Dr. Pamela Gore
Georgia Perimeter College


  1. Describe the forces that cause the formation of volcanoes.
  2. Describe internal (inside the Earth) and external (on the surface of the Earth) processes associated with volcanoes.
  3. Explain how plate movement relates to the formation of volcanoes.
  4. Construct models that simulate faults, folds, volcanoes, mountains, valleys, etc.

Other things to know:

  1. Describe the three major types of volcanoes, telling which erupt gently and which erupt explosively.
  2. Describe the factors controlling the type of volcano.
This section addresses the following Georgia QCC standard(s)
  1. Uses maps to illustrate the "ring of fire".
  2. Recognizes constructive and destructive forces change the earth's surface.

This section addresses the following National Science Education Standard(s):
  1. Content Standard D:
    As a result of their activities in grades 5 - 8, all students should develop an understanding of:
    1. Structure of the Earth system
    2. Earth's history
    3. Earth in the solar system
  2. Content Standard F:
    1. Personal health
    2. Populations, resources, and environments
    3. Natural hazards
    4. Risks and benefits
    5. Science and technology in society

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 also a shiled volcano. It 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. Ranier

Factors affecting the formation of volcanoes

Viscosity of the magma controls the type of volcano.
Viscosity is controlled by:
  1. Composition (silica (SiO2) content) of the magma.
  2. Temperature of the magma
Granitic/sialic magma is more viscous (stiffer) than basaltic/mafic magma.
Basaltic or mafic magmas tend to be fairly runny.
Also hotter lavas are less viscous (more runny).

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
    • pahoehoe

      Pahoehoe flows (from circa 1993) near Kalapana Black Sand Beach.

      Pahoehoe at the end of Chain of Craters Road (from circa 1995).

    • aa

      Weathered aa lava flow, Kalapana Region, Big Island of Hawaii

      1974 Aa lava flow, Chain of Craters Road, Hawaiian Volcanoes National Park.

  2. Pyroclastic debris
    • volcanic bombs
    • ash and dust
    • lapilli (walnut-sized) and cinders

    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
    • H2O (70%)
    • CO2 (15%)
    • N2 and nitrogen compounds (5%)
    • sulfur compounds (H2S, SO4, SO2, etc.) (5%)
    • minor amounts of Cl, H2, Ar, etc.
      NO free oxygen

    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)

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

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

October 1995
Modified March 4, 1998
Modified September 21, 1998
Modified July 17, 1999
Modified May 12, 2000
Modified June 4, 2000
Modified February 24, 2001