Environmental Geology lecture outline
What are compositional
variations and physical properties of lavas?
- increasing silica content: komatiite,
basalt, andesite, rhyolite.
- deacreasing extrusion temperatures with
silica content from circa 1400°C typically to 650°C.
- relative viscosities?
- gas content can be very important.
How do you melt
- decrease pressure on hot rocks, e.g. mantle
plumes of solid rock produces upper portion of partial melting.
- add fluids and lower melting temperature,
e.g. wet subduction slab contribution.
- increase temperature, e.g. inside thickened
What are the
sources of magma?
- partial melting of ultramafic (very rich
in Fe and Mg) mantle rocks for basalts, typically at depth of
- 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
eruptions, flood basalts.
- Figure of Columbia River flood basalts, two thick flows,
one stacked on top of the other. Two red lines marke 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.
in terms of increasing violence these are Strombolian, Vulcanian,
Vesuvian, Plinian, Pelean.
- Pelean: role of mass wasting, role of
water, asymmetric sideways blast.
- 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.
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.
- volcanic gases: death gullies.
- volcanic ash loading and collapse of structures.
- volcanic ash and aviation safety, real time
- associated earthquakes.
- possibility of jokulhaups (breakout floods).
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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