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Geosciences LibreTexts

4: Volcanism (Exercises)

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    Q4.1 How thick is the oceanic crust?

    The magma available to create oceanic crust at this setting is approximately 10% of the volume of the 60 km thick part of the mantle from which it is derived, so the oceanic crust should be about 6 km thick.

    Q4.2 Under pressure

    No answer possible

    Q4.3 Volcanoes and subduction

    The volcanoes are between 200 and 300 km from the subduction boundary, about 250 km on average. If the subducting crust is descending at 40 km per 100 km inland, the depth to the Juan de Plate beneath these volcanoes is between 80 and 120 km, or 100 km on average.

    Q4.4 Kilauea’s June27th lava flow

    1) The flow front advanced at a rate of about 160 m/day or just under 7 m/hour between June 27th and October 29th 2014. That doesn’t mean that the lava only flowed at rates of a few m/hour over that time. It likely flowed much faster (probably 10s to 100s of m/hour), but it advanced in fits and starts, and the advancing front changed locations many times. At other times the flow spread out across the area.

    2) Between January 2015 and January 2016 the flow did not extend any further northeast towards Pahoa. Instead it spread out across the plain to the north of Pu’ u’ o’ o.

    Q4.5 Volcanic Hazards in Squamish

    Hazard Risk
    Tephra emission Yes, but much of the tephra from a large eruption would extend up into the atmosphere, and would not affect Squamish.
    Gas emission Yes, There could be dangerous amounts of sulphurous or acidic gases flowing down the mountainside into Squamish.
    Pyroclastic density current Yes, a pyroclastic density current that flows down the western or southwestern sides of Garibaldi could easily reach Squamish.
    Pyroclastic fall Yes, in the later stages of a large eruption some tephra (or pyroclastic fragments) cold rain down on Squamish
    Lahar Yes, Squamish is definitely at risk from a lahar on the western side of the mountain. The risk would be increased if the eruption takes place in winter or spring when the amount of snow is at a maximum.
    Sector collapse Yes, this is possible. The western side of Mt. Garibaldi has already collapsed several times since the last glaciation.
    Lava flow Yes, Squamish is at risk from lava that flows on the southern and western sides of the mountain. There is a Pleistocene-aged lava flow clearly evident in the photograph. It flowed down the southern flank, and then turned west towards where Squamish is situated today.

    Q4.6 Volcano alert

    The most important tools for monitoring volcanoes are seismometers, and while there is a good network of seismometers in southwestern BC, there are not enough in close proximity to Mt. Garibaldi to be able to accurately define the locations and depths of earthquakes around the volcano. So the first project would be to establish about 5 additional seismic stations in the Squamish region. They don’t have to be right on the mountain, but can be placed near to existing roads and highways in the area. They need to be secured to bedrock. Every effort should be made to have them located on all sides of the mountain. The second project would be to establish some means of measuring deformation of the mountain itself. This could be done with tiltmeters or GPS stations, but GPS would be better. The GPS receivers have to be placed on the flanks of the mountain, and they also have to be installed right on bedrock. That could be a real challenge in winter or spring, when there is lots of snow. While this work is going on, we should charter a helicopter to fly around the mountain to see if there is any sign of eruptive activity or melting snow, and to look for convenient places to install GPS stations. We may want to land in a few different places.

    There isn’t a lot that we can say to the public at this stage, except that this sudden increase in seismic activity could mean that Garibaldi is getting ready to erupt, that the Geological Survey and all emergency measures organizations are working together on it, and that residents of the Squamish area, and anyone using highway 99, should keep listening to local radio stations for further updates. We could also establish a system to send out alerts via text message.

    Q4.7 Volcanoes down under

    We would expect to see composite volcanoes on the North Island, some 200 to 300 km inland (northwest) from the Kermadec Trench, and within the ocean along the same trend to the northeast of NZ. There is also the potential for composite volcanism to the south of the South Island, east of the Macquarrie fault zone, although there appears to be some doubt about whether subduction is actually taking place in this region.