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NatureSpeak Articles

A short story of lava and ice: It’s written in the rocks

The ebb and flow of glaciers, the eruption of volcanoes – all are part of our history. For most of geological time no one was around to keep records but the story is written in the rocks. The trick is learning to read the clues – something geologists have been striving to do ever since scientists and creationists agreed to disagree on the age of the earth.

Our present knowledge of geological history is pieced together from the observations and deductions of thousands of different people. Each tiny "flash of insight" contributing to a better understanding of the world around us. Here at Whistler the insight of my old professor, Bill Mathews, has unravelled may a geological mystery.

He was, for example, the first to recognize subtle differences among scraps of columnar basalt scattered along Highway 99 between Powerline Hill and Rubble Creek. At Brandywine Falls the lava flows are flat lying and extend over a large area. They rest on a thick layer of glacial till and their surface has been gouged and striated by the passage of younger glacier ice. In contrast the flows in and around the B.C. Rail quarry form narrow, winding ridges with no lateral extent, and there are significant differences in the type of columns.

Columnar jointing develops as basaltic lava flows cool and shrink. The fluid lava solidifies from the top down and the bottom up and the columns develop at right angles to the cooling surface. Columns at the base of the flow are referred to as the collonade, those at the top the entabliture. In the Brandywine flows both the collonade and entabliture consists of regular hexagonal columns at right angles to their base and top respectively. In flows at and around the quarry site the collonade is normal while the entabliture is a confused maze of random fractures.

To Bill it all made sense. The thick till layers under the Brandywine flows were left behind by glaciers that had come and gone before the first lava was erupted and the flows spread out in thin sheets across the dry surface of old till deposits. The ice-free period was followed by the Fraser Glaciation, that last great surge of ice that filled the valley until about ten thousand years ago. The Fraser ice covered and scoured the surface of the Brandywine Flows and it was during this same time, when ice filled the valley, that the "Quarry" lava flows issued during a second episode of eruption. Confined by ice these younger lavas piled up in sinuous meltwater channels thawed beneath the glacier. As the flows cooled a regular collonade of columns formed along the base but the entablature, quenched by water from the melting ice, developed the random fracture pattern typical of what we now recognize as ice-contract jointing.

It all seems so simple and logical when you have someone like Bill to interpret the story written in the rocks.

Written by: Jack Souther


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