sometimes before earthquakes or ground deformation occur.

Gas Geochemistry

Gas emissions, especially of SO2, tend to increase with increasing magmatic activity. The emission rate of SO2 and other gases is monitored on suspicious volcanoes detect potential hints for eruption.

The best way to minimize the effects of an eruption is to incorporate hazards information in land-use planning to avoid high-density development in hazardous volcanic areas. We might not be able to tame the beast, but we can learn to predict its behavior and attempt to stay out of its way.

Assignment: Part 3

Search the Internet and find some historic descriptions of  volcanic eruptions. Pick four and compare and contrast them to each other.  Was destruction and /or loss of life due to primary or secondary effects?  A few suggested eruptions: Krakatau, Pinatubo, Etna, Vesuvius, Nevado del Ruiz, Laki, Katmai, El Chichon.  Don't feel limited to these!


http://www.avo.alaska.edu/   Alaska Volcano Observatory

http://volcanoes.usgs.gov/   USGS Volcano Hazards Program

http://vulcan.wr.usgs.gov/home.html   Cascades Volcano Observatory

http://quake.wr.usgs.gov/VOLCANOES/LongValley/ Long Valley Observatory

http://www.volcano.si.edu Smithsonian Global Volcanism Program

http://volcano.und.nodak.edu/ Volcano World

http://www.dartmouth.edu/~volcano/ The Electronic Volcano

Find out about some Deadly Volcanoes:

Mont Pelee, West Indies

Vesuvius (PBS Nova)

http://www.pbs.org/wgbh/nova/vesuvius/deadliest.html More PBS information

Cascades Volcano Observatory -- Menu of the world's Deadliest Volcanoes

Craters of the Moon National Monument Nodak's page on this famous National Park

End of Module 6