A meteoroid is a naturally occurring, relatively
small, solid body. Bodies larger than 1 kilometer in diameter are usually
called asteroids. Meteoroids and asteroids move quickly, at speeds of
greater than 10 km/second, about 58,000 miles per hour!
When a meteoroid enters through the Earth's
atmosphere, it is slowed by friction with atmospheric molecules. This causes
the meteor to heat up. It often glows, and leaves a trail of vaporized
material and ionized air behind it. A glowing meteoroid moving through the
atmosphere is called a meteor. Meteoroids enter the Earth’s atmosphere
continuously. Most of these objects, however, are dust to sand sized
particles, which burn up, or vaporize, high in the atmosphere.
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Regmaglygt texture of meteorite
from Sikhote Alin, Russia
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If a meteor survives its passage through the
atmosphere and strikes the Earth’s surface, it is called a meteorite. This
can happen in two ways. Some meteors disaggregate or explode in the
atmosphere, and their remnants fall to the surface. If a meteor travels all
the way to the surface, it usually creates a crater. The energy of the
meteor compresses the ground where it strikes, forming a hemispherical
cavity. In addition, much of the meteor, and some of ground is vaporized.
Material is also ejected from the crater, landing in
piles all around it. This material may include bits of the meteor and
surface rocks, as well as molten rock formed by the energy of the impact.
This molten rock is similar in appearance to terrestrial magma. In some
impacts, large blobs of molten rock are flung outward. These strike the
surface and quickly cool, forming glassy objects called tektites. Tektites
in this scenario are considered primarily melted Earth materials that
sprayed upward and outward during an exceptional meteorite impact.
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Camel Donga meteorite from Australia
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Meteorites and tektites are rare for three reasons.
First, much of a meteorite is vaporized during its impact with the surface.
Second, after they reach the Earth’s surface, meteorites and tektites are
subjected to weathering and breakdown. Third, these objects may be buried by
the deposition of sand, mud, or other sediment.
Meteorites have three general compositions:
- Iron meteorites are composed mainly of metallic
iron and nickel, often mixed together as an alloy.
- Stony meteorites contain mainly silicate minerals,
such as pyroxene, plagioclase feldspar, and olivine. They also contain
minor amounts of metals, particularly nickel and iron alloys. Stony
meteorites account for 95% of all meteoritic material.
- Stony-iron meteorites contain a mixture of silicate
minerals and nickel-iron alloy. Stony meteorites are similar to igneous
Earth rocks like basalt. Iron meteorites probably resemble the material
in the Earth’s core.
You may wish to tell the students that their tektite
samples are from Thailand, and are about 1.2 million years in age. They came
from a crater that has not been found, but was probably someplace in
Cambodia. Tektites from this impact have been found as far away as central
Australia.
PROCEDURE:
In this lab the students will compare meteorites with
Earth rocks and then see what happens to a meteorite when it hits the
Earth's surface.
- The students have samples of granite, basalt,
obsidian, sandstone, and schist from Earth. They also have a small
meteorite to use for comparison. This sample is a tektite from
Indonesia. Tell students to compare and contrast the samples.
- In this lab the students will compare samples of
tektites with five Earth rocks: granite, basalt, obsidian, sandstone,
and schist. They will probably conclude that the tektite is most similar
to basalt or obsidian because both samples will be black in color and
contain visible minerals. However the composition of tektites is more
similar to obsidian. Basalt is actually quite different; the tektites
contain much more silica. The granite, sandstone, and schist do not
resemble the meteorite at all.
