Tracking the Course of Evolution
by Richard Cowen
THIS ESSAY, written in 1999, is a chapter from my book History of Life, published by Blackwell Science, Boston, Massachusetts, 2000. © Richard Cowen. You may print out a copy for personal or educational use, and you may link to this site. Illustrations are missing from this Web version of the chapter.
Cowen, R. 1994. History of Life. 2nd edition. 460 pp. Blackwell Scientific Publications, Cambridge, Massachusetts. This is a freshman-level textbook published by Blackwell Science. Copyright Richard Cowen 1994. Available from Blackwell Science, 238 Main Street,Cambridge, Massachusetts 02142, telephone 800-215-1000. Information and updates on the 3rd edition.
See also a separate essay devoted to the general topic of major extinctions, and for an outline of Richard Cowen's oral presentation.At the Geology Department at the University of California, Davis, Richard Cowen tries to maintain other Web pages of interest:
- Updates and Web links for the essay on the KT Extinction
- New references on the KT Extinction that have appeared since History of Life was published.
- Updates and Web links for the essay on Extinction
- New references on Extinction that have appeared since History of Life was published.
- Paleontology in the News: Web pages of current interest.
The End of the Dinosaurs: The K-T extinction
Almost all the large vertebrates on Earth, on land, at sea, and in the air (all dinosaurs, plesiosaurs, mosasaurs, and pterosaurs) suddenly became extinct about 65 Ma, at the end of the Cretaceous Period. At the same time, most plankton and many tropical invertebrates, especially reef-dwellers, became extinct, and many land plants were severely affected. This extinction event marks a major boundary in Earth's history, the K-T or Cretaceous-Tertiary boundary, and the end of the Mesozoic Era. The K-T extinctions were worldwide, affecting all the major continents and oceans. There are still arguments about just how short the event was. It was certainly sudden in geological terms and may have been catastrophic by anyone's standards.Despite the scale of the extinctions, however, we must not be trapped into thinking that the K-T boundary marked a disaster for all living things. Most groups of organisms survived. Insects, mammals, birds, and flowering plants on land, and fishes, corals, and molluscs in the ocean went on to diversify tremendously soon after the end of the Cretaceous. The K-T casualties included most of the large creatures of the time, but also some of the smallest, in particular the plankton that generate most of the primary production in the oceans.RETURN TO TOP
There have been many bad theories to explain dinosaur extinctions. More bad science is described in this chapter than in all the rest of the book. For example, even in the 1980s a new book on dinosaur extinctions suggested that they spent too much time in the sun, got cataracts, and because they couldn't see very well, fell over cliffs to their doom. But no matter how convincing or how silly they are, any of the theories that try to explain only the extinction of the dinosaurs ignore the fact that extinctions took place in land, sea, and aerial faunas, and were truly worldwide. The K-T extinctions were a global event, so we should examine globally effective agents: geographic change, oceanographic change, climatic change, or an extraterrestrial event. The most recent work on the K-T extinction has centered on two hypotheses that suggest a violent end to the Cretaceous: a large asteroid impact and a giant volcanic eruption.An Asteroid or Cometary Impact?
A meteorite big enough to be called a small asteroid hit Earth precisely at the time of the K-T extinction. The evidence for the impact was first discovered by Walter Alvarez and colleagues. They found that rocks laid down precisely at the K-T boundary contain extraordinary amounts of the metal iridium (Figure 18.1). It doesn't seem to matter whether the boundary rocks were laid down on land or under the sea. In the Pacific Ocean and the Caribbean the iridium-bearing clay forms a layer in ocean floor sediments; it is found in continental shelf deposits in Europe; and in North America, from Canada to New Mexico, it occurs in coal-bearing rock sequences laid down on floodplains and deltas. The dating is precise, and the iridium layer has been identified in more than 100 places around the Earth. Where the boundary is in marine sediments, the iridium occurs in a layer just above the last Cretaceous microfossils, and the sediments above it contain Paleocene microfossils from the earliest part of the Cenozoic.The iridium is present only in the boundary rocks and therefore was deposited in a single large spike: a very short event. Iridium occurs in normal seafloor sediments in microscopic quantities, but the iridium spike at the K-T boundary is very large. Iridium is rare on Earth, and although it can be concentrated by chemical processes in a sediment, an iridium spike of this magnitude must have arisen in some unusual way. Iridium is much rarer than gold on Earth, yet in the K-T boundary clay iridium is usually twice as abundant as gold, sometimes more than that. The same high ratio is found in meteorites. The Alvarez group therefore suggested that iridium was scattered worldwide from a cloud of debris that formed as an asteroid struck somewhere on Earth.An asteroid big enough to scatter the estimated amount of iridium in the worldwide spike at the K-T boundary may have been about 10 km (6 miles) across. Computer models suggest that if such an asteroid collided with Earth, it would pass through the atmosphere and ocean almost as if they were not there and blast a crater in the crust about 100 km across. The iridium and the smallest pieces of debris would be spread worldwide by the impact blast as the asteroid vaporized into a fireball. If indeed the spike was formed by a large impact, what other evidence should we hope to find in the rock record? Well-known meteorite impact structures often have fragments of shocked quartz and spherules (tiny glass spheres) associated with them (Figure 18.2). The glass is formed as the target rock is melted in the impact, blasted into the air as a spray of droplets, and almost immediately frozen. Over geological time, the glass spherules may decay to clay. Shocked quartz is formed when quartz crystals undergo a sudden pulse of great pressure. If they are not heated enough to melt, they may carry peculiar and unmistakable microstructures (Figure 18.2, top).RETURN TO TOP
All over North America, the K-T boundary clay contains glass spherules (Figure 18.2, bottom), and just above the clay is a thinner layer that contains iridium along with fragments of shocked quartz. It is only a few millimeters thick, but in total it contains more than a cubic kilometer of shocked quartz in North America alone. The zone of shocked quartz extends west onto the Pacific Ocean floor, but shocked quartz is rare in K-T boundary rocks elsewhere: some very tiny fragments occur in European sites. All this evidence implies that the K-T impact occurred on or near North America, with the iridium coming from the vaporized asteroid and the shocked quartz coming from the continental rocks it hit.The K-T impact crater has now been found. It is a roughly egg-shaped geological structure called Chicxulub, deeply buried under the sediments of the Yucatán peninsula of Mexico (Figure 18.3). The structure is about 180 km across, one of the largest impact structures so far identified with confidence on Earth. A borehole drilled into the Chicxulub structure hit 380 meters (more than 1000 feet) of igneous rock with a strange chemistry. That chemistry could have been generated by melting together a mixture of the sedimentary rocks in the region. The igneous rock under Chicxulub contains high levels of iridium, and its age is 65 Ma, exactly coinciding with the K-T boundary.On top of the igneous rock lies a mass of broken rock, probably the largest surviving debris particles that fell back on to the crater without melting, and on top of that are normal sediments that formed slowly to fill the crater in the shallow tropical seas that covered the impact area.Well-known impact craters often have tektites associated with them as well as shocked quartz and tiny glass spherules. Tektites are larger glass beads with unusual shapes and surface textures. They are formed when rocks are instantaneously melted and splashed out of impact sites in the form of big gobbets of molten glass, then cooled while spinning through the air.Haiti was about 800 km from Chicxulub at the end of the Cretaceous (Figure 18.3). At Beloc and other localities in Haiti, the K-T boundary is marked by a normal but thick (30 cm) clay boundary layer that consists mainly of glass spherules (Figure 18.2). The clay is overlain by a layer of turbidite, submarine landslide material that contains large rock fragments. Some of the fragments look like shattered ocean crust, but there are also spherical pieces of yellow and black glass up to 8 mm across that are unmistakably tektites. The Beloc tektites apparently formed at about 1300°C from two different kinds of rock; and they are dated precisely at 65 Ma. The black tektites formed from continental volcanic rocks and the yellow ones from evaporite sediments with a high content of sulfate and carbonate. The rocks of the Yucatán around Chicxulub are formed dominantly of exactly this mixture of rocks, and the igneous rocks under Chicxulub have a chemistry of a once-molten mixture of the two. Above the turbidite comes a thin red clay layer only about 5-10 mm thick that contains iridium and shocked quartz.One can explain much of this evidence as follows: an asteroid struck at Chicxulub, hitting a pile of thick sediments in a shallow sea. The impact melted much of the local crust and blasted molten material outward from as deep as 14 km under the surface. Small spherules of molten glass were blasted into the air at a shallow angle, and fell out over a giant area that extended northeast as far as Haiti, several hundred kilometers away, and to the northwest as far as Colorado. Next followed the finer material that had been blasted higher into the atmosphere or out into space and fell more slowly on top of the coarser fragments.RETURN TO TOP
The egg-shape of the Chicxulub crater shows that the asteroid hit at a shallow angle, about 20°-30°, splattering more debris to the northwest than in other directions. This accounts in particular for the tremendous damage to the North American continent, and the skewed distribution of shocked quartz far out into the Pacific.Other sites in the western Caribbean suggest that normally quiet, deep-water sediments were drastically disturbed right at the end of the Cretaceous, and the disturbed sediments have the iridium-bearing layer right on top of them. At many sites from northern Mexico and Texas, and at two sites drilled on the floor of the Gulf of Mexico, there are signs of a great disturbance in the ocean at the K-T boundary. In some places, the disturbed seafloor sediments contain fossils of fresh leaves and wood from land plants, along with tektites dated at 65 Ma (Figure 18.4). Around the Caribbean and at sites up the Eastern Atlantic coast of the United States, existing Cretaceous sediments were torn up and settled out again in a messy pile that also contains glass spherules of different chemistries, shocked quartz fragments, and an iridium spike. All this implies that a great tsunami or tidal wave affected the ocean margin of the time, washing fresh land plants well out to sea and tearing up seafloor sediments that had lain undisturbed for millions of years. The resulting bizarre mixture of rocks has been called "the Cretaceous-Tertiary co*cktail."Once Chicxulub was identified, it became possible to calculate that shocked quartz had been launched into a high-angle spray from the impact. This first hot fireball blew vaporized and molten debris (including glass spherules and iridium) high above the atmosphere to be deposited last and globally as it slowly drifted downward. The larger fragments, solid and molten, were blasted outward at lower angles, but not very far, and were deposited first and locally (about 15 minutes travel time to Colorado!). At the same time, smaller fragments, including shocked quartz, were blown upward between the hot fireball and the larger fragments, and were deposited second and regionally (about 30 minutes to reach Colorado). The impact energy, for comparison with hydrogen bomb blasts, was around 100 million megatons.A Giant Volcanic Eruption?
Exactly at the K-T boundary, a new plume (Chapter 6) was burning its way through the crust close to the plate boundary between India and Africa. Enormous quantities of basalt flooded out over what is now the Deccan Plateau of western India to form huge lava beds called the Deccan Traps. A huge extension of that lava flow on the other side of the plate boundary now lies underwater in the Indian Ocean (Figures 18.3 and 18.5). The Deccan Traps cover 500,000 km2 now (about 200,000 square miles), but they may have covered four times as much before erosion removed them from some areas. They have a surviving volume of 1 million km3 (240,000 cubic miles) and are over 2 km thick in places. The entire volcanic volume that erupted, including the underwater lavas, was much larger than this (Figure 18.5).Furthermore, the Deccan eruptions began suddenly just before the K-T boundary. The peak eruptions may have lasted only about one million years (± 50%), but that short time straddled the K-T boundary. The rate of eruption was at least 30 times the rate of Hawaiian eruptions today, even assuming it was continuous over as much as a million years; if the eruption was shorter or spasmodic, eruption rates would have been much higher. The Deccan Traps probably erupted as lava flows and fountains like those of Kilauea, rather than in giant explosive eruptions like that of Krakatau. But estimates of the fire fountains generated by eruptions on the scale of the Deccan Traps suggest that aerosols and ash would easily have been carried into the stratosphere. The Deccan plume is still active; its hot spot now lies under the volcanic island of Réunion in the Indian Ocean.Thus there is strong evidence for short-lived but gigantic volcanic eruptions at the K-T boundary. Some people have tried to explain all the features of the K-T boundary rocks as the result of these eruptions. But the evidence for an extraterrestrial impact is so strong that it's a waste of time to try to explain away that evidence as volcanic effects. We should concentrate instead on the fact that the K-T boundary coincided with two very dramatic events. The Deccan Traps lie across the K-T boundary and were formed in what was obviously a major event in Earth history. The asteroid impact was exactly at the K-T boundary. Certainly something dramatic happened to life on Earth, because geologists have defined the K-T boundary and the end of the Mesozoic Era on the basis of a large extinction of creatures on land and in the sea. An asteroid impact, or a series of gigantic eruptions, or both, would have had major global effects on atmosphere and weather.There is a feeling, particularly among physical scientists, that if we can show that a physical catastrophe occurred at the K-T boundary, we have an automatic explanation for the K-T extinctions. But this connection has to be demonstrated, not just assumed. We still have to ask which catastrophe, if either, caused the K-T extinctions, and if so, how?