Exercise 4
Petrography
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# Exercise 6: Distribution of Active Volcanoes

Volcano World was one of the first geoscience resources on the Internet. For this exercise you will need a map showing theplate boundaries [if you print from the browser, turn the image on its side and reduce the size to about 75% for the best effect), or you can copy the larger figure on the inside cover of the text.

This exercise will take about a month the way it is designed. Your instructor may elect to modify the exercise to better fit the class schedule.

Look at the Update on Current Volcanic Activity. Prepare a table in which you list the 10 most recent volcanic events prior to March 1, 2001:

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LocationCompositionExplosivity IndexRemarks

Under the Remarks column tell whether the volcano is located at a plate boundary or within a plate. If the volcano occurs at a plate boundary tell what kind of boundary.

Locate each of these events with a red circle on your reference map.

Questions

1. What is a hot spot? Mark the location of a proposed hot spot with a purple circle on your reference map.

2. Describe the plate tectonic setting of the Cascade Mountains.

3. Describe the orientation of the volcanoes of the Cascades. How far (on average) are the Cascades from the subduction zone(s)? Speculate as to what controls the distance between a subduction zone and the volcanoes?

4. At the Volcano World site is a list of volcanic activity from about 1995 to the present. About how many eruptions have been recorded each month since 1995?

5. What is the range of the numbers of eruptions : ______ (lowest) to ______ (highest). Predict how many eruptions you think will occur between March 1, and April 1, 2,001.

6. add the month of March eruptions to the table and locate them in blue

In general, the viscosity of a melt (a measure of its resistence to flow) is inversely proportional to its temperature. At high temperatures the melt tends to flow more readily than at low temperatures. Water can drastically modify the viscosity of a melt; the higher the water content, the lower the viscosity.

7. Relate the type (quiet versus explosive) of eruption to the viscosity of a melt.

8. In your own words, compare and contrast rhyolites, andesites and basalts. Which, other things being equal, tend to have the highest viscosity?

9. At what types of plate settings do you tend to find these three lava types?

## A Virtual Field Trip

Field trips are the geologists counterpart of a case history. Virtual Field trips are not a substitute for being in the field in person. However, it rarely rains on a virtual field trip and I am not aware of a student suffering a snakebite.

Take some time and Virtually Climb Stromboli. Answer the following questions upon your safe return:

1. Where is Stromboli? Locate it on your map in orange.

2. What type of volcano is Stromboli?

3. How many meters will you climb by the time you get to the top?

Something Extra

1. If you enjoy this way to learn, you might want to Virtually climb the Vancori-summit!