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Physical Science Notes

Volcanes

Introduction

Mount St. Helens - May 18th, 1980 - Largest volcanic eruption in North America.

Not all volcaic eruptions are as violent. The Hawaiian Volcanoes Observatory operates on the summit of Kiluaea.

What causes some eruptions to be violent and others to be tame?

Factors that determine the violence of an eruption

  1. Composition of the magma
  2. Temperature of the magma
  3. Dissolved gases in the magma

Viscosity of magma

  1. Viscosity is a measure of a material's resistance to flow
  2. Factors affecting viscosity
    1. Temperature (hotter magmas are less viscous)
    2. Composition (silica content)
      1. High silica = high viscosity (e.g., felsic lava)
      2. Low silica = more fluid (e.g., mafic lava)
    3. Dissolved gases
      1. Gas content affects magma mobility
      2. Gases expand near the surface and extrude lava
      3. Violence of an eruption is related to how easily gases escape from magma
        1. Fluid basaltic lavas are generally quiescent
        2. Highly viscous magmas produce explosive eruptions

Fluid lava allows gases to escape easily. Highly viscous magma traps gases until pressure builds to explosive proportions.

Materials extruded during an eruption

  1. Lava flows
    1. Basaltic lavas are more fluid
    2. Types of basaltic lava
      1. Pahoehoe lava (resembles braids in ropes)
      2. Aa lava (rough, jagged blocks)
  2. Gases
    1. One to 6 percent of magma by weight
    2. Mainly water vapor and carbon dioxide
  3. Pyroclastic materials
    1. "Fire fragments"
    2. Types of pyroclastic material
      1. Ash and dust-fine, glassy fragments
      2. Pumice-from "frothy" lava
      3. Lapilli-"walnut-sized"
      4. Cinders-"pea-sized"
      5. Particles larger than lapilli
        1. Blocks-hardened lava
        2. Bombs-ejected as hot lava

Volcano Features

  1. Opening at summit (top): two types
    1. Crater
    2. Caldera - larger than one kilometer
  2. Vent - a tube that connects the top of a volcano to the magma chamber beneath the earth's surface

Types of volcanoes

There are three main types of volcanoes. Real volcanoes may consist of mixtures of types.

  1. Shield volcano - Mauna Loa in Hawaii
    1. Broad, slightly domed
    2. Primarily made of basaltic (fluid) lava
    3. Generally large
    4. Generally produce a large volume of lava
  2. Cinder cone - Paracutin
    1. Built from ejected lava fragments
    2. Steep slope angle
    3. Rather small size
    4. Frequently occur in groups
  3. Composite cone - stratovolcano - Vesuvius
    1. Most are adjacent to the Pacific Ocean (e.g., Fujiyama, Mount Shasta)
    2. Large size
    3. Layers of lavas and pyroclastics
    4. Most violent type of activity
    5. Often produce nuée ardente
      1. Fiery pyroclastic flow made of hot gases infused with ash
      2. Flows down sides of a volcano at speeds up to 200 kilometers per hour
    6. May produce a lahar, a type of volcanic mudflow

Volcanoes are responsible for shaping the land just like glaciers. These shapes and characteristics are known as landforms. Some form as part of the volcano on its surface and others form underneath the earth and are revealed as erosion wears the land away and leaves the more resistant rock visible.

Landforms

  1. Calderas
    1. Steep-walled depression at the summit
    2. Size exceeds 1 kilometer in diameter
    3. Types of calderas
      1. Crater Lake-type
      2. Hawaiian-type
      3. Yellowstone-type
  2. Fissure eruptions and lava plateaus
    1. Fluid basaltic lava extruded from crustal fractures called fissures
    2. Produces flood basalts
    3. Example: Columbia Plateau
  3. Volcanic pipes and necks
    1. Pipes are short conduits that connect a magma chamber to the surface
    2. Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone

    Intrusive igneous activity

    1. Most magma is emplaced at depth
    2. An underground igneous body is called a pluton
    3. Plutons are classified according to
      1. Shape
        1. Tabular (sheetlike)
        2. Massive
      2. Orientation with respect to the host (surrounding) rock
        1. Discordant-cuts across sedimentary beds
        2. Concordant-parallel to sedimentary beds
    4. Types of igneous intrusive features
      1. Dike, a tabular, discordant pluton
      2. Sill, a tabular, concordant pluton (e.g., Palisades Sill, NY)
      3. Laccolith
        1. Similar to a sill
        2. Lens-shaped mass
        3. Arches overlying strata upward
      4. Batholith
        1. Largest intrusive body
        2. Surface exposure 100+ square kilometers (smaller bodies are termed stocks)
        3. Frequently form the cores of mountains

    Origin of magma

    1. Magma originates when essentially solid rock located in the crust and upper mantle melts
    2. Factors that influence the generation of magma from solid rock
      1. Role of heat
        1. Earth's natural temperature increase with depth (geothermal gradient) is not sufficient to melt rock at the lower crust and upper mantle
        2. Additional heat is generated by
          1. Friction in subduction zones
          2. Heating of crustal rocks during subduction
          3. Rising, hot mantle rocks
      2. Role of pressure
        1. Increase in confining pressure causes an increase in melting temperature
        2. Drop in confining pressure can cause decompression melting
          1. Lowers the melting temperature
          2. Occurs when rock ascends
      3. Role of volatiles
        1. Primarily water
        2. Cause rock to melt at a lower temperature
        3. Play an important role in subducting ocean plates
      4. Partial melting
        1. Igneous rocks are mixtures of minerals
        2. Melting occurs over a range of temperatures
        3. Produces a magma with a higher silica content than the original rock

    Volcanoes and Plate Tectonics

    1. Global distribution of igneous activity is not random
      1. Most volcanoes are located on the margins of the ocean basins (intermediate, andesitic composition)
      2. Second group is confined to the deep- ocean basins (basaltic lavas)
      3. Third group includes those found in the interiors of continents
    2. Plate motions provide the mechanism by which mantle rocks melt to form magma
      1. Convergent plate boundaries
        1. Deep-ocean trenches are generated
        2. Descending plate partially melts
        3. Magma slowly rises upward
        4. Rising magma can form
          1. Volcanic island arcs in an ocean
            1. Basaltic composition
            2. Example: the Aleutians
          2. Continental volcanic arcs
            1. Andesitic or rhyolitic composition
            2. Example: Andes Mountains
      2. Divergent plate boundaries
        1. The greatest volume of volcanic rock is produced at oceanic ridges
          1. Lithosphere pulls apart
          2. Less pressure on underlying rocks
          3. Partial melting occurs
          4. Large quantities of fluid basaltic magma are produced
      3. Intraplate igneous activity
        1. Activity within a rigid plate
        2. Plumes of hot mantle material rise
        3. Form localized volcanic regions called hot spots
          1. Example: Hawaii
          2. Examples: Columbia Plateau in the northwestern United States