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Basalt (also called "Trap Rock")

Basalt is a plain, gray rock with an exciting history -- it formed as lava erupted from a volcano, cooled, and hardened.

In our area, we find basalt chunks along some railroad tracks and basalt pebbles on beaches and in rivers.  We can also buy basalt gravel by the bag at a few landscape centers.  (It's been at least a billion years since there were volcanoes around here!)

To learn more about basalt, scroll farther down this page.

Basalt collected along a railroad track.
Basalt purchased by the bag as "Trap Rock."
   
 
bulletHow to recognize basalt
bulletOther rocks that look like basalt
bulletWhere basalt came from
bulletHow basalt formed
bulletOther names for basalt
bulletLinks to Web sites about volcanoes
  

How to recognize basalt

bulletBasalt is mostly dark gray and dull looking, even in the sunlight.  (Weathered pieces may look yellowish or reddish on the outside and along cracks.)
bulletBasalt is made mostly of tiny crystals, but it may contain scattered larger crystals.
bulletBasalt breaks into chunks with smooth sides.
bulletMost pieces of basalt are hard to scratch with a nail, but a knife may scratch some pieces.  (See more about the scratch test.)
bulletThe basalt doesn't make bubbles in acid or vinegar.  (However, some basalt has patches or lines of white calcite crystals that do bubble in acid.)
    

Other rocks that look like basalt

Limestone:
bulletLimestone is sometimes dark gray, smooth, and dull-looking, like basalt.
bulletHowever, limestone makes bubbles when you put a drop of acid or vinegar on it.  (Basalt does not bubble in acid or vinegar.)
bulletIt's easy to scratch limestone with a nail or knife.
  
Obsidian:
bulletObsidian is mostly black and looks glassy.  (Basalt is dark gray and dull-looking.)
  
Gabbro:
bulletGabbro often looks dark gray, and dirty pieces may look dull.
bulletHowever, if you look close you'll see that gabbro is made of lots of small crystals, some of which shine in the sun.

 

    

Where basalt came from

There are no volcanoes in Chicago, so the basalt
we find in our area had to come from somewhere
else:
bulletBasalt that we find on beaches, in rivers, or in
the soil was carried here by glaciers.  The glaciers
plucked the basalt from ancient, eroded volcanoes 
that were hundreds of miles to the north of us.
bulletBasalt found along railroad tracks and in parking 
lots was carried here by trains or trucks.  We're not
sure were this basalt was originally found. 

     

How basalt formed

Basalt formed as lava from a volcano cooled and
hardened.  

The lava cooled pretty fast, so the crystals in basalt 
didn't have time to grow very large.  However, some 
lava contained crystals that had already grown pretty
big before the mostly-melted rock was pushed out
of the volcano.  These crystals are now embedded
in the fine-grained rock.

Some basalt hardened with a few bubble holes in
the rock.  These bubbles holes often filled in later 
with crystals of calcite or other minerals.  (When 
basalt cooled with lots of bubble-holes, we call it 
scoria
.)

Basalt-producing volcanoes are found today in
Hawaii and along the Mid-Ocean Ridges, far below
the surface of the ocean.  Much of the Moon's surface
is made of basalt that formed billions of years ago.

  

Other names for basalt

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

bulletWe've seen 50-pound bags of basalt gravel labeled "trap rock." 
bulletPeople who mine, quarry, or use basalt rock often call it "trap rock."  (However, the term trap rock can also be used to refer to almost any type of hard, dark rock.)
  
Here are some ways to classify basalt (by grouping it with similar types of rocks):
bulletBasalt is an igneous rock, because it formed from melted rock.
bulletBasalt is an extrusive igneous rock, because it formed from lava "extruded" onto the surface of the Earth.

   

Links to Web sites about volcanoes

VolcanoWorld has lots of information and activities for kids:
   < http://volcano.und.nodak.edu/vwdocs/kids/kids.html >

  


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Copyright 2001-2002 Eric D. Gyllenhaal                                        Search this Site
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This page was created on October 10, 2001, and it was last updated on July 27, 2002.