Until about 1980, geologists concentrated on defining the main strata and their corresponding geologic eras and periods. The five geologic eras end in the suffix "-zoic," referring to life, because they are defined by the types of life forms found within those strata. Geologists didn’t pay much attention to the transitions from one geological period to another, which showed marked changes in the relative numbers of different species. No one knew if these changes occurred gradually or abruptly.
Earth's Geological Timescale with Milestones. Click here for original source URL
As fossil and geological evidence accumulated, geologists decided that some of the changes were very dramatic. Modern studies show that the Paleozoic Era ended in a huge catastrophe about 250 million years ago, when about 90% of the existing plant and animal species died out in less than half a million years. After that, reptiles rose rapidly and dominated the ensuing 200 million years. Then, about 65 million years ago, another catastrophe wiped out about the majority of existing species, including the giant reptiles, or dinosaurs. Because this occurred between the Cretaceous and Tertiary geologic periods, it is referred to as the K/T boundary extinction (where K is from the German spelling for Cretaceous). Similarly, the end of the Paleozoic Era is also called the Permian-Triassic boundary. These events are mass extinctions: relatively brief intervals in which a large fraction of species vanish. The end of the Paleozoic and Mesozoic Eras are the two best examples of mass extinctions. They mark such dramatic breaks in the fossil record that geologists define geologic eras as beginning or ending with them. Smaller examples of mass extinctions define boundaries between other geologic periods as well.
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.
The first mass extinction to be explained by direct scientific evidence was the one at the end of the Mesozoic, 65 million years ago. Approximately 75% of all species of plants and animals disappeared within a few million years, including the dinosaurs! In the early 1980s, geochemists made an interesting discovery about the thin layer of sediments at the boundary between Cretaceous and Tertiary rocks. It contained an excess of iridium, an element that is extremely rare in Earth’s crustal rocks, but present in higher levels in meteorites. This discovery suggested that a giant meteorite impact might have been connected with the end of the Mesozoic. Further evidence supported this theory: the iridium layer was mixed in with glassy spheres formed by melted rock and quartz grains that had been heated and shocked suddenly. The extreme pressures required to do this can only be reached during a high-velocity impact. Scientists also discovered concentrations of soot that indicated worldwide forest fires.
For years, scientists argued about how to interpret this evidence. The father and son team of Walter Alvarez, a geologist, and Luis Alvarez, a Nobel Prize-winning physicist, advocated the hypothesis that an impact from space killed off the dinosaurs and other species. This was a controversial idea. In the scientific method, there are almost always rival explanations for any set of data. Because radioactive dating has limited time resolution, scientists could not prove that the extinction was catastrophic - occurring over a period of days or years rather than tens of thousands of years. Volcanism could have produced the glassy spheres of melted rock, and it could have triggered forest fires. But volcanism can’t produce the high pressures needed to shock quartz.
The "smoking gun" in this detective story was still missing. If an impact was to blame for a mass extinction, where was the impact crater? In what is now North America, scientists found that the 65 million-year-old layer contained deposits from large tsunamis, or tidal waves. The tsunami deposits suggested an impact near the Gulf of Mexico. Eventually scientists discovered a large crater completely buried under sediments, straddling the coastline of the Yucatan peninsula in Mexico. The crater, named Chicxulub after a nearby town, had a diameter of about 160 to 180 kilometers (100 to 110 miles) from rim to rim. It was created by impact of an asteroid about 10 kilometers across (6 miles) — the size of a small town! But did the crater have the right age? After a delicate period of negotiations involving core samples taken by a Mexican oil company, scientists had the evidence they needed. The age of the crater was indeed 65 million years.
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 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 imact, and other craters of similar age have been found, so life may have been subject to multiple &quo;punches.&quo; 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.