Dinosaurs became extinct 65 million years ago, and there has been much speculation as to the cause of their demise. The most developed theory is that the extinction resulted from the collision of a huge asteroid with the Earth. A primary figure behind this theory is the American geologist Walter Alvarez. He describes the scientific adventure leading to the theory in a beautifully written book, T rex and the Crater of Doom (Penguin, 1998).
Geologists study the Earth, its history, and the physical, chemical and climatic conditions that explain the structure and activity of the planet. The Earth's history is written in the rocks. Rocks are formed by various processes. Igneous and sedimentary rocks are two main types. Igneous rocks are formed when molten magma wells on to the surface from the interior of the Earth, cools and solidifies. Sedimentary rocks are formed at the bottom of oceans and lakes when material from the water slowly settles out and compacts.
Over time, rocks are laid down on top of each other in layers, with the oldest on the bottom, the next oldest on top of them, and so on up to the youngest rocks in the top layer. By studying these layers geologists build up a picture of the Earth's history.
The age of each rock layer is gauged by measuring the radioactive elements in the layer. The fossilised plants and animals in each layer show the types of life that flourished when the layer formed. The geological record is divided into broad eras, each sub-divided into periods and stages. The Earth was formed about five billion years ago and life arose about 3.5 billion years ago.
Several massive extinctions of life forms are seen in the geological record. The extinction of 65 million years ago occurred at the boundary between the Mesozoic and Cenozoic eras, the KT boundary, when about half the life forms on Earth, including the dinosaurs, vanished for ever. The dinosaurs had been the top animals on Earth for 150 million years.
Some scientists believe the extinction of 65 million years ago was gradual, developing slowly over thousands of years. I am no expert in the area, but it seems to me that the evidence for a sudden extinction is more convincing. What could cause a sudden massive extinction of life? A catastrophe is the obvious answer, but geologists hate invoking a catastrophe to explain anything. One of the fathers of geology, Charles Lyell (1797-1875), introduced the doctrine of uniformitarianism. This states that the forces that now operate to shape the Earth are the same forces that always operated, and these slow processes, operating over deep time, are sufficient to explain the present features of the Earth. This doctrine excluded the idea that sudden catastrophes contributed significantly to forming the Earth's features.
Uniformitarianism has been very useful in facilitating the orderly development of geology. It has explained most things about the Earth and has prevented a profusion of "quick-fix" catastrophic hypotheses arising to explain the questions posed by the stories written in the rocks. But uniformitarianism is too rigid in completely dismissing the significance of catastrophes.
The KT boundary is marked by a centimetre-thick layer of clay. Alvarez's team found that this layer contains more iridium than other rocks in the Earth's crust. Where did this worldwide dusting of iridium come from? This question sparked the idea of collision with an asteroid. Asteroids contain a higher concentration of iridium than rocks in the Earth's crust. If a huge asteroid slammed into Earth and vapourised, one would expect a noticeable worldwide deposition of iridium.
How could an asteroid wipe out life on a global scale? This would result from the secondary effects of impact. Enormous amounts of dust would be blown into the atmosphere and carried around the globe. The searing heat generated by the asteroid would start huge forest fires, belching thick black smoke up to join the dust. This blanket of soot and smoke would blot out the sun, plunging the Earth into freezing darkness for months.
The freezing darkness would devastate plant life and animal life with vegetarian feeding habits. Large carnivores would succumb next, deprived of large herbivores to feed on. It is easy to visualise how a large number of life forms could be wiped out. A massive search was undertaken for an impact crater sufficiently large to fit the asteroid-extinction hypothesis. It was found on the Yucatan Peninsula of Mexico, and announced to the world in 1991.
It is estimated that the asteroid was about 10 kilometres in diameter and smashed into the Earth at a speed of about 30 km per second. If it were placed quietly on the surface of the Earth it would stand taller than Mount Everest. Its volume would be equal to the volume of all buildings in the US. The energy released on impact was equivalent to an explosion of 10,000 times the present world nuclear arsenal.
The impact also released massive amounts of carbon dioxide from limestone (a storage form of carbon dioxide). Carbon dioxide is a powerful greenhouse gas and, after the dust and smoke were washed out of the atmosphere, it raised the temperature of the Earth to sweltering heat.
The heat of the incoming asteroid would also split nitrogen molecules, allowing nitrogen to combine with oxygen to produce nitrous oxide (NO). The NO would combine with water in the atmosphere to produce nitric acid. This acid then rained from the skies, killing animals and plants.
When dinosaurs ruled the Earth mammals were small and occupied a minor role. Large animals suffered disproportionately from the asteroid impact, but many small mammals survived. Their descendants became the dominant land animals, eventually producing human beings. One wonders what Tyrannosaurus rex felt as he watched the fireball light up the sky on the last day of the Mesozoic world.
William Reville is a senior lecturer in biochemistry at UCC