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Pumice ("PUM-iss")

		Pumice is a type of lava rock that's full of bubble holes. The bubbles formed as the lava was blasted out of a volcano, and were trapped as the lava cooled and hardened. To learn more about pumice, scroll farther down this page.

	How to recognize pumice
	Other rocks that look like pumice
	Special things to look for
	Where pimice came from
	How pumice formed
	Other names for pumice
	Links to Websites about pumice

How to recognize pumice

	Pumice is mostly light gray or tan in color.
	Pumice is mostly bubble holes. However, some of the holes may have filled in with clay or crystals.
	If you look closely at the fresh rock between the bubble holes, it looks glassy.
	Dry pumice often floats in water.

Other rocks that look like pumice

Scoria:

	Like pumice, scoria is full of bubble-holes.
	However, scoria is also heavier in weight -- it never floats in water.
	Scoria is usually darker in color than pumice -- darker gray, deeper red.

Special things to look for

	Some pumice floats! Actually, almost all fresh, gray pumice floats. But our reddish brown pumice has been exposed to the weather for so long that it has started to turn into clay. Because clay plugs many of the bubble holes, some of our pumice doesn't float anymore.

There are no volcanoes in Chicago, so the pumice
we find in our area had to come from somewhere else.
Somebody must have shipped it here by truck or train.
However, so far we haven't found out where it came from.

How pumice formed

Pumice was a bubble-filled lava that exploded out
of a volcano. It then cooled and hardened quickly
that the bubbles could not escape.

In fact, pumice cooled so quickly that crystals
couldn't form -- the most of the rock between the
bubble-holes is volcanic glass. Volcanic glass
without bubble holes is called obsidian.

Other names for pumice

We use the scientific name "pumice" for this rock,
but it is also known by other names:

We've seen 50-pound bags of pumice gravel labeled "Golden Nuggets."

Here are some ways to classify pumice (by grouping it with similar types of rocks):

	Pumice is an igneous rock, because it formed from melted rock.
	Pumice is an extrusive igneous rock, because it formed from lava "extruded" onto the surface of the Earth.

Links to Web sites about Pumice

At the Children's Museum of Indianapolis Web site, you can help
solve the "Mystery of the Floating Rock." Go to:
< http://www.childrensmuseum.org/geomysteries/floatingrock/a1.html >

Here's a Franklin Institute Web page about pumice:
< http://www.fi.edu/fellows/payton/rocks/expert/pumice.htm >

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This page was created on May 2, 2001, and it was last updated on July 27, 2002.