Earth image Volcanoes

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.
  5. Describe the three major types of volcanoes, telling which erupt gently and which erupt explosively.
  6. Describe the factors controlling the type of volcano.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E5. Students will investigate the scientific view of how the Earth's surface is formed.
  • S6E5c. Describe processes that change rocks and the surface of the Earth.
  • S6E5d. Recognize that lithospheric plates constantly move and cause major geological events on the Earth's surface.
  • S6E5e. Explain the effects of physical processes (plate tectonics, erosion, deposition, volcanic eruptions, gravity) on geological features including oceans (composition, currents, and tides)

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • The interior of the earth is hot. Heat flow and movement of material within the earth cause earthquakes and volcanic eruptions and create mountains and ocean basins. Gas and dust from large volcanoes can change the atmosphere.
  • Some changes in the earth's surface are abrupt (such as earthquakes and volcanic eruptions) while other changes happen very slowly (such as uplift and wearing down of mountains). The earth's surface is shaped in part by the motion of water and wind over very long times, which act to level mountain ranges.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Lithospheric plates on the scales of continents and oceans constantly move at rates of centimeters per year in response to movements in the mantle. Major geological events, such as earthquakes, volcanic eruptions, and mountain building, result from these plate motions.
  • Land forms are the result of a combination of constructive and destructive forces. Constructive forces include crustal deformation, volcanic eruption, and deposition of sediment, while destructive forces include weathering and erosion.
  • Internal and external processes of the earth system cause natural hazards, events that change or destroy human and wildlife habitats, damage property, and harm or kill humans. Natural hazards include earthquakes, landslides, wildfires, volcanic eruptions, floods, storms, and even possible impacts of asteroids.
  • Students should understand the risks associated with natural hazards (fires, floods, tornadoes, hurricanes, earthquakes, and volcanic eruptions), with chemical hazards (pollutants in air, water, soil, and food), with biological hazards (pollen, viruses, bacterial, and parasites), social hazards (occupational safety and transportation), and with personal hazards (smoking, dieting, and drinking).

A volcano is a conical landform or mountain built from lava and ash. The lava and ash erupt from the volcano through a vent that connects with reservoirs of hot molten rock or magma deep within the Earth.

More than 50 volcanoes in the United States have erupted one or more times in the past 200 years. The most volcanically active regions of the Nation are in Alaska, Hawaii, California, Oregon, and Washington.

There are more than 1500 volcanoes on the Earth which have the potential to become active because they have erupted within the past 10,000 years. Approximately 10% of the Earth's population live close enough to a volcano to be at risk of volcanic hazards.

Volcanoes produce a wide variety of natural hazards that can kill people and destroy property. This simplified sketch shows a volcano typical of those found in the Western United States and Alaska, but many of these hazards also pose risks at other volcanoes, such as those in Hawai`i. Some hazards, such as lahars and landslides, can occur even when a volcano is not erupting.

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 or ropy lava.
      As the lava flows, the surface cools and hardens. The lava continues to move underneath the hardened skin, and the skin wrinkles and may resemble twisted ropes. Pahoehoe may flow at 30 km/hr (about 20 mi/hr).

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

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

    • Aa or blocky lava.
      Aa is a slow-flowing, rough, blocky basaltic lava with sharp, jagged projections. As the molten interior advances, the outer crust is broken, giving the flow an appearance of advancing rubble. Aa flows may advance at 5 to 50 meters/hr. Aa is basaltic lava which has flowed farther from the vent than pahoehoe, and has cooled, become more viscous, and lost much of its dissolved gas.

      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

    The word "pyroclastic" refers to any fragmental material released from a volcano. It is derived from the Greek, pyro, meaning "fire" and klastos, meaning "fragments". Pyroclastic fragments range from dust-sized up to blocks that weigh several tons.

    • Volcanic bombs
      Volcanic bombs are ejected as hot, glowing lava. They become streamlined in shape as they sail through the air. They are generally 64 mm or more in length.

      Volcanic bomb

    • Volcanic ash and dust
      Volcanic ash and dust are formed as gases expand in rising magmas within the volcanic vent. Volcanic ash is less than 2 millimeters in diameter. Volcanic dust is about the consistency of flour, and can travel far from an erupting volcano. Hot volcanic gases expand explosively, and the frothy lava (pumice) is blown into fine-grained fragments.

      Image of ash-fall tephra deposit about 9 cm thick at former U.S. Clark Air Base, Philippines, about 25 km east of Mount Pinatubo. The pumice and ash fell to the ground on June 15, 1991. Photograph by R.P. Hoblitt on June 16, 1991. Image courtesy of U.S. Geological Survey.

    • Lapilli (walnut-sized) and cinders
      Lapilli are pyroclastic fragments ranging between 2 and 64 millimeters in diameter. Some lapilli are roughly the size of a walnut.

    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).

  5. Lahars
    Lahars are debris flows or mudflows, which flow rapidly down the side of a volcano. They are mixtures of pyroclastic debris and water, originating on the slopes of a volcano. These flows typically form because of rapidly melting snow and ice on the top of the volcanic mountain, or as a result of intense rainfall on pyroclastic debris.

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

Volcanoes can occur in a number of different tectonic settings.

Map showing distribution of active volcanoes around the world, along with the tectonic plate boundaries. The Ring of Fire is labeled.

Volcanoes at Convergent Plate Boundaries - The Ring of Fire

Most volcanoes are located at convergent plate boundaries (such as around the rim of the Pacific Ocean, called the Ring of Fire), where the Earth's tectonic plates move toward one another. Approximately 80% of the Earth's volcanoes are in the Pacific Rim region. The volcanoes that occur around the Pacific rim are the result of subduction occurring at convergent tectonic plate boundaries, where one tectonic plate moves beneath another at a deep sea trench. When an oceanic plate is overridden by another plate, the oceanic plate begins to melt, and the molten rock begins to rise, eventually providing the magma source for a volcano. These volcanoes may be in island arcs (such as the Aleutian Islands off Alaska, or Japan, New Zealand, or Indonesia), or they may be a chain of volcanoes along the edge of a continent (such as the volcanoes in the Cascade Range of the northwestern US and Canada, or the Andes Mountains in South America).

Volcanoes at Divergent Tectonic Boundaries

Volcanoes also occur at divergent tectonic plate boundaries or spreading centers (such as Iceland along the Mid-Atlantic Ridge or Mt. Kilamanjaro along the East African Rift Valley in Africa), where the plates are pulling apart and magma wells up into the gap, producing volcanoes and lava flows.

Intra-plate Volcanoes - Volcanoes at Hot Spots

Still other volcanoes occur within a tectonic plate (i.e., "intra-plate" volcanoes), as a result of mantle hot spots or mantle plumes (such as the Hawaiian hot spot).

Map of the Pacific basin showing the location of the Hawaiian Ridge-Emperor Seamount Chain. Base map reprinted by permission from World Ocean Floor Panorama by Bruce C. Heezen and Marie Tharp, Copyright 1977. Image courtesy of U.S. Geological Survey.

Artist's conception of the northwestward movement of the Pacific Plate over the fixed Hawaiian "Hot Spot" to illustrate the formation of the Hawaiian Ridge-Emperor Sea- mount Chain. The distinctive linear shape of the Hawaiian-Emperor Chain reflects the progressive movement of the Pacific Plate over a deep immobile hot spot. This hot spot partly melts the region just below the overriding Pacific Plate, producing small, isolated blobs of magma. Less dense than the surrounding solid rock, the magma rises buoyantly through structurally weak zones and ultimately erupts as lava onto the ocean floor to form volcanoes. The progressive northwesterly drift of the islands from their point of origin over the hot spot is well shown by the ages of the principal lava flows on the various Hawaiian Islands from northwest (oldest) to southeast (youngest), given in millions of years: Kauai, 5.6 to 3.8; Oahu, 3.4 to 2.2; Molokai, 1.8 to 1.3; Maui, 1.3 to 0.8; and Hawaii, less than 0.7 and still growing.

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

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 8, 2005
Image links updated June 12, 2008