| The Tswaing crater has a diameter
of approximately 1 kilometre and has salt deposits and a highly saline
pond in its centre, which gave rise to the name Tswaing (“place of salt”). The excellently preserved crater rim is elevated 60 metres above the surrounding
plains and has a maximum elevation of 119 metres above the rather flat
crater floor. The Tswaing crater is strikingly similar in size and
structure to the much more famous meteorite crater in Arizona which was
formed approximately 50 000 years ago.
Throughout earth history, the earth's
surface has permanently been bombarded with meteorites from outer space. Large and small rock or iron projectiles frequently hit the earth's surface,
sometimes resulting in the formation of impact craters such as the Tswaing
meteorite impact crater. Movies such as Armageddon and
Deep Impact have made people more aware of the potential danger of a cosmic
body colliding with our planet, causing the possible mass extinction of
all life on our planet. This is not as far-fetched as it seems, since
meteorite impacts with catastrophic effects have occurred throughout earth
history. In one specific case, approximately 65 million years ago,
the direct and indirect effects of a meteorite impact led to the mass extinction
of not only the dinosaurs, but almost all the large animals on our planet.
This meteorite impact site has been identified by scientists to be the
180 kilometers wide Chicxulub crater on the Yucatan peninsula of Mexico, caused
by a meteorite about 50 kilometers in diameter.
In comparison, the Tswaing meteorite
crater is tiny, only 1,1 kilometers in diameter, and probably caused by a 30 to
50 meter wide meteorite, a projectile consisting of metallic rock. Nevertheless,
Tswaing is one of the best preserved craters of the 150 known impact structures
on earth and is one of the very few meteorite craters which are easily
accessible. The catastrophic effects on the environment were still
spectacular and it is rather fascinating to imagine that the early Stone
Age people inhabiting the Magaliesberg at the time of the impact event
could have watched this event. In how far they were able to enjoy
it remains questionable, as a shock wave and wind speeds of several hundred
km/h must have hit any spectator as a result of the impact. Within
an area of approximately 35 x 35 kilometers no life would have survived this explosion,
which was 100 times the force of the atomic bomb that hit Hiroshima and
ended World War II in 1945.
This meteorite pierced the atmosphere
at a speed of 20 – 30 km/sec, which is amazingly fast when compared to
the speed of a bullet (which exits a gun at less than 1 km/sec). The catastrophe
happened almost without warning – it took only ten seconds for the meteorite
to penetrate the earth's atmosphere, causing a bright flash in the sky
as the outer parts of the meteorite melted due to the enormous frictional
heat of several thousand degrees centigrade. In the case of the relatively
small Tswaing meteorite crater, the crater-forming event, which led to
the formation of the crater wall as a result of huge amounts of rock material
being ejected laterally from the site of the impact, and the typical internal
depression, took less than 30 seconds. Temperatures of several thousand
degrees centigrade and the immense pressures (several 100,000 bars) during
the impact leave impact-specific mineralogical and geological marks in
the affected rocks but generally also destroy most of the meteorite. Only in rare cases such as the famous Hoba iron meteorite in Namibia, is
a meteorite fragment still preserved and accessible to scientific studies.
The three most commonly asked questions
asked with respect to meteorite impacts are:
-
Why is the moon's surface riddled with
meteorite craters whereas earth apparently has only relatively few?
-
Where do all these projectiles come
from?
-
Can scenarios such as depicted in the
movies Deep Impact and Armageddon become reality in the near future?
The first question
is relatively easy to answer. Several factors do actually play a
role in preventing the earth from being as badly damaged as the moon. One is the existence of our atmosphere, which is firstly responsible for
the vaporization and explosion of most projectiles due to the frictional
forces during the projectile's passage through the atmosphere. In
spite of this, thousands of tons of meteoritic material are added each
year to the earth's mass. The atmosphere is also responsible for
weathering on our planet. The forces of wind and water are fierce
enemies working on the destruction of anything in their way. The
moon is void of wind and water which explains partly the fantastic state
of preservation of even the oldest meteorite craters which are probably
more than 3000 million years old.
Another very
important reason is the fact, that our earth is geologically active, which
means that we experience volcanism, earth quakes and continental drift,
the latter being the concept of rigid so-called plates moving on top of
a plastic underlying layer which results in the ongoing generation and
destruction of parts of the earth's crust. The majority of our earth's
crust is less than 200 million years old, which is relatively young compared
with the overall age of our planet (4 500 million years). Two thirds
of our planet is covered by water, which also adds to the scarcity of visible
impact craters as on the earth's surface.
The answer
to the question regarding the origin of the meteorites is slightly more
difficult. Most meteorites are assumed to stem from the asteroid
belt which is situated between Mars and Jupiter. This belt consists
of fragments which are thought to originate from a planet, compositionally
similar to our earth, which at some stage during the history of our solar
system was hit itself by a large projectile, breaking this planet into
millions and millions of smaller fragments of which some are on collision
course with the earth. Some meteorites have been identified to originate
from our moon or even from mars, probably due to meteorite impacts which
catapulted fragments out into space on collision course with the earth.
With respect
to the third question statistics have to be evoked. By using all
available information with respect to meteorites and meteorite impacts
it can be stated that every 1 600 years the earth will be hit by a meteorite
with the size of the Tswaing meteorite. But this figure represents
only the average calculated time period and in fact earth can be hit catastrophically
today, tomorrow, or at any time, and in any place. |
