Unit 34


Unit Overview

This unit examines volcanic activity across the globe. The main sections are as follows:

  • Distribution of volcanic activity
  • Volcanic mountains
  • Calderas
  • Landscapes of volcanism

Volcanism is the eruption of molten rock at the Earth's surface, which is often accompanied by rock fragments and explosive gases. Besides being associated with seafloor spreading at midoceanic ridges, volcanism also occurs at subduction zones, specifically convergent plate boundaries. Volcanism is also associated with intraplate hot spots┬┐subcrustal, stationary plumes of extraordinarily high heat over which lithospheric plates move.

Volcanism produces lava, and the mineral content of the lava determines its viscosity and gaseous content. For example, basaltic lava, which occurs in oceanic plates, has a much lower viscosity and contains fewer gases than other types of lava because of its low silica content and high iron and magnesium content. Consequently, basaltic lava flows more easily and is less explosive than other types of lava. Landforms forming from lava are a function not only of the composition of the lava but also of the vent, the thickness of the lava flow, and the nature of the surface over which the lava spreads.

The four types of volcanic landforms are composite volcanoes, lava domes, cinder cones, and shield volcanoes. Composite volcanoes are steep-walled "mountains" that form over subduction zones; they are characterized by pyroclastics and viscous lava. Lava domes are relatively small mounds formed by the oozing of lava with minimal pyroclastic activity. Cinder cones, which are also relatively small, consist almost entirely of pyroclastics. Shield volcanoes are formed from basaltic lavas associated with a hot spot and can exhibit a considerable horizontal extent.

The magma reservoir inside a volcano can be reduced, at which point it will cease to support the volcano. The volcano collapses and a caldera forms. The magma reservoir can also be penetrated by water, thereby producing explosive events known as phreatic eruptions.

Unit Objectives

  • To relate volcanic activity to plate boundary types
  • To discuss typical landforms produced by volcanic eruptions
  • To cite some dramatic examples of human interactions with volcanic environments

Glossary of Key Terms

Aa Angular, jagged, blocky-shaped lava formed from the hardening of not especially fluid lavas.
Caldera A steep-walled, circular volcanic basin usually formed by the collapse of a volcano whose magma chamber emptied out; can also result from a particularly powerful eruption that blows off the peak and crater of a volcano.
Cinder cone Volcanic landform consisting mainly of pyroclastics; often formed during brief periods of explosive activity, they normally remain quite small.
Composite volcano Volcano formed, usually above a subduction zone, by the eruption of a succession of lavas and pyroclastics that accumulate as a series of alternating layers; the larger and more durable composite volcanoes are called stratovolcanoes.
Hot spot A place of very high temperatures in the upper mantle that reaches the surface as a "plume" of extraordinarily high heat; a linear series of shield volcanoes can form on lithospheric plates moving over this plume, as happened in the case of the Hawaiian island chain.
Lahar A mudflow largely comprised of volcanic debris. Triggered high on a snowcapped volcano by an eruption, such a mudflow can advance downslope at high speed and destroy everything in its path; frequently solidifies where it comes to rest.
Lava dome A small volcanic mound produced when acidic lava penetrates and oozes out onto the surface.
Nuee ardente A cloud of high-temperature volcanic gas that races downslope following a spectacular explosion associated with unusually high pressures inside the erupting volcano; incinerates everything in its path.
Pahoehoe Ropy-patterned lava; forms where very fluid lavas develop a "smooth" skin upon hardening that wrinkles as movement continues.
Phreatic eruption An extraordinarily explosive volcanic eruption involving the penetration of water into a superheated magma chamber; such explosions of composite volcanoes standing in water can reach far beyond a volcano's immediate area.
Pyroclastics The collective name for the solid lava fragments that are erupted explosively from a volcano.
Shield volcano Formed from fluid basaltic lavas that flow in sheets that are gradually built up by successive eruptions; in profile their long horizontal dimensions peak in a gently rounded manner rounded manner that resembles a shield. The main island of Hawai'i has some of the world's most active shield volcanoes.
Vent An opening through the Earth's crust from which lava erupts; most eruptions occur through pipe-shaped vents that build volcanic mountains, but fissure eruptions also occur through lengthy cracks that exude horizontal sheets of lava.

Unit Outline

  • Distribution of volcanic activity
    • Most volcanism is related to seafloor spreading or subduction zones
    • Active, dormant, and extinct volcanoes
      • active volcano has erupted in recorded history
      • dormant volcano has not been seen erupting, but shows evidence that it has
      • extinct volcanoes show no signs of eruption, and are weathered and eroded
    • Lava and landforms
      • viscosity of lava and magma varies
        • basaltic lavas quite fluid
        • silica-rich lava and magma are viscous
        • volcanic bombs are globs of lava thrown into the air that solidify in mid-air
        • pyroclastics (volcanic cinders, ash, and dust) are formed from smaller fragments of lava
        • fissure eruptions do not create mountains, make plateaus
    • Volcanic mountains
      • Composite volcanoes
        • large mountains formed in subduction zones
        • stratovolcanoes of great height
      • extremely dangerous, erupt with little or no warning
    • Lahars are deposits of hot ash on snowcapped volcanoes, creating mudslides
    • Nuee ardente is a cloud of hot gas and dust associated with a volcanic eruption; can descend rapidly downslope, incinerating all in its path
    • Risk prediction of volcanic eruption is improving, but is by no means error free
    • Volcanic domes are produced by oozing lava
    • Cinder cones are composed mainly of pyroclastics (fragments), generally small in size
    • Shield volcanoes are formed from fluid basaltic lavas
      • peaks are unimpressive, horizontal dimensions greater
      • pahoehoe is the slightly hardened lava, which is still moving
    • Hot spots are unusually hot "plumes" in a fixed location; as plates move over it, shield volcanoes then form
      • can calculate speed and direction of plate movement from observing hot spots
  • Calderas
    • Formed when a volcano is no longer supported by a source of magma, and it crumbles
      • earthquakes can contribute to caldera formation
      • Crater Lake, Oregon
      • Ngorongoro Crater, Tanzania
    • Phreatic eruption
      • when water enters a magma chamber, can blow top off of volcano, creating huge craters
        • Krakatau (1883), phreatic eruption created tsunamis (seismic sea waves)
        • Tambora (1815), phreatic eruption created so much dust, that solar radiation interfered with; temperatures plunged, crops ruined
        • Santorini (around 1645 B.C.E.), seawater may have entered magma chamber, cataclysmic explosion, sky dark with dust for days, huge caldera created
  • Landscapes of volcanism
    • Volcanic features dominate an area's physical and mental landscapes

Review Questions

  1. What is pahoehoe, and how does it differ from Aa?
  2. List the four types of volcanic landforms.
  3. How does lava flow during a fissure eruption? Use Fig. 34.2 as a reference.