Lecture outline - Answer portion
What are compositional
variations and physical properties of lavas?
- increasing silica content: komatiite (presently rare),
basalt, andesite, rhyolite.
- deacreasing extrusion temperatures with
silica content from circa 1400°C typically to 650°C.
- decreasing densities with increasing silica content.
- relative viscosities?
- gas content can be very important.
How do you melt
rocks? Three recipes:
- decrease pressure on hot rocks, e.g. mantle
plumes of solid rock produces upper portion of partial melting.
- associated with rift zones, sea floor spreading, and hot spots.
- mainly produces basalts.
- add fluids and lower melting temperature,
e.g. wet subduction slab contribution.
- associated with subduction zones.
- produces wide range of compositions, but the average composition is andesite.
- increase temperature, e.g. inside thickened
- produces rhyolites.
What are the
sources of magma? Multiple sources, mixing and segregation all play a role.
- partial melting of ultramafic (very rich
in Fe and Mg) mantle rocks for basalts, typically at depth of
- partial melting - only a portion melts, and certain elements go slectively into the melt, so that the melt has a different composition than the source rock that is melting and there is a residue. Basalts represent some 10-20% partial melt.
- mixture of crust and mantle sources for
- partial melting of continental rock for
What are types
of volcanic eruptions or phenomenon?
- flows vs. pyroclastic eruptions.
- flows: point vs. fissure
- domes: build up of viscous, usually more silica rich magma at throat of volcano.
in terms of increasing violence these are Strombolian, Vulcanian,
Vesuvian, Plinian, Pelean.
- Pelean: role of mass wasting, role of
water, asymmetric sideways blast.
- pyroclastic debris flows (glowing avalanches, hot incandescent and
swiftly moving debris flows often generated by dome collapse)
during Vesuvian and Pelean eruptions.
- ash falls.
- lahars: volcanic mudflows. Mix volcanic
debris with water and watch it flow - long distances.
Columbia River flood basalts
Map showing the overall extent of the Columbia River Flood basalts. Humans have not witnessed eruptions on this scale. 170,000 cubic kilometers of basalt are included int his province. Image source: http://vulcan.wr.usgs.gov/Volcanoes/ColumbiaPlateau/Maps/map_columbia_plateau.html
Steamboat rock mesa along the Columbia River. The dark layers stacked one on top of each other are individual flows. SImage source: http://3dparks.wr.usgs.gov/grandcoulee/html2/gc711.htm
Figure of Columbia River flood basalts, two thick flows,
one stacked on top of the other. Two red lines mark contact
between flows. Medium sized trees at base for scale. these basalts
represent part of a large igneous province that was emplaced
some 16 million years ago. Taken during 1987 field trip.
What is determining
these two different types of volcanic constructs? Melt viscosity and gas content!
What plate settings
do different types of volcanism occur in?
- subduction related:
- andesites, composite cones, more explosive
- examples - Andes, Cascades, Aleutian,
- divergent - continental rift related.
- primarily basalts, quieter eruptions.
- East African rift zone
- oceanic rift (spreading ridge related).
- oceanic intraplate - hot spot related.
- continental intraplate - hot spot related.
- of what use is this information?
So what's the concern?
What should be included in a hazard map for a volcanic region?
- lateral blasts and associated pyroclastic
- lahars - where and how far?
- caldera lake stability, and possible gas release.
- volcanic gases: death gullies.
- volcanic ash loading and collapse of structures.
- volcanic ash and aviation safety.
- associated earthquakes.
- possibility of jokulhaups (breakout floods).
- possible contribution to dam failure
- Crucial question is how do each of these influence local human features such as roads, dams, towns, bridges, etc..
Can an eruption
be controlled or prevented?
- magnitude of forces involved.
- dependent on type of eruption.
- attempts to redirect lava flows: in Italy,
Hawaii and Iceland have had some limited success.
© Harmon D. Maher Jr..
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H.D. Maher Jr., 3/6/200
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