Volcanoes on the planets! Shapes of Volcanoes 
Photograph of pahoehoe flow from Puu Oo vent on Kilauea, July 8, 2002.  Photo by Karen S. Bartels What factors influence the shapes of volcanoes on earth?

How do conditions differ on other planets?

This page describes an activity to explore these questions.

Factors that influence the shapes of volcanoes include the type of lava that erupts, as well as the eruption style.  A program called ERUPT , developed by Ken Wohletz, allows users to model different types of volcanic eruptions.  A nice feature of this program is that one can change the force of gravity from normal Earth gravity, to gravity on Mars, to up to eight times Earth gravity. 

To see how volcanoes of different shapes are produced, download the ERUPT program and simulate Hawaiian, Peleean, and Strombolian eruption types under different gravitational conditions.

Window that appears when ERUPT is started.  Suggested Activity
Use the ERUPT software to investigate the effects of gravity on the shape of volcanic landforms produced by the eruption of highly viscous lava and by eruption of pyroclastic material. 

Begin by considering two planets, one that is a little smaller and much less dense than earth, and another that is a little larger and much more dense than earth.  What would a cinder cone look like on these two planets?  Assume that the same type and volume of material erupted at the same rate in both cases.  Would the two cinder cones be the same size and shape?  Would they be the same size, but different shapes?  Or the same shape but different sizes?

After making your prediction, use the ERUPT program to simulate a Strombolian eruption under two different gravity conditions, one with the gravity of Mars (which is about 40 percent of Earth's gravity), and one with gravity 4 times Earth's gravity.  Under each condition, form at least three cinder cones of the same volume, and be sure to print out or save your results. 

What do you observe?  Do your observations of the results of the model match up with your earlier predictions?  What causes the cinder cones to be shaped the way they are?

Next, repeat this exercise for eruptions of viscous lava forming a lava dome on these two hypothetical planets.  You can do this by simulating a Peleean type eruption under the two different gravity conditions.


Choose eruption type using buttons in lower left hand corner:

Change force of gravity using Options menu in upper left:

Some equations to help explain the results:

Cinder cones form as pyroclastic material is thrown from a volcanic vent to fall down and pile up around the vent.  How far a piece of pyroclastic material is thrown depends on the velocity at which it erupted (including speed and angle of ejection), the velocity of wind, and the force of gravity. 

You may recognize this as a physics problem of "projectile range."  The range of a projectile can be determined by the equation 

R = (v2 sin 2 theta)/g

where R is the range, v is the initial velocity, theta is the angle, and g is gravity.

What happens to the range when g is decreased?  Does this help explain your observations?

The height and radius of a lava dome depends on the yield strength and density of the lava as well as gravity.

h = 1.76 [r(tau)/g(rho)]½

where h is height of dome, r is radius of dome, tau is yield strength, rho is density and g is gravity.

For a given radius, what happens to dome height as g is decreased?  Does this help explain your observations of lava dome shape?

To save images of your work, hit the "Print Screen" button on your keyboard, open WORD or an image editor, and execute a "paste" command (such as Ctrl-v).  This should paste the image of your computer screen into a new document or image file.
Note to Educators:  You can conduct the suggested exercise in your classes using concept test and jigsaw techniques.  Having the students think about a problem, predict what they think will happen, and then observe what actually happens is a type of concept test.  You can use the cooperative group learning jigsaw technique when you have the students run the ERUPT software.  Divide the class into groups of four members.  One member can simulate strombolian eruptions on Mars, another the same but with 4x Earth gravity, another Peleean eruptions on Mars, and the last the same but with 4x Earth gravity.  These "experts" on their eruption type can get together with the same experts from other groups to share their findings, and then rejoin their original base groups to educate each other on their areas of expertise and discuss the meaning of their collective findings.
The NASA/UNCF Project at NEIU | Northeastern Illinois University

Text of this page © 2003 The NASA/UNCF Project,  Northeastern Illinois University
Last updated March 16, 2004.