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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."

	How to recognize basalt
	Other rocks that look like basalt
	Where basalt came from
	How basalt formed
	Other names for basalt
	Links to Web sites about volcanoes

How to recognize basalt

	Basalt is mostly dark gray and dull looking, even in the sunlight. (Weathered pieces may look yellowish or reddish on the outside and along cracks.)
	Basalt is made mostly of tiny crystals, but it may contain scattered larger crystals.
	Basalt breaks into chunks with smooth sides.
	Most pieces of basalt are hard to scratch with a nail, but a knife may scratch some pieces. (See more about the scratch test.)
	The 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:

	Limestone is sometimes dark gray, smooth, and dull-looking, like basalt.
	However, limestone makes bubbles when you put a drop of acid or vinegar on it. (Basalt does not bubble in acid or vinegar.)
	It's easy to scratch limestone with a nail or knife.

Obsidian:

Obsidian is mostly black and looks glassy. (Basalt is dark gray and dull-looking.)

Gabbro:

	Gabbro often looks dark gray, and dirty pieces may look dull.
	However, 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:

	Basalt 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.
	Basalt 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:

	We've seen 50-pound bags of basalt gravel labeled "trap rock."
	People 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):

	Basalt is an igneous rock, because it formed from melted rock.
	Basalt 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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This page was created on October 10, 2001, and it was last updated on July 27, 2002.