Volcanoes

Pamela J. W. Gore
Georgia Perimeter College

Objectives

Upon completion of this module, the student should be able to:

1. List and describe the factors that determine the nature of an igneous eruption.
2. List the categories of materials that may be emitted during a volcanic eruption.
3. Compare and contrast the three basic types of volcanoes (shield, composite, and cinder cone) as to size, shape and eruptive style.
4. Name a prominent example of each of the three basic types of volcanoes.
5. Relate the distribution of volcanic activity to the plate tectonics model.


Types of volcanoes

  1. Shield
    basaltic composition
    runny, low viscosity lava
    sides slope at 15 degrees or less
    (resembles a Roman shield lying on the ground, hence its name) characterized by relatively quiet eruptions with lava flows
    relatively little explosive activity


    Mauna Loa Volcano, Hawaii, a shield volcano, as viewed from the summit of Kilauea, about 33 miles to the southeast.

    Mauna Loa, on the Big Island of Hawaii, is the largest active volcano in the world. It last erupted in 1984. Mauna Loa erupted 14 times in the 20th Century, and 37 times since 1832. Mauna Loa is the most massive mountain on Earth, rising to an elevation of 13,677 feet above sea level, or 31,677 feet above the sea floor. Its volume is 10,000 miles3.
    The tallest mountain on Earth is located nearby, also on the Big Island of Hawaii. It is Mauna Kea, rising to an elevation of 13,796 feet above sea level, or 31,796 feet above the sea floor.
    Both Mauna Loa and Mauna Kea are shield volcanoes.

    In comparison, Mt. Everest (in the Himalayas), the highest point on Earth above sea level, rises to an elevation of 8848 m (or 29,028 ft). Mt. Everest is NOT a volcano, however.

    The largest volcano in the Solar System is located on the planet Mars. Its name is Olympus Mons (or Mount Olympus), and it is three times as high as the largest volcanoes on Earth (nearly 27 km high). It is about 100 times as massive as one of the Hawaiian volcanoes.


  2. Cinder Cone
    relatively small (less than 300 m or 1000 ft high)
    relatively steep slopes (30 - 40 degrees)
    made of pyroclastic material


    Cinder cone, Puu Puai, created by eruption in 1959, Devastation Trail, Kilauea, Hawaii Volcanoes National Park.

    The volcano Paracutin, in Mexico, is a well-known example of a cinder cone.


  3. Composite Volcano or Strato-volcano

    Eruption of Mt. St. Helens

    large (1 - 10 km across)
    layered structure, consisting of alternating layers of lava and pyroclastic material
    high silica content (sialic or intermediate) with composition of andesite, dacite, and occasionally rhyolite
    These volcanoes make up the largest perentage of the Earth's volcanoes (about 60%)
    Examples: Mt. Vesuvius, Cascade Range volcanoes such as Mt. St. Helens and Mt. Rainier

According to Volcano World, there are six major types of volcanoes:

  1. shield volcanoes
  2. strato volcanoes (also called composite volcanoes)
  3. large rhyolite caldera complexes
  4. monogenetic fields
  5. flood basalt provinces
  6. mid-ocean ridges
"Shield volcanoes--the largest of all volcanoes on Earth (not counting flood basalt flows). The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep (you can't pile up a fluid that easily runs downhill). These volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosivity fountaining that forms cinder cones and spatter cones at the vent, however, 90% of the volcano is lava rather than pyroclastic material. Shield volcanoes are the common product of hotspot volcanism but they can also be found along subduction-related volcanic arcs and out by themselves as well."(Volcano World, http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp12/question1401.html).

"Strato volcanoes--making up the largest percentage (~60%) of the Earth's volcanoes, these are characterized by eruptions of cooler and more viscous lavas than basalt. The usual lavas that erupt from strato volcanoes are andesite, dacite, and occasionally rhyolite. These more viscous lavas allow gas pressures to build up to high levels (they are effective "plugs" in the plumbing), therefore these volcanoes often suffer explosive eruptions. They are usually about half-half lava and pyroclastic material, and the layering of these products gives them their other common name of composite volcanoes. Strato volcanoes are commonly found along subduction-related volcanic arcs."(Volcano World, http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp12/question1401.html).

"Large rhyolite caldera complexes--the most explosive of Earth's volcanoes. These are volcanoes that often don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure. The collapsed depressions are called calderas, and they indicate that the magma chambers associated with the eruptions are huge. Fortunately we haven't had to live through one of these since 83 AD when Taupo erupted. Yellowstone is the most famous US example of one of these. Their origin is still not well-understood. Many folks think that Yellowstone is associated with a hotspot, however, a hotspot association with most other rhyolite calderas doesn't work. " (Volcano World, http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp12/question1401.html).

"Monogenetic fields. These also don't look like a "volcano", rather they are a collection of sometimes hundreds to thousands of separate vents and flows. These are the product of very low supply rates of magma. The supply rate is so slow and spread out that between the times of eruptions the plumbing doesn't stay hot so the next batch of magma doesn't have any preferred pathway to thesurface and it makes its own path. A monogenetic field is kind of like taking a single volcano and spreading all its separate eruptions over a large area. There are a number of monogenetic fields in the American southwest, and there is a famous one in Mexico called the Michoacan-Guanajuato field. "(Volcano World, http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp12/question1401.html).

"Flood basalt provinces--another strange type of "volcano". Some parts of the world are covered by thousands of square kilometers of thick basalt lava flows--some flows are more than 50 meters thick, and individual flows extend for hundreds of kilometers. The old idea was that these flows went whooshing over the countryside at incredible velocities. The new idea is that these flows are emplaced more like pahoehoe flows--slow moving, with most of the great thickness being accomplished by injecting lava into the interior of an initially thin flow. The most famous US example of a flood basalt province is the Columbia River Basalts, covering most of SE Washington State, and extending all the way to the Pacific and into Oregon. The Deccan Traps of NW India are a much larger flood basalt province."(Volcano World, http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp12/question1401.html).

"Mid-ocean ridge volcanism occurs at plate margins where oceanic plates are created. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins--some folks consider this the largest volcano on Earth. Here, the plates are pulled apart by convection in the upper mantle, and basalt lava intrudes to the surface to fill in the space. Or, the basalt intrudes to the surface and pushes the plates apart. Or, better yet, it is a combination of these two processes. Either way, this is how the oceanic plates are created. A recent mid-ocean ridge eruption took place along the Gorda Rise--the mid-ocean ridge that separates the Juan de Fuca plate from the northern part of the Pacific plate. "(Volcano World, http://volcano.und.nodak.edu/vwdocs/frequent_questions/grp12/question1401.html).


Viscosity of the magma controls the type of volcano.
Viscosity is controlled by the composition and temperature of the magma.
Silica (SiO2) content controls viscosity.
Granitic/sialic magma is more viscous (stiffer) than basaltic/mafic magma.
Hotter lavas are less viscous.
(Remember Bowen's Reaction Series and the relative temperatures of sialic vs mafic magmas

Shapes of volcanoes are due to the viscosity of the magma (or lava).
Runny basaltic lava will not form a steep cone; forms relatively flat shield volcanoes.
Mafic lavas are low in silica (only about 50% SiO2)
Sialic lavas may have more than 70% SiO2.

Explosivity of the volcano is also controlled by the viscosity (and chemistry) of the lava or magma.
Gases are easily released from low viscosity (runny) lavas.
Ex. = vesicular basalt.


Olivine crystals in vesicular basalt. In building stone at Hawaiian Volcano Observatory.
The larger olivine crystals are several millimeters in diameter.

Gases are not easily released from stiff, viscous magmas or lava.
The pressure builds up.
When the pressure builds high enough, a violent explosion can occur.
Typical of composite cones or strato-volcanoes


Basic parts of a volcano


Looking down into Diamond Head, Island of Oahu, Hawaii


Looking into Caldera of Kilauea Volcano, which is about 2 to 2.5 miles in diameter, and about 400 feet deep. A road around the crater rim is 11 miles long. Steam is rising from the inner crater, Halema'uma'u, near the center of the left photo.

Kilauea is the world's most active volcano, and it has been erupting continuously since January 3, 1983, with lobes of lava threatening (and destroying) housing subdivisions, and entering the sea through lava tubes. Kilauea rarely erupts from its summit. Instead it erupts from vents on its flanks, particularly along its east and southwest rift zones. The summit of Kilauea is about 4000 feet above sea level.
See explanatory diagram of Kilauea caldera on sign at Volcanoes National Park.


Things that come out of volcanoes

  1. Lava flows


  2. Pyroclastic debris


    Volcanic bomb


    Pyroclastic material from Kilauea volcano, Hawaii.


    Olivine crystals (green) and Pele's Tears (black oval) in pyroclastic debris along Devastation Trail, Hawaii Volcanoes National Park. (In the palm of a hand.)


  3. Gases

    Between 100 and 2000 metric tons of sulfur dioxide (SO2) are released per day from Kilauea. The rain is so acidic that a desert has formed downwind from the summit of the volcano.


    Sulfur Banks, Hawaii Volcanoes National Park. Sulfur and other minerals are being deposited here from gases rising from the hot magma below. The steam contains mostly water vapor, with lesser amounts of carbon dioxide and hydrogen sulfide.


    Warning signs about gases in Hawaii Volcanoes National Park.


  4. Nuee ardentes
    Glowing clouds of volcanic gases (steam) and pyroclastic debris (ash) which avalanche down the side of a volcano. They can reach speeds of 125 mi/hr (200 km/hr).

    A nuee ardente from Mount Pelee, on the Caribbean island of Martinique, destroyed the town of St. Pierre in 1902, killing almost all of its 28,000 inhabitants at once (a prisoner in a dungeon, a shoemaker, and a few people on ships in the harbor survived).


Major eruptions

  1. 1883 Krakatoa, Indonesia
    explosion heard around the world (4800 km away)
    18 km3 of volcanic debris ejected

  2. 1815 Mt. Tambora, Indonesia
    ejection of 30 km3 of volcanic debris
    caused "year without a summer"
    global temperature drop
    ash in atmosphere blocks incoming sunlight and reflects it back into space.
    another side effect of ash in the atmosphere is vivid sunsets.

  3. May 18, 1980 Mt. St. Helens, Washington State
    for comparison, only 1 - 2 km3 of volcanic debris was ejected.

  4. 5000 BC, Mt. Mazama, Oregon
    like Mt. St. Helens, part of the Cascade Mountain chain
    explosion ejected 40 km3 of volcanic debris
    magma chamber empited and collapsed to form a caldera,
    now filled with water, it is Crater Lake.

  5. 1902 Mt. Pelee, Martinique (Carribbean)
    See above description of nuee ardente

  6. April 4, 1982 El Chichon, Mexico
    associated with El Nino climatic abberation

  7. June 15, 1991 Mt. Pinatubo, Philippines
    ejection of 2 miles3 of dust and fine ash
    destroyed more than 42,000 homes and 100,000 acres of cropland
    killed approximately 900 people
    evacuation and abandonment of Clark Air Force Base (US)
    eruption was predicted 1 month in advance and monitored
    SO2 aerosol cloud circled Earth in just 21 days
    stratospheric haze caused a 1 degree temperature drop.


Distribution of volcanoes

  1. Ring of Fire
    Surrounds Pacific Ocean.

  2. Hot spots
    Hawaiian Islands
    mantle plumes

  3. Spreading centers
    mid-ocean ridges (Iceland)
    rift valleys (Mt. Kilamanjaro, Africa)


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Volcano diagrams used with permission of Bruce E. Herbert, Texas A & M University, Big Bend Virtual Field Trip

This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, Clarkston, GA

October 1995
Modified March 4, 1998
Modified July 17, 1999
Photo added Feb 3, 2006
Typo corrected Aug 22, 2007