Geology Reference
In-Depth Information
In this chapter, I explain the most common theories for what leads to mass extinctions
and describe the five biggest extinction events in earth's history, as well as a few minor
ones.
Explaining Extinctions
Each mass extinction in earth's history has been recorded by the sudden absence of
fossils of certain organisms in the geological record. These events or periods of extinc-
tion affected the entire planet. Scientists think that in each case, some change in the en-
vironment resulted in conditions that could no longer support the organisms that had
adapted to it. Thus, the organisms died off in great numbers, and some never reap-
peared.
Scientists have not yet determined, unquestionably, what led to each mass extinction,
but they have some good ideas, expressed as theories, that are still being tested by
modern scientific research. I introduce four such theories in this section.
Heads up! Astronomical impacts
Earth is only one of many objects moving through the universe. Occasionally, as eviden-
ced by the craters on the moon, flying objects in space may hit one another. When this
occurs, it is called an
impact event.
Scientists have found evidence for impacts on earth,
such as craters resulting from meteorites that have hit earth's surface. The span of hu-
man history has not recorded any impact large enough to cause a dramatic change in
global conditions, but evidence exists that such major events occurred in the past.
While it may seem obvious that being struck by a meteor devastates life in the areas sur-
rounding the impact zone, what is not so obvious is the continued after-effects that are
experienced all around the globe. The following sequence of events explains how global
ecosystems could be negatively affected through an impact event:
1. A large object hits earth.
The impact sends large amounts of rock and other colli-
sion debris into the atmosphere, and it starts fires, which add smoke and ash to the
atmosphere.
2. The atmosphere is polluted.
The particles of ash and rock in the atmosphere do
three things:
•
Block sunlight, which plant life depends on
•
Block sun warmth, leading to global cooling
•
Create conditions for acid rain
