Rocks on the Move

By Claire Eamer

Whitehorse, where I live, is in the middle of the southern Yukon. It lies in a broad valley cut through thousands of years’ worth of glacial till by the mighty Yukon River.

To the east, hundreds of kilometres of rocky outcrops, pothole lakes, rivers, and boreal forest stretch all the way to the Mackenzie Mountains.

To the west, a half-day’s drive through dry, patchy forest, across rivers and streams, and around the southern end of a huge lake will take you to the Kluane Range, steep mountains weighed down by massive icefields.

Still farther west are the peaks of the St. Elias Range, the highest mountains in Canada. Only beyond them might you finally reach the Pacific Ocean.

Things were very different 500 million years ago. A peaceful sea lapped against the shore of North America, on the eastern edge of today’s Yukon. The rock beneath my feet, here in Whitehorse, was probably part of an island arc somewhere off the coast of China. And the St. Elias Mountains, well…

The rocks that make up the St. Elias Mountains are weird, says JoAnne Nelson of the British Columbia Geological Survey. “It would be like coming around the corner and seeing a flock of penguins.”

The St. Elias Mountains are a dramatic example of plate tectonics, the combination of forces that sends Earth’s land masses drifting around the globe, combining and recombining in new patterns like a giant kaleidoscope. For a long time, the St. Elias rocks were a puzzle, Nelson says. Recently, however, clues from an ancient sea bottom hint at a journey almost beyond belief.

The clues are fossils, the remains of some odd sea-bottom creatures that once lived in only one small bit of ocean, nowhere near the modern Yukon. Based on that evidence, 430 million years ago the rocks of the St. Elias Range lay beneath the Atlantic Ocean (or its ancestor) between Scandinavia and Russia.

What brought them almost half a world away? The lithosphere, the outer layer of the planet we live on, is constantly moving. Bits of it slide over or under other bits or jam up against them, squeezing rocks together like an accordian or pushing bits of seabed into towering mountains. What we now call the Yukon was created and moulded by those processes.

“The entire lithosphere of the Yukon is being picked up and pushed on top of the Northwest Territories,” explains Nelson.

That’s the kind of pressure that drove a bit of ocean bottom across the top of the world, shoved it against an island arc from the China Sea, and jammed both against the Pacific shores of ancient northern North America with a force that flung up immense mountains.

But, dramatic as they are, the mountains won't last. “These landscapes are fleeting,” Nelson says. “It’s only that we, the living, are so much more fleeting than they.”

For information on plate tectonics, and some great animations, go to http://www.ucmp.berkeley.edu/geology/tectonics.html