
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 = (v^{2} 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 Ctrlv).
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. 