Volcanoes and Igneous Activity

Volcanoes: Features and Eruptive Styles

Volcanoes are features created by lava erupting on the surface. The mountain peak is formed by continued eruptions. Volcanic features are a result of lava eruption, ranging from stratovolcanoes to lava flows to gas eruptions (fumaroles).

Main Styles of Volcanoes

  1. Shield Volcanoes

    • Broad, shallow slopes (e.g., Hawaiian Islands - Mauna Loa, Mauna Kea, Kilauea).
    • Formed by lava flows.
    • Mauna Loa and Mauna Kea are 13,000+ feet above sea level.

  2. Composite Cones (Stratovolcanoes)

    • Steeper slopes.
    • Classic explosive eruptions (e.g., Cascade volcanoes - Mount Hood, Mount St. Helens).
    • Stratovolcanoes is an older term, composite cones is more recent terminology, but they're interchangeable.

  3. Cinder Cones

    • Smaller, short-lived (months to years of activity).
    • Lava fountain-like eruptions.
    • Sometimes occur on the flanks of larger volcanoes.
    • Loose cones of material with a concave divot at the top.

Lava Composition, Viscosity, and Gas Content

  • Key differentiators: type of lava, viscosity, and gas content.
  • Composition: Low silica (basaltic) vs. High silica (andesitic and granitic).
  • Viscosity: Resistance to flow (thickness).
    • High viscosity: thick (syrup, honey).
    • Low viscosity: fluid (water, mineral oil).
  • Gas content: Dissolved gas builds pressure, leading to explosive potential.

Effusive vs. Explosive Eruptions

  • Controlled by viscosity and gas content.
  • Effusive (non-explosive): Lava flows, low viscosity, low gas, basaltic compositions.
  • Explosive: Ash clouds, high viscosity, high gas, andesitic to granitic compositions.

Effusive Eruptions and Basalt Flows

  • Involve flowing lava; low viscosity, low gas pressure, basaltic.
  • Shield volcanoes, repeated effusive eruptions - basalt flows.
  • Hawaiian Islands are a prime example.

Columbia River Flood Basalts

  • Fissure eruptions that occurred 17 to 5 million years ago, with the majority around 15 million years ago.
  • Covered Oregon and Washington, originating near the Idaho border.
  • Basalt was very hot and low silica, allowing it to spread over vast areas.

Types of Basalt Flows

  1. Pahoehoe

    • Ropey texture.
    • Thin skin that wrinkles as lava flows underneath.

  2. Aa

    • Thicker skin that cracks and breaks.
    • Pile of superheated rock bulldozing forward.

Videos show the differences in flow behavior between pahoehoe and aa.

Lava Tubes

  • Caves formed in lava flows.
  • The exterior of a flow solidifies creating an empty voided shell, allowing lava to flow through, and spread for miles.
  • Lava flows re-use lava tubes.
  • Common in Hawaii.
  • Examples: Ape Caves (WA), lava caves outside Bend (OR).
  • Restrictions may be in place due to bat fungus concerns.

Vesicular Texture: Scoria and Pumice

  • Porous texture due to gas bubbles in lava.
  • Scoria: Vesicular basaltic rock.
  • Pumice: Vesicular high silica rock; can float in water due to porosity.
  • Formation: Gas bubbles pop, leaving cavities as lava cools and solidifies.

Columnar Jointing

  • Seen in Columbia River flood basalts.
  • Pillars of lava form as massive flows cool from the outside in.
  • Cracks develop at 90 degrees to the flow, typically hexagonal.
  • Cooling causes contraction and cracking; also observed in drying mud.

Pillow Lavas

  • Result of basalt erupting or flowing underwater.
  • Lava insta-cools when exposed to water, forming balloon-shaped pillows.
  • Basalt is exposed to water, it basically insta cools into a layer of basalt or basaltic glass

Explosive Eruptions and Stratovolcanoes

  • High viscosity and high gas pressure. Mid to high silica compositions (andesitic to rhyolitic/granitic).
  • Associated with composite cones (stratovolcanoes).
  • Stratovolcano is an older term.

Composite Cones (Stratovolcanoes) Characteristics

  • Steeper slopes, the height will vary, varying compositions.
  • Layers of lava flows, lahars (volcanic mudflows), and pyroclastic material (ash).
  • Example: Mount St. Helens (basalt lava tubes, occasional basalt flows, but generally andesitic).

Lava Flows with Stratovolcanoes

  • Obsidian flow in the Newberry volcanic system (Bend, OR) - high viscosity, low temperature, high silica, doesn't flow far.
    *Volcanic Dome or Plug: Lava reaches the surface but doesn't flow (e.g., Mount St. Helens after 1980, lava with very little gas).

Pyroclastic Material (Tephra)

  • Exploded hot rocky pieces.
  • Ash and dust (smaller).
  • Lapillae (pebble-sized).
  • Bombs (larger rock chunks, lava bombs).
  • Bombs, ash, lapel these are lava.
  • Ash is lava exploding into microscopic pieces of volcanic glass, pieces of lava that would have been stuck between a bunch of air or gas bubbles.
  • Volcanic bombs have football/teardrop shapes, formed by lava blobs flying through the air.

Pyroclastic Flows

  • Hot ash, gas, and material that flow down the side of a volcano.
  • Move at tens to hundreds of kilometers per hour, following valleys.

Mount St. Helens Eruption (1980)

  • Lateral blast due to a bulge in the mountain caused by shallow magma.
  • Landslide triggered the depressurization and explosion.
  • Classic ash cloud with pyroclastic flows.

Pompeii and Mount Vesuvius

  • Buried by pyroclastic flows in 79 AD.
  • Preserved cavities of villagers in ash.
  • Pliny the Younger's observational account is an early record of volcanic eruption.

Calderas

  • Collapsed magma chambers.
  • Example: Crater Lake (OR).
  • Mount Mazama erupted so much material that its magma chamber emptied, causing the mountain to collapse on itself.
  • Wizard Island - several eruptions occurred after the culminating eruption.
  • Yellowstone also has a caldera.

Lahars

  • Volcanic mudflows, mix of water, ash, and debris.
  • Melted snow and ice from volcanic eruptions create lahars.
  • Hazardous in areas around Mount Rainier and Mount Hood, can bury areas in mud by following river valleys.
  • Can be caused by small eruptions or even just heavy rain on weathered volcanic material.

Cinder Cones

  • Short-lived (months to years), basaltic pimples of the volcano world.
  • Start as lava fountains, then dwindle to lava flows.
  • Can exist alone or as part of larger volcanic areas (e.g., on Mauna Kea).
  • Cinder cones form from piles of cinders (scoria).
  • Example: Lava Butte (Bend, OR).
  • Weathered and eroded easily due to loose rock composition.