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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.

  
bulletHow to recognize pumice
bulletOther rocks that look like pumice
bulletSpecial things to look for
bulletWhere pimice came from
bulletHow pumice formed
bulletOther names for pumice
bulletLinks to Websites about pumice
  

How to recognize pumice

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

Other rocks that look like pumice

Scoria:  
bulletLike pumice, scoria is full of bubble-holes.
bulletHowever, scoria is also heavier in weight -- it never floats in water.
bulletScoria 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.

   

   

Where pumice came from

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:

bulletWe'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):
bulletPumice is an igneous rock, because it formed from melted rock.
bulletPumice 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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Copyright 2001-2002 Eric D. Gyllenhaal                                           Search this Site
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This page was created on May 2, 2001, and it was last updated on July 27, 2002.