Geoscience Reference
In-Depth Information
iridium, an element abundant in some types of meteorite. But there was no sign of an im-
pact crater of that age. Then evidence began to emerge, not from the land but from the sea
just off the Yucatan Peninsula of Mexico, of a buried crater 200 kilometres across. There is
evidence of debris from a much wider area. If, as is calculated, it marks the point where an
asteroid or comet, maybe 16 kilometres across, hit the Earth, the results would indeed have
been devastating. Apart from the effects of the impact itself and the tsunami that resulted,
so much rock would have been vaporized that it would have spread round the Earth in the
atmosphere. At first it would have been so hot that its radiant heat would have triggered
forest fires on the ground. The dust would have stayed in the atmosphere for several years,
blocking out sunlight, creating a global winter, and causing food plants and plankton to die.
The sea bed at the impact site included rocks rich in sulphate minerals and these would
have vaporized, leading to a deadly acid rain when it washed out of the atmosphere again.
It is almost surprising that any living creatures survived.
The menace within
It was once hard to understand how any mass extinctions could have occurred. Now, there
are so many competing theories that it is difficult to choose between them. They mostly
involve severe climate change, whether triggered by a cosmic impact, changing sea levels,
ocean currents and greenhouse gases, or a cause from within the planet such as rifting or
major vulcanism. It does seem that most of the mass extinctions we know coincided at least
approximately with major eruptions of flood basalts. In the case of the late Cretaceous, it
was the eruptions that produced the Deccan Traps in western India. There has even been
a suggestion that a major asteroid impact caused shock waves to focus on the other side
of the Earth, triggering eruptions. But the times and positions do not seem to line up well
enough to prove that explanation. Whatever the reason, the history of life and of the planet
has been punctuated by some catastrophic events.
Chaos reigns
We can all remember climatic events that stand out, say over the last decade, as the worst
winter, flood, storm, or drought. Take the record back for a century and the likelihood is
that an even bigger one will stand out. Authorities often use the concept of a '100-year'
flood in planning coastal or river flood defences; they are designed to withstand the sort
of flood that only happens once a century. It's likely to be more severe than the sort which
happens only once a decade. But, if you extend the same idea to a thousand years or a milli-
on years, there is likely to be one that will be bigger still. According to some theorists, that
is likely to be true of anything from floods, storms, and droughts to earthquakes, volcanic
eruptions, and asteroid impacts. Over geological time we had better watch out!
