Chapter 5: The Earth-Moon System

For a long time, the two largest mass extinctions got surprisingly little attention in geology textbooks. Part of the reason was the fragmentary nature of the fossil record. The layering of rocks and fossils is not neat and orderly throughout geological time. Volcanism, erosion, or metamorphosis of rocks can destroy some of the fossil evidence. Also, radioactive dating has limits. Let's say we could determine the age of 100-million-year-old rocks with a precision of 0.1%. The measurement error is still 100,000 years. This means that 100,000 years is the limit of our ability to resolve time. If we see the disappearance of many species over that interval, should we consider it gradual or catastrophic change? But as geologists found more fossils, their precision improved, and the extinctions seemed to be more dramatic.

What happened after the asteroid hit, to cause the extinction of so many species? The details are exceedingly complex, and they're still poorly understood. The huge impact probably blasted a cloud of dust and other debris into space, which then fell back into the atmosphere and spread all over the world in the following hours. If you were standing on the Earth a few minutes after the impact, you would have observed the sky light up as falling debris formed brilliant fireballs and shooting stars. The molten debris caused a strong heat pulse that may have killed many land animals instantly and ignited forest fires. This injected an enormous amount of fine soot into the stratosphere, which probably floated there for some time and blocked sunlight for weeks or months. Sediments at the site of the crater were rich in carbonates and sulfur, and these materials were vaporized by the impact. Carbon dioxide released into the atmosphere may have

resulted in long-term global warming. The sulfur would have combined with water in the atmosphere to form acid rain. All of these changes would have wreaked havoc on the entire food chain. Reptiles had ruled the Earth for 150 million years. Now they died out due to a catastrophic change in their environment. Previously minor species, including certain small mammals, had less competition. They proliferated into this ecological vacuum, evolving into new species, including a certain upright mammal that has evolved to be able to study astronomy.

In the spectacular mass extinction that ended the Paleozoic Era roughly 250 million years ago, even more of the existing species were destroyed. In the oceans, the fraction has been estimated to be as high as 95% of all species! This event was so dramatic, geology textbooks refer to it as "The Great Dying.” The cause is still uncertain. No positive proof of an impact has been found. Most hypotheses center on internally-caused

geological events. One leading hypothesis is that large plumes of hot magma rise from the mantle sporadically, like blobs rising in a lava lamp. Upon reaching the crust, they may have caused major episodes of volcanism and considerable plate tectonic shifts, altering the Earth's climate. An alternate hypothesis involves a sudden turnover or disturbance in the ocean, which brought oxygen-poor water to the surface and increased CO₂ concentrations in the air. These might not be sudden disasters like the transient climate effects of large impacts, but they could still increase the rates of extinction and the emergence of new species in new environmental niches.

In 1996, Oregon paleontologist Gregory Retallack presented evidence of shocked quartz grains from the layer at the end of the Paleozoic. Shocked quartz grains are usually accepted as evidence of an impact. Conceivably, an impact in the oceans or continental margins might have disturbed the oceans, and the crater could have been subsequently destroyed by plate tectonic activity. The fossil record this far back in time is quite patchy. Also, since radioactive techniques cannot date rocks with very high precision, it's difficult to distinguish between the rival hypotheses of a sudden catastrophe and a somewhat slower geological change. Even the story of the K-T extinction has got muddier in recent years. Geologists have pointed to a surge in volcanism that slightly preceded the impact and other craters of similar age have been found, so life may have been subject to multiple "punches." It's a lesson in the scientific method not to confuse correlation with causation. The race to explain the most dramatic episodes of dying in Earth's history continues.

It's a sobering fact that we're in the midst of another mass extinction, this time by our own hand. Unlike the other mass extinctions caused by geological processes of impacts from space, the rapid loss of species currently is caused by environmental degradation, pollution, and the expanding human "footprint" on the planet. The current geological is called the Holocene, covering the past 12000 years since the dawn of civilization. Time will tell whether we can avert an environmental catastrophe.

Author: Chris Impey
Editor/Contributor: Ingrid Daubar