Volcanism II: Styles of Volcanic Eruptions, Volcano - Atmosphere Interactions

Mt. Redoubt, AK from Wikipedia
Magma composition and behavior: Depending on their composition, magmas behave differently:
  • Basic principles:

    Basalt flow with pahoehoe texture on Fernandina - Galapagos Islands
  • Mafic lava characteristic textures:

    Katmai volcano, AK from USGS
  • Felsic to intermediate lava characteristic textures:

    Additional Information.

    Volcano types:

    Associated structures:

    Styles of volcanic eruption:

    Effusive eruptions: These produce low-viscosity (i.e. highly liquid) basaltic lava.

    Volcano - water interactions:

    Volcano - atmosphere interactions:

    On an airless world like Io, volcanic material simply travels in a ballistic trajectory. On worlds with atmospheres, the situation is more interesting. We have already discussed pyroclastic flows, avalanches formed when an aerosol of volcanic gas and particles glides downhill over a layer of trapped air.

    Plinian eruption of Mt. Spurr, AK from USGS
  • Plinian eruption:

    Occurs when the heat of the eruption creates a rising column of volcanic-ash laden air. Such a column may distribute pyroclasts to high altitude winds, resulting in widespread ash fall.

    In different regions of a plinian column, erupted material ascends for different reasons. Three zones from lowest to highest elevation:

    Terminal velocity: The laws of fluid mechanics tell us that the amount of drag that an object experiences increases with velocity, such that an object falling through an atmosphere will reach a velocity at which drag overwhelms gravitational acceleration - its terminal velocity. This velocity varies with the size and roughness of the object. Smaller or rougher objects have a lower terminal velocity. Volcanic particles will ascend in a plinian column as long as the column is rising faster than their terminal velocity.

    What controls the column's height? Three factors:

    Expressed as:

    HT = k(MΔT)1/4


    Based on this we predict that all other things being equal, we predict that HT will be greater when:

    Thus, we would expect a plinian column on Mars to rise higher than on Earth, because Mars:

    Of course, M changes over the course of the eruption, eventually dropping to zero. So, plinian columns eventually collapse under their weight as they cool.

    Key concepts and vocabulary: