4

• Viscosity


  The resistance of a fluid to flow
• Factors affecting viscosity of magma

  • Composition (silica content)
  • Temperature
  • Amount of dissolved gases
• Silica content of magma

  Directly related to viscosity - more silica, greater viscosity
• Dissolved gases in magma

  Tend to increase fluidity by reducing polymerization
• Movement of magma from source to surface

  1. Partial melting in asthenosphere

  2. Ascent of basaltic magma through lithosphere

  3. Ponding and differentiation in magma chamber

  4. Ascent of silica-rich magma to surface
• Trigger for Hawaiian-type eruptions

  Arrival of new batch of melt into near-surface magma reservoir, causing inflation and fracturing of rock above
• Role of volatiles in explosive eruptions

  Dissolved gases separate from melt as pressure decreases, forming bubbles that can cause explosive ejection of magma
• Viscosity of magma and gas content

  Determine nature of volcanic eruption - more viscous, gas-rich magma leads to more explosive eruptions
• Basaltic magmas


  • They have a low viscosity which allows gases to escape with relative ease
  • They produce "gentle" outflows of fluid lava in places like Hawaii
• Andesitic and rhyolitic magmas

  • They have a high viscosity which leads to explosive and sometimes catastrophic eruptions in places like Mount St. Helens, Mount Pinatubo, and Soufriere Hills
• Chile, Peru, and Ecuador boast the highest volcanoes in the world, with dozens of cones exceeding 20,000 feet
• Chimborazo and Cotopaxi in Ecuador were once considered the world's highest mountains until the Himalayas were surveyed in the 19th century
• Factors that determine the nature of a volcanic eruption

  • Magma composition
  • Magma viscosity
  • Magma gas content
• Hawaiian-type eruption


  Generally triggered by the escape of dissolved gases from fluid basaltic magma
• Volcano fed by highly viscous magma

  Likely to be a greater threat to life and property than a volcano supplied with very fluid magma
• More than 90 percent of the total volume of lava on Earth is estimated to be basaltic in composition
• Andesites and other lavas of intermediate composition account for most of the rest, while rhyolitic (felsic) flows make up as little as 1 percent of the total
• Basaltic lavas


  They are usually very fluid and generally flow in thin, broad sheets or streamlike ribbons
• Rhyolitic lavas


  Their movement may be too slow to perceive and they seldom travel more than a few kilometers from their vents
• Andesitic lavas


  They exhibit characteristics that are between the extremes of basaltic and rhyolitic lavas
• Aa flows


  They have surfaces of rough jagged blocks with dangerously sharp edges and spiny projections
• Pahoehoe flows


  They exhibit smooth surfaces that often resemble the twisted braids of ropes
• Pahoehoe lavas form


  At higher temperatures and are more fluid than aa flows
• Cooling as a pahoehoe flow moves away from the vent

  Increases viscosity and promotes bubble formation, transforming the flow into an aa lava
• Lava tubes


  Cave-like tunnels that develop in the interior of a hardened basaltic flow, serving as insulated pathways that facilitate the advance of lava great distances from its source
• Some lava tubes exhibit extraordinary dimensions, like Kazumura Cave on Hawaii's Mauna Loa volcano which extends for more than 60 kilometers
• Block lavas


  Andesitic and rhyolitic magmas tend to generate relatively short prominent flows, with an upper surface consisting largely of vesicle-free, detached blocks
• Pillow lavas


  Tube-like structures formed when molten basalt is extruded underwater, with the outer skin quickly congealing while the lava continues to move forward
• Pillow lavas indicate that the lava flow formed in an underwater environment
• Volcanic gases


  They make up from 1 to 6 percent of the total weight of magmas, with most in the form of water vapor
• Occasionally, eruptions emit colossal amounts of volcanic gases that rise high into the atmosphere and may impact Earth's climate
• Composition of volcanic gases

  About 70% water vapor, 15% carbon dioxide, 5% nitrogen, 5% sulfur dioxide, with lesser amounts of chlorine, hydrogen, and argon
• Volcanoes are natural sources of air pollution, emitting large quantities of sulfur dioxide which forms sulfuric acid and other sulfate compounds
• Formation of volcanic conduits

  1. Swelling of the magma body fractures the rock above

  2. Hot blasts of high-pressure gases expand the cracks and develop a passageway to the surface

  3. Erosive forces enlarge the conduit
• Lava bombs 20 feet long and weighing over 200 tons have been thrown 2000 feet from the vent during eruptions of Japan's Asama volcano
• Pyroclastic materials


  Pulverized rock, lava, and glass fragments ejected from a volcanic vent, ranging from fine ash to large blocks and bombs
• Ash and dust particles


  Produced when gas-rich viscous magma erupts explosively, forming a froth-like melt that is blown into very fine glassy fragments
• Lapilli


  Somewhat larger pyroclasts ranging in size from small beads to walnuts
• Blocks and bombs


  Larger pyroclasts over 64 mm in diameter, with bombs being ejected while still molten and acquiring streamlined shapes
• Scoria


  Vesicular ejecta that is a product of basaltic magma