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VolcanismWhy is some lava and volcanism so runny?QuestionsExplosive VolcanismDifferences in eruptive styleThe answer lies in a review of igneous processesQuestions"Exceptional" VolcanismIllustrationsQuestions:Comments on intraplate volcanismThe San Francisco Volcanic FieldOther Volcanism ExamplesPacific Northwest VolcanismAdditional Cascade Range Volcanoes InformationCase StudiesTwo More ReadingsVolcano Observatories and Monitoring
Overview of this page: Your chapter reading contains a nice exposition of all the terminology associated with volcanism and volcano types. This page is meant to highlight some particular ideas to complement/supplement the chapter reading.
Look at the videos below for a couple of examples of runny lava from Hawaii (videos "should" play on the webpage, otherwise use the links):
Hawaiian Lava Flow Again:
In cartoon/sketch form, we imagine this type of volcanism like this:
Here is an image of such volcanism form Iceland in 2021 where people are walking right up to the lava flow path:
What color is this lava? What does this suggest to you about its composition? How does that relate to the silica tetrahedra bonding style?
What is the best word to describe this lava:
High viscosity
Low viscosity
In the United States perhaps the most famous volcanic eruption is the 1980 Mt. St. Helens eruption in Washington (part of the Cascade Range volcanoes of the United States Pacific Northwest).
A good informational page to check out is the US Geological Survey's Mt. St. Helen's website.
Below is an image of Mt. St. Helens before and after its 1980 eruption:
Here are some video resources:
Another with some human interest perspective
Mount St. Helens Eruption, May 18, 1980. The complete Gary Rosenquist sequence in HD. - YouTube
Here is quite beautiful picture some years after the explosion:
Here is an illustration of a new lava dome building up after an explosion:
Here are two links exploring lava dome growth at Mt. St. Helens in the years following the 1980 explosion:
2004-2008 Renewed Volcanic Activity | U.S. Geological Survey (usgs.gov)
Timelapse Reveals How Quickly Mount St. Helens Grows | PetaPixel
To what can we attribute the difference in volcanic eruption style?
Turns out it relates to our handful of mineral friends and the manner in which the silica tetrahedra bond. Why? Because the effects the viscosity of the resulting magma/lava.
Bowen's Reaction Series:
Silica tetrahedra bonding style of our common rock forming minerals:
Mineralogy and Viscosity - note the arrow (2nd from the bottom) trend in relation to the words silica content, mafic, and felsic.
How would you make an annotated sketch that illustrates the mineralogical reason for the two bookend volcanism styles?
Related to the above, how would you make an annotated sketch that illustrates why magmas evolve to a more felsic composition (over geologic time)?
In what tectonic environment (i.e. "where") do we expect to find low-viscosity volcanism? How would you describe the composition of the lava?
In what tectonic environment (i.e. "where") do we expect to find high-viscosity volcanism? How would you describe the composition of the lava?
Do all volcanoes occur in the tectonic environments you note above? In other words, is all volcanism associated with plate boundaries? If not, then this would represent an exception to the theory of Plate Tectonics as we currently understand it in this class.
By exceptional it is meant that the volcanism is not predicted by the Plate Tectonic ideas we have developed. What are some examples? This type of volcanism is called intraplate volcanism.
Probably the best known example is the Hawaiian Islands. These volcanoes are in the middle of the massive Pacific Plate very far away from any plate boundaries. How is volcanism explained here? The concept of a mantle hotspot or mantle plume was invoked decades ago, in the early days of Plate Tectonics general acceptance. However, it is important to note that to this day, the concept of a mantle plume is not universally even defined and accepted.
Image source
Another famous location of intraplate volcanism is Yellowstone National Park in NW Wyoming. Also very far from a plate boundary.
You can check out the US Geological Survey's website on Yellowstone: Yellowstone | U.S. Geological Survey (usgs.gov)
Would you expect the mineralogy of the magma/lava at Hawaii to be the same or different than at Yellowstone? If different, in what ways? If the same - why?
How do the arrangement of the volcanoes in Hawaii or the Snake River Plain Hotspot track 1 , 2, 3, 4 tell us something about the plate motions over these hotspot locations?
Note that the depth from which the mantle plumes originate from is not defined in the above images. Some geologists believe they originate at the core-mantle boundary (which is 3,000 km deep - look at our slides - or your reading - on earth's basic layered structure). There is definitely evidence for this, but it is not as unambiguous as we'd like it to be.
Others favor an upper mantle source (say in the asthenosphere and above). There is evidence for this as well.
What is unambiguous is that intraplate volcanism occurs. The scientific debate goes on about how it all works. In fact, there is a website devoted to this very topic that researchers working in this field use to debate the process: Do plumes exist? (mantleplumes.org). Here the question is posed as do mantle plumes that originate at the core-mantle boundary exist?
Although not as well known as the Hawaii and Yellowstone examples, the San Francisco Volcanic Field is another example of intraplate volcanism the United States. The name is a bit deceptive - the location is not in San Francisco, but in northern Arizona near and around Flagstaff.
Please read the following links:
The San Francisco Volcanic Field, Azrizona. A US Geological Survey publication.
Sunset Crater, San Francisco Volcanic Field
Travels in Geology: Cones and craters in Flagstaff, Arizona
Understanding origins of Arizona's Sunset Crater eruption of 1,000 years ago (phys.org)
Below are more examples of volcanism to help you put things into perspective.
The Cascades Range of volcanoes (from northern California to northern Washington and southern British Columbia. Mt. Baker is the northernmost one in Washington state). What type of plate tectonic boundary causes these volcanoes?
Learn about the volcanoes of the Pacific Northwest Cascade Range mountains by scrolling over individual volcanoes on this interactive map. Gives height, latest eruption, and other notes. Also shows the 4,000-year eruptive history on a US Geological Survey chart of volcanoes link
55 million year history of the Cascades Range.
If you have been to Mt. Lassen National Park (southernmost volcano in the Cascades Range - still in CA), you may find this website useful. Contains information about the 1915+ Lassen Peak eruptions.
Mt. Rainier outside of the Seattle, WA, area is one of the more well-known volcanoes of the Cascade Range.
More on the Mt. St. Helens eruption of 1980. This story has a human interest angle to it. More US Geological Survey links about Mt St Helens: 1, 2, 3, 4.
General Volcanic Hazards site from the US Geological Survey.
Observing a Galápagos Volcano from Buildup to Eruption. Note that the behavior of the mafic/basaltic lava and volcanic system is observed to be different than typical. What are these anomalous behaviors and what are some reasons postulated for it?
Why was the Sunset Crater eruption so powerful?. This is long video talk, but if you are really into volcanoes you might try it. Just for fun😉.
Please do the following two readings. They are taken from the terrific little book called The Story of the Earth in 25 Rocks.
Igneous Dikes. Think of this as related to the Chapter 3 material on melting.