The causes of any given collapse are different and poorly understood

Volcanic collapses can occur due to many reasons. It is difficult for scientists to determine the exact causes of any given collapse, which makes them difficult to predict.

Causes of volcanic instability (courtesy of Tibaldi, A.)

Factors that influence slope stability include 1) Gravity, 2) Water, 3) Regional and local structures, 4) Weak materials, and 5) Triggering events. All of these factors are either inherent problems, causes that increase the shear stress, or causes that reduce the shear stress (Voight and Elsworth, 1997). Read more complete summaries about slope instability factors here.

1) Gravity

  • Static loading is a constant force pushing volcanic flanks down
  • Uneven weight distribution due to eruption materials being preferentially on one side of the volcano due to vent orientation or wind direction
  • Gradual volcano spreading

2) Water

  • Pore fluid pressure enhancement
  • Addition of water from rainfall or snow melt adds weight to the slope, making it unstable
  • Water has the ability to change the angle of repose, such as what is explained in the following images:

Dry sand can remain stacked to its angle of repose (http://www.tulane.edu/~sanelson/geol204/slopestability.htm)

Damp sand can go beyond the angle of repose because water acts as a glue between the grains (http://www.tulane.edu/~sanelson/geol204/slopestability.htm)

Completely wet sand causes the grains to sit too far apart from each other, causing the material to be fluid and fail (http://www.tulane.edu/~sanelson/geol204/slopestability.htm)

  • Water can dissolve the mineral cements that hold grains together
  • Liquefaction, or when loose sediment becomes oversaturated with water and individual grains loose grain to grain contact with one another as water gets between them. This often occurs during seismic events.

    Liquefaction of grains causes the particles to become loose and fluid, resulting in landslides (http://www.tulane.edu/~sanelson/geol204/slopestability.htm)

3) Regional and local structures

  • Bedding attitude relative to slope face
  • Discontinuity systems- faults, joints, dykes, bedding planes
  • Slope forming process history; movement history; orientation of movement
  • Volcanoes tend to fail perpendicular to the direction of maximum horizontal compression (MHC), or the direction in which the earth is being squeezed the hardest. This causes dikes to preferentially rise and the edifice to be elongated parallel to the direction of MHC, such as in the example below. The white arrows indicate expansion and the black arrows indicate compression, mimicking the way squeezing a rubber ball would cause the sides to move out

    Preferred orientation of collapses is caused by regional stresses (Voight, B. and Elsworth, D., 1997)

4) Weak materials

  • Erosion or undercutting

One example of how undercutting can destabilize a slope (http://www.tulane.edu/~sanelson/geol204/slopestability.htm)

  • Clay minerals that can hydrothermally alter or stack in compression, allowing for easy slip surfaces to develop

Clay minerals can stack in compression or after the cement that holds them together is dissolved, allowing for the formation of slip surfaces (http://www.tulane.edu/~sanelson/geol204/slopestability.htm)

5) Triggering events (see big ideas: Triggers)

  • Seismicity
  • Magmatic or phreatic eruptions
  • Glacial melt which add water to the edifice and cause decompression of the magma chamber, leading to eruptions

One thought on “9.7

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