Environmental Geology lecture outline:
Science of volcanology
We can start with broad questions
about volcanism in general. These are important for understanding the exact nature of related geohazards and the associated risks and possible mitigation steps.
Shield vs. Composite Volcanoes (Stratovolcanoes)
There are two fundamentally different types of volcanic constructs with
contrasting behavior, shield volcanos and stratovolcanos:
- shield volcanos:
basaltic, low angle slopes and often very large, quieter eruptions,
small summit calderas, example Hawaiian islands.
- Example above is of the small Belknap shield volcano that is part of the Cascade chain of volcanism. The foreground is relatively young basalt lava flow. Below is a small image of the top of the Mauna Loa shield volcano. Note the low angle slopes Image source USGS website: http://pubs.usgs.gov/gip/volc/types.html
- strato or composite volcanos: andesitic, high angle slopes, sometimes very large
calderas, potential for very devestating eruptions, example Mt.
factor helps explain the traits of these two different types of volcanic constructs?
- Composite volcanos can build up ( a constructive phase) and then literally blow themselves apart (a destructive phase).
- large calderas and very extensive ash beds are the remanants of such catastrophic eruptions. Calderas are depressions where a very explosive eruption ejected material and formed a depression. Since they are depressions lakes often form in them.
- The classic historic example is Krakatoa (more on that later).
USGS map of Yellowstone volcanic center. Yellowstone does not look like a volcano, yet there are volcanic rocks and hotsprings everywhere, and geophysically we can detect the magma underneath it. It is a super volcano with very large caldera complexes. The purple and green lines mark the edges of the calderas from the most recent explosive eruption, and older ones. Image source: USGS U.S. Geological Survey Fact Sheet 2005-3024 2005 - https://pubs.usgs.gov/fs/2005/3024/images/fs2005-3024_fig_03_large.jpg (accessed 11/13/2017).
plate settings do different types of volcanism occur in?
Iceland - land of fire and ice, and lots of basalt
of Mount Hecla in the background (low white ridge) taken during
1986 UNO field trip to Iceland. Mt Hecla is one of the most active
volcanic constructs in Iceland (most recent flows in 1970s), and
is also the site of descent into the interior in Jules Verne's
Journey to the Center of the Earth.
Iceland is an anomalous emergent portion of a seafloor spreading
ridge, so basalt dominated.
Map below shows the position of the active rift and related volcanoes (as red triangles). Image from USGS website: http://pubs.usgs.gov/gip/dynamic/understanding.html
- case history of Heimay eruption.
Images below of houses destroyed by 1973 Heimay flow and air fall material from USGS web site: http://pubs.usgs.gov/gip/heimaey/
- fire + ice (eruptions under and into glaciers) = jokulhaups (discussed earlier), more explosive eruptions and ash clouds.
- April-May 2010 eruptions of Eyjafjallajökull. Image source USGS site - https://www.usgs.gov/media/images/eyjafjallaj-kull-eruption , accessed 11/14/2017 (Credit: Oddur Sigurðsson, Iceland Meteorological Office. Public domain.)
- The greyish coloration distinguishes the ash cloud from the 2010 eruption (April 15th) from normal water vapor clouds. As weather positions shifted the ash cloud shifted with them. Satellites help keep track of such ash clouds so that planes can avoid them. Abundant ash clouds closed northern European air traffic a times. This and a suite of ash cloud pictures form the eruption at https://www.nasa.gov/topics/earth/features/iceland-volcano-plume-archive1.html (accessed 11/14/2017).
- hot springs and geothermal energy.
Photo of a geysir in Iceland, with a jet
of steam just erupting through the overlying water and initiating
an eruption. This feature erupted about every 5 minutes. Note
the light colored cinter around that represents silica precipitating
out of the hot waters. This activity is fed by magma and hot
volcanic rocks at depth. Iceland takes good advantage of this geothermal energy source.
Related web Resources:
Next lecture outline on volcanos.
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H.D. Maher Jr., 3/6/2017
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