Richard A.F. Grieve
As we go about daily living, we are reminded in many ways that we do not exist in isolation. We are part of a much larger symbiotic system of life on earth and it is even popular these days to talk of the earth as if it were a single, closed system. The earth, however, does not exist in isolation.
It is a planet in the solar system and, as such, is subjected to solar system processes. One such process is the impact of interplanetary asteroids and comets.
The intensity of this impact bombardment over geologic time is evident on the oldest lunar surfaces, which are literally saturated with impact craters of all sizes and ages. This is, however, not the case for the earth. Not because it escaped these asteroidal and cometary impacts, but because the earth’s surface is constantly being renewed by such active geologic processes as erosion, sedimentation and tectonism. As a result, impact craters, which are surface features, are removed, buried or destroyed. The evidence of impact is there, however, for those geologists who know where to look.
The study of terrestrial impact craters is a relatively new facet of of the geological sciences, having been inspired in large part by the discoveries of impact craters on other planetary bodies.
Canada was a leading participant in the early study of impact craters on earth through the foresight of the late Dr. C.S. Beals. Dr. Beals, who was the Dominion Astronomer, reasoned in the 1950’s that, if there were impact craters on the moon, there must be impact craters on earth. He initiated a program to search for evidence of these interplanetary collisions on the Canadian Shield; an area of 6x106 km2 of relatively stable crystalline rocks with low erosion rates that make up a large part of the Canadian landmass. This study of terrestrial impact and its effects continues today within the Geological Survey of Canada.
Currently, we know of approximately 130 impact craters of earth, of which 26 are in Canada (Figure 1). Terrestrial impact craters appear as circular structures and it was the identification of anomalous round shaped lakes on the Canadian Shield, where most lakes are linear or irregular features, that led to the discovery of many of the Canadian impact craters. The smallest and youngest crater in Canada is New Quebec in Ungava, Quebec, which is approximately 3 km in diameter and 1 million years old. The largest and oldest is the Sudbury Structure in Ontario, which has an estimated original diameter of 200 km and is 1.8 billion years old.
Impacts on earth, however, are more than scientific curiosities. They can have significant economic potential. In Canada, for example, uranium ores are exposed by the Carswell crater, Saskatchewan, the Cu-Ni sulphide and platinum group ores at Sudbury, Ontario, North America’s largest mining camp, are the result of the melting of 10,000 km3 of crustal rocks by the Sudbury impact event. Several buried structures of probable impact origin in the prairies are oil producers and at Manicouagan, Quebec, a massive hydro-electric reservoir fills the annular depression of the crater. Large-scale impacts are also the most catastrophic of geologic processes and can have extreme effects on the local, and even global, environment.
The mean terrestrial impact velocity of asteroidal bodies is 25 km per second or 90,000 km per hour. Certain types of comets can have impact velocities as high as 70 km per second. With such ultra-high velocities, even relatively small impacting bodies contain vast amounts of kinetic energy. For example, the energy released on forming New Quebec was on the order of 1018 Joules. This is equivalent to the energy in 250 megatons of TNT, greater than the energy in the largest nuclear device.
The kinetic energy of the impacting body is transferred to the earth through a shock wave. At the point of impact, the peak shock pressures can reach 1000 Gigapascals (10 million times normal atmospheric pressure) or higher and temperatures are thousands of degrees Celsius.
As the shock wave propagates into the earth, it diminishes in intensity, but not before large volumes of rock are vaporized, melted and changed, or metamorphosed by high pressure deformation. Thanks, in part, to the pioneering studies undertaken at the Earth Physics Branch, now part of the Geological Survey of Canada, it is now well established that impact is the only natural process with sufficiently high shock pressures to produce these irreversible changes, known as shock metamorphism (Figure 2). Although other geologic processes, such as explosive volcanism, can produce circular landforms, circular structures, do to impact, can be identified uniquely through the occurrence of shock metamorphism. Not only are the pressures and temperatures on impact way off-scale compared to other geologic processes, they also occur on a vastly different time scale.
Geologic processes are generally slow processes that occur over years or even millions of years. In the impact event that resulted in New Quebec, the duration of the shock pulse was a fraction of a second and the entire 3 km diameter crater formed in approximately 10 seconds.
It is difficult to conceive of the catastrophic power of major impact events. For example, the energy released in the formation of Manicouagan, Quebec, originally 100 km in diameter and formed 212 million years ago, was equivalent to 10 times the annual output of internal energy of the earth, from volcanoes, earthquakes and general heat losses being released in a few seconds on one spot on the earth’s surface. There has long been speculation on the potential effects of such massive assaults to the earth, but it was not until the 1980’s that geologists discovered evidence that a large impact event may have led to the global mass extinction of many lifeforms, including dinosaurs, 66.4 million years ago at the so-called Cretaceous-Tertiary (K/T) boundary.
The evidence for a major impact at the K/T boundary is global and consists of shock metamorphosed minerals and glasses, as well as an unusual chemical signature from the vaporized impacting body. Models of the impact suggest that the impacting body was on the order of 10 km in diameter. Based on estimates of the terrestrial cratering rate, derived from counting the number and ages of craters in areas such as the Canadian Shield, impacts of this magnitude occur on earth every 100 million years or so. The proposal that a major impact was the cause of the K/T mass extinctions has been controversial. The evidence for impact, however, has never been seriously challenged, because of the well documented body of knowledge on impact craters, such as the ones in Canada. Most recently, a major impact crater has been discovered buried beneath 1 km of sediments on the Yucatan peninsula, Mexico. This crater, called Chicxulub, was known as a major geophysical anomaly but identified as a crater, in large part, through the scientific efforts of D.A. Hildebrand, who now works at the Geological Survey of Canada. It is 180 km in diameter and appears to be of K/T age. Various other lines of evidence, including the discovery of metre-thick K/T beds with abundant impact glass in nearby Haiti, suggest that Chicxulub may be the crater responsible for the K/T mass extinctions.
With K/T-sized impact events occurring every 100 million years or so, it would seem that impact, although a global hazard on geological time-scales, is of little concern to our survival. It was models of the K/T event, however, that led atmospheric scientists to develop models of a nuclear winter following a nuclear exchange, involving about one-third (5,000 megatons TNT equivalent) of the world’s strategic arsenal. Impact events smaller than the K/T event occur much more frequently than K/T-sized events and it is calculated that one, sufficient to effect damage equivalent to a nuclear winter, occurs every half million years. Given the highly organized and interdependent infrastructure of modern civilization, such an event would be a global catastrophe.
An impact-produced nuclear winter every half million years may not appear to be much of a risk to civilization, but impacts are random events and may occur anytime. Such an event may occur tomorrow. Indeed, a 400 meter diameter asteroid with an estimated kinetic energy of 1000 megatons TNT equivalent missed the earth by only 600,000 km in March, 1989. Although, this distance is large, the high relative velocity with which the earth and the asteroid were travelling means it missed the earth by only six hours. The most recent historical impact event that caused significant damage was in 1908 at Tunguska, in Siberia, Russia. Here, a small piece of cometary material exploded in the atmosphere at a height of about 10 km with an estimated energy of 10 megatons. The explosion could be heard a thousand km away, trees were felled and animals killed with 20 km of ground-zero, and people as far away as 60 km were blown off their feet. The isolation of the area meant there was no loss of human life. Events such as Tunguska occur on time scales of every hundred years or so. Fortunately, most of the earth’s surface is essentially empty of people, being, in fact, ocean. Nevertheless, as the human population continues to grow even these relatively minor impact events will become of increasing concern.