About 600 million years ago, the earth's dry land was a single giant supercontinent, surrounded by ocean. Then it began to break apart.

Manipulated by unimaginably strong forces deep below the planet's surface, the core of the continent that is now North America became a separate piece of land, lapped on the west by an early version of the Pacific Ocean.

The geological nature of this new western continental margin of North America and how it has evolved are puzzles Don Murphy and his colleagues are trying to solve.

Murphy, a geologist with the Yukon Geology Program, is part of an international team mapping the Ancient Pacific Margin of northern North America. The project is part of the Geological Survey of Canada's National Mapping Program (NATMAP) and also involves geological agencies in British Columbia and Alaska and several Canadian universities.

The ancient core of the continent lies far to the east of the current Pacific shoreline, Murphy says. It extends from the interior of the continent generally as far west as the Tintina Fault, which runs from eastern Alaska to central British Columbia, cutting diagonally through the southern Yukon.

Rock east of the Tintina Fault clearly belongs to the ancient core of the continent. Much further west are rock formations that originated somewhere else in the Pacific region and slowly moved eastwards until they collided with North America.

Between those two groups of rock is a mysterious, complex assembly of rock that extends at least half the length of the continent and underlies much of the southern Yukon and east-central Alaska -- where it is called the Yukon-Tanana terrane.

"This belt of rocks is probably the last outstanding regional question in the geology of the Canadian Cordillera," says Murphy.

The rocks puzzled geologists because they didn't seem to belong to the continental core, but neither did they carry clear evidence of having originated somewhere else.

Murphy says geologists originally suspected that this complexly deformed and metamorphosed band of rock was part of the Pacific margin of the ancient continent, but they had no proof. However, recently developed techniques for determining the age of some rock have made it possible to draw a clearer picture.

After several years of mapping and piecing together the geological history of the rocks in this terrane, the project geologists have come up with an explanation of what happened in the distant past.

Murphy says the story began 600 million years ago when the earth's single continent broke apart. By about 550 million years ago, a proto-Pacific Ocean had filled the depression between the new North American continent and the continental landmasses of Asia and Australia.

For about 200 million years, things settled down along the Pacific coast.

"It was just a nice passive continental margin like the North Atlantic," says Murphy.

About 360 million years ago, things began to get lively again. The ocean basin was cooling and getting heavier. Eventually, the edges became too heavy for the molten rock beneath them, and a crack developed along the edge of the continent.

The ocean bed sank and began pushing under the continental plate. The continental edge, with nothing to support it, stretched and thinned until a long "ribbon continent" broke off from the main core and began moving outward into the ocean in a process called back-arc rifting.

The ocean plate slid beneath the ribbon continent and triggered a volcanic arc on top of the continent. A back-arc basin opened up between the ribbon continent and the North American mainland. Today, Murphy explains, the islands and Sea of Japan are a similar formation.

"We seem to be finding evidence that this process happened more than once," says Murphy.

Possibly two or more ribbons of rock, broken off from the continental core, moved about the Pacific Ocean for 100 million years. Fossil evidence from rock as far north and inland as Faro shows that at least one of the ribbon continents must have traveled south to much warmer regions.

Then, about 260 million years ago, geological forces began reshaping the Pacific basin yet again, pushing the ribbon continents back north and east until they collided with North America. By now, however, they had been folded, compressed, melted, and strained to a point where they no longer resembled the land they had broken away from.

The changes the Yukon-Tanana rocks went through made it difficult to piece together their history, Murphy says, but he's confident the team has succeeded. A synthesis of their work, tracing the history and location of the Ancient Pacific Margin, will be published next spring.

For more information about the Ancient Pacific Margin NATMAP program, contact Don Murphy, Yukon Geology Program, at (867) 667-8516.