SCIENCE

Birth of a continent

Scientists unearth Canada’s long history

JAMES DEACON December 7 1992
SCIENCE

Birth of a continent

Scientists unearth Canada’s long history

JAMES DEACON December 7 1992

Birth of a continent

SCIENCE

Scientists unearth Canada’s long history

JAMES DEACON

Since the birth of the planet about 4.6 billion years ago, the Earth’s surface has been in a state of sometimes violent transition. Whole new continents were formed by the collisions of migrating landmasses carried along the surface by the slow but constant movement of underground tectonic plates. According to a major study by Canadian geoscientists, huge mountain ranges once rose in what is now Eastern Canada, only to sink out of sight millions of years later. In the distant past, great seabeds were thrown up and now he high and dry in the Rocky Mountains. Then, the land that was to become Canada was nearly split in two by a great continental rift. Those and other earthly insights have emerged from a study called Lithoprobe, an eight-year-old investigation into the makeup of Canada’s geological crust. Said Lithoprobe’s director, Ronald Clowes, a geophysicist at the University of British Columbia: “In many ways, we probably know more about outer space than we do about inner space—the planet on which we live. This project is designed to give us a better understanding than we have ever had before.”

At a cost of more than $50 million so far, Lithoprobe is the largest geoscientific study ever undertaken in Canada. The scientists involved are trying to map the lithosphere, a rigid outer crust and mantle of rock that extends about 60 miles below the earth’s surface. Based in Vancouver, Lithoprobe has enlisted hundreds of specialists in three earth-science disciplines—geophysics, geochemistry and geology—from universities, the federal and provincial governments and industry.

For generations, scientists could only speculate on the makeup and configuration of the lithosphere. Now, Lithoprobe is gathering hard evidence on its nature and origins. “We have had to make intelligent guesses about what the Earth’s crust is made of, and how it got that way,” said Peter Savage, a geologist who recently retired from his job as exploration manager for Calgary-based PanCanadian Petroleum Ltd. Added Savage: “Lithoprobe has given us a great deal of evidence to support those suppositions, while contradicting others.”

Some of the project’s earliest discoveries were made on the West Coast. Lithoprobe’s findings indicate that much of British Columbia’s landmass originated at some point during the past 180 million years, when huge islands migrated out of the Pacific Ocean and collided with the then-western shore. The resulting upheaval caused the land and seabed to fold, and created the successive ranges of mountains that run like waves across the province. The formation of the Rocky Mountains threw up pieces of seabed from an inland ocean that produced the celebrated Burgess Shale deposit in the Rocky Mountains in Yoho National Park near Field, B.C. Since it was discovered in 1909, about 140 species of marine invertebrates have been discovered in the deposit’s fossil-bearing rock formations.

In the course of their investigations, Lithoprobe scientists have mapped some of the underground faults that produce much of the earthquake and volcanic activity on North America’s West Coast. A chain of volcanoes in the region includes Washington’s Mount St.

Helens and Mount Baker. Clowes says that Lithoprobe’s findings will not make it easier to predict exactly when earthquakes or volcanic eruptions will occur. Still, he said that from the broad knowledge scientists have gained of the underground structure, “we have been able to say where we think an earthquake would occur, and how big it would be.”

Earth scientists first came up with the concept for Lithoprobe in 1981 at a meeting sponsored by Canada’s Natural Sciences and Engineering Research Council, but the project did not begin until 1984. Among the techniques employed in the survey is an adaptation of conventional seismic exploration, in which

sound waves are used to detect the shape and makeup of underground geological formations. Traditionally, the technique was a staple of the petroleum and mining industries, but its use was usually restricted to relatively shallow depths of up to 15,000 feet. To create the necessary sound waves, Lithoprobe scientists use 25-ton trucks designed for petroleum exploration. They are equipped with heavy platforms and a mechanism that causes the platforms to vibrate, sending shock waves into the Earth that rebound and are measured by seismic sensors. In areas covered by water, technicians use bursts of compressed air to create vibrations.

At the same time, the scientists acquired sensitive electronic measuring devices to record the returning seismic waves and developed sophisticated computer software to interpret the readings. As a result, they have routinely been able to see 50 miles below the Earth’s surface. Said Clowes: “The big thing about the geophysical aspect of this is that, for the first time, it has given us a third dimension—depth.” Ray Price, a former head of the Geological Survey of Canada and now a professor of geological sciences at Queen’s University in Kingston, Ont., said the development of highly sensitive seismic equipment was, for geoscientists, akin to the discovery of the telescope. Said Price: “Instead of relying on the naked eye, we are now looking through telescopes or, better, through radio telescopes that let us see into the darkness.”

Participants say that one of the keys to the project’s long-term success is the sheer number of scientists involved. Lithoprobe has at its disposal more than 400 scientists and students at 28 universities, a dozen federal and provincial ministries and 17 private companies. While scientists examine the underlying structures, others carry out an analysis of the rock formations near the surface. According to Price, one of the geologists’ principal roles is to advise where geophysicists should carry out seismic tests to reveal important underground rock formations.

The project is financed by an annual budget of $6.3 million that is provided jointly by the federally supported Natural Sciences and Engineering Research Council ($4.5 million) and the Geological Survey of Canada ($1.8 million). According to Clowes, Lithoprobe is the largest geoscientific project currently under way anywhere in the world—and the only one that embraces all of the earth sciences.

In the course of the study, scientists working on the East Coast of Canada have charted the violent arrival about 1.2 billion years ago of a floating continent about the size of India, which collided with the existing North American landmass. The impact was so powerful, they say, that it pushed up a vast slab of land against the unyielding shoulder of the Canadian Shield, one of the oldest rock formations in the world. The result was a 4,000-km-long range of mountains, equal in height to the Himalayas, that towered above what is now southern Ontario

and Quebec. Millions of years of erosion by ice ages, wind and rain have whittled those mountains down to the point where they are no longer visible.

According to Clowes, Lithoprobe has also turned up a major geological puzzle in the form of a 2,000-km-long Keweenawan Rift System, which extends from Kansas to Lake Superior and then loops down under Lake Michigan. According to scientists, the rift is 22 miles deep in places, making it the deepest on the planet. Project scientists say that they do not know exactly what caused the fissure, but they add that it nearly cracked the Earth’s crust in places, an event that could have divided North America into two completely separate parts. “It did not keep rifting, and we don’t know why,” Clowes says. Over aeons of time, the rift has been filled in by volcanic lava seeping up from below and sediments pushed down from the surface. Said Price: “The rift was known to be there before, but Lithoprobe showed that its dimensions were grossly underestimated. It is a massive rift.”

Some scientists say that the findings may help petroleum and mining companies to find valuable new deposits. Hugh Morris, chairman of Vancouver-based Imperial Metals Corp., said that tests by Lithoprobe at existing mines near Sudbury, Ont., showed that there was potential for mineral discoveries in nearby areas where mining companies had not thought to explore. Declared Morris: “The days of easy exploration are past. We are now facing the challenge of searching for resources that are buried and hidden.”

The petroleum industry stands to gam unexpected advantages from the new findings. The project has produced a broad picture of where the so-called basement of oil-producing sedimentary rock lies beneath western North America. Savage said that the techniques may not be of use in Alberta, which has been heavily explored, but it may assist exploration in mostly untapped areas such as northeastern British Columbia. Said Savage: “It’s difficult to relate this new information directly to where we drill for tomorrow’s oil well. But it should lead to that eventually.”

Clowes estimates that it will take at least 10 years to complete Lithoprobe. With much of the actual testing completed, the remaining time will largely be spent analysing the data and reaching new conclusions about the development of the continent. Scientists say that at its halfway mark, Lithoprobe has already delivered more than they had expected. Said Savage: “I think the process that Lithoprobe is going through will virtually rewrite the geological history of North America.” And that, in turn, may give the people who have settled here a new appreciation of the land on which they live.

JAMES DEACON