1. Understanding Earth: Chapter 12 - Volcanoes
  2. film Earth Revealed: movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view #13 Volcanism and #14 Intrusive igneous rocks.
  3. Lab: Volcanism (pdf)
  4. Online resources:
Terms: extrusive, intrusive, aphanitic, phaneritic, volcano, stratovolcano, shield volcanoe, caldera, pyroclastic, lahar, harmonic tremors

Significance of volcanism

Volcanic eruptions can be deadly (The Deadliest Volcanic Eruptions). Eruptions cause local devastation, emit poisonous gases and can generate tsunami. Colossal eruptions cause short- or long-term climatic changes leading to crop failures, famine, and possibly extinction.   The eruption of Mount Toba 70,000 years ago nearly wipe out Humankind. (See Ambrose, 2005, Bradshaw Foundation).  The eruption of Krakatoa in 535 brought on a sustained period of cold lasting nearly 10 years which brought on the famine, The Plague, civil unrest and the Dark Ages.  

Towering volcanoes, broad plains and plateaus, and deep basins are just some of the landforms created by volcanism.   The type of landform and the ejecta that blankets it reflected the eruptive style of the volcano.  Yellowstone National Park lies within a large basin within a plateau built of volcanic ash.  Both these features indicate that this sleeping giant with its geyser basins has the eruptive potential to change civilzation as we know it.

Extrusive (volcanic) Landforms

Extrusive landforms are created by igneous activity at the surface of the earth. There is wide range of volcanic landforms that includes large depressions (calderas and Maars), conical mountains (volcanoes), and broad lava plains and plateaus. The size and shape of these landforms is controlled by a number of factors most of which are listed below.

Figure 1. Common volcanic landforms from least explosive (top) to most explosive (bottom). The increasing violence of a volcanic feature is largely related to magma viscosity which is a function of magmas composition. (Mondified from Topinka, USGSICVO, 1997, obtained from the U.S.G.S. Cascade Volcanic Observatory)

Factors governing the shape of a volcanic feature

1. Type of eruption (eruptive style):

Volcanoes may produce voluminous outpourings of fluid lava, like the Hawaiian volcanoes, or explode violently emitting huge quantities of tephra (ash and the like) and gas.  The deadly nature of an eruption, related to the exposiveness and shear volume of material produced, is quantified using the Volcanic explosivity Index (VEI).  Fortunately supervolcanos (VEI >7) don't erupt often.  The factors that most influence the eruptive style are silica composition and gas content (fig. 2).

Effects of Temperature and Gas Content: T

  • Basaltic and andesitic flows become less fluid as lava cools and gas content decreases. (Note these videos of the 2006 eruption of Mount Etna, the flows are fluid near the vent and pasty farther on after the magma has cold and lost much of its gas.)
  • Felsic (silica-rich magmas) are explosive when charged with gas. Later gas-poor eruptions produce pasty flows of glassy rubble.  (Observe this video of the 1944 eruption of Mt. Vesuvius in Italy. Note the initial pyroclastic eruption is followed by thick, slow, but  unstoppable flows.)

    A continuum exists between non-violent effusive eruptions and violent explosive eruptions of pyroclastic flows.  This is because there is a continuum between the composition of magmas (fig. 2).
  • Eruptive style governed by:
    • Silica composition of the magma (controls viscosity) learn more from VolcanoWorld
    • Temperature
    • gas content (propels lava into the air)
      • Water Vapor (50=80%)
        • Magmatic eruptions having a high water content can be violently explosive such as Indonesian volcano Krakatoa, which erupted in 1883. These eruptions are often referred to as phreatic or phreatomagmatic.
        • Water only Phreatic eruptions such as geysers emit primarily steam and/or hot water.
      • Other gasses include SO2, CO2, H2S, Cl2

2. Vent characteristics, such as the size, shape (i.e. pipes and fissures), and number can control whether a the landform develops into a volcano (conical mountain) or a broad plateau. (vent: USGS photo glossary)

3. Volume of material erupted over time governs the size and extent of th feature.

Classification and description of volcanic landforms

1. Cylindrical landforms

2. Plains and Plateaus

  • Basalt flows, plains and plateaus: Typically composed of fluid basalts (flood basalts) extruded from fissures (dikes); may be dotted with cinder cones
    • Examples: Snake River Plain, Columbia River Plateau, Iceland
  • Ignimbrite sheets and plateaus: Composed of welded pyroclastic flows from one or more violent caldera eruptions
    • Examples :Katmai ignimbrite sheet; Yellowstone Plateau, Bishop Tuff from the Long Valley Caldera, CA

Volcanic Hazards

Primary Hazards
Secondary Hazards

Intrusive Igneous Features


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