When you look out into space on a dark night, all seems peaceful. Stars twinkle in the clear black sky and the vast emptiness of space soars above your head. But the reality is quite different. The Earth is actually being bombarded with a constant stream of space matter, mostly drawn from the asteroid belt between Mars and Jupiter.
This onslaught is incessant and, while it is thought that the number of interplanetary projectiles is much lower today than it was when the Earth was being formed, it is estimated that 30 000 tonnes of interplanetary debris still enters our atmosphere every year.
Most of these bits and pieces burn up as they plummet through the atmosphere, heated beyond their melting points by the air resistance. The bright light emitted by these objects make them visible as meteors, or shooting stars. If the meteor is big enough, however, part of it may survive the heat and land on earth as a meteorite.
FYI, the largest meteorite found thus far is the Hoba meteorite, discovered in 1920. It hit the ground near Grootfontein, northern Namibia, over 80 000 years ago and hasn’t been moved since. Hoba is a solid piece of iron and nickel, measuring nearly three metres square and one metre high.
But what if something really big came thundering towards Earth? What would happen if a bolide measuring 10 or even 100 kilometers across smashed into the ground? Studies of previous impact events don’t paint a very pretty picture.
Large impact events seem to have the potential to devastate not just the localised impact site, but the entire planet. Monstrous earthquakes, titanic tsunamis and floods of volcanic activity resulting from the impact would scour the land of life.
Furthermore, gigantic quantities of dust and debris blown into the atmosphere may block out the sun, plunging the planet into a catastrophic winter that lasts for months or years. In short, it is postulated that impact events could cause extinctions on a global scale.
Extinction is a natural part of the evolutionary process. Over billions of years, innumerable species have died out because of climatic changes, natural disasters and competition with other species. This is considered normal (or background) extinction and is still continuing today – helped along by modern humans as they pillage the planet.
It is estimated that, under normal conditions, between two and five taxonomic families will become extinct every million years but, with a bit more pollution, we may be able to get that average right up.
However, the fossil record indicates that, at certain points in the past, there were mass extinctions in which a hefty percentage of all the life on Earth disappeared quite suddenly.
These are sometimes called boundary events and they are evidenced by discontinuities in both the rocks and fossils, where one type of life form is abruptly replaced by another. These boundary events are usually evident in contemporary rock layers at various sites across the planet.
As we rely on the fossil record to identify these boundaries, the progression of mass extinction events only becomes apparent after the proliferation of life in the Cambrian period, about 550 million years ago (Ma).
Since then, there have been 5 major mass extinctions: the End-Ordovician transition (444 Ma: 84% species loss), the Late Devonian (360 Ma: 79% species loss), the End-Permian (250 Ma, 95% species loss – the planet’s worst mass extinction), End-Triassic (200 Ma, 79% species loss) and the End-Cretaceous (65 Ma, 70% species loss).
Some modern researchers further claim that we are currently in the grip of another mass extinction, the fastest to have been recorded. Helped along by our callous treatment of the environment, this so-called Holocene extinction event could mean that up to half the species of life on Earth will die within the next 100 years (according to one rather gloomy scientist).
The reasons for these mass extinctions are difficult to determine. Only one (the End-Cretaceous, or K-T Boundary) has been definitively linked to an impact event. The others have a less certain origin, as no corresponding impact structures have been found thus far.
Apart from impacts, proposed causes include: climatic changes (such as ice ages), mammoth volcanic events (which poison the atmosphere), gamma-ray bursts (which fry the living organisms on the surface) and plate tectonics (which may bring continents together and thus unite previously isolated species of life, creating the sudden emergence of new species).
It is considered unlikely, however, that any one of these hypotheses could alone account for all the mass extinction events, and a combination of factors is a more probable conclusion.
In any case, research has shown that mass extinctions occur on a cyclical basis, perhaps as often as once every 30 to 60 million years. It should also be noted that just because we don’t have significant evidence for a mass extinction event before 500 Ma, it doesn’t mean that they didn’t occur.
Before the Cambrian period, life was still rather rudimentary, consisting of simple forms such as bacteria, brachiopods and trilobites. These tiny organisms do not leave behind extensive fossils, and their numbers are thus difficult to track.
Furthermore, rocks dating back more than 600 million years are quite rare, since they tend to get recycled in the tectonic process. This removes the evidence of boundary layers which may indicate periods of global crisis. Suffice it to say that, if there were any complex life forms on Earth at the time of the Vredefort impact, they would have been wiped out!
By David Fleminger