Hi:
I'm a children's science book author currently working on a project for
Scholastic containing many SEMs. I have an SEM of fractured pumice from
Mt. St. Helens mag x3500 showing the many tiny air holes. I need help
interpreting the photo. How were these air holes formed? What is the
original source of the rock? (How deep in the earth?)
Vicki Cobb
Dear Vicki,
The pumice is made of rocks that were melted probably at a depth of about
100 km (60 miles) beneath St Helens. As the magma rose towards the
surface (especially very close to the surface) the reduced pressure
(because fewer rocks were above it pushing down), the gas that was part
of the magma came out of solution and made bubbles. AS pressure was
reduced more the bubbles continued to grow. Finally when the rock was
erupted out of the volcano the cool air forced it to quickly cool and the
bubbles were frozen into place. Hope this helps!
Chuck Wood, University of North Dakota
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