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Pelican Rapids

The granite ridge forming the Pelican Rapids is one of the most accessible out-crops of Canadian Shield west of the Slave River.  The Canadian Shield is the core of the North American continent and contains rocks nearly four billion years old, the oldest known in the world.  These ancient Precambrian rocks of the Shield represent over 85 percent of our planet's entire geological history.  The granite of Pelican Rapids tells a story of collisions between continents two billion years ago.

The Canadian Shield is exposed at the surface over about 50 percent of Canada and it underlies an even larger portion of the North American continent.  To many Canadians, the Shield is a frontier -- a vast, empty, inaccessible land of barren rock, muskeg, lakes and, in the south, impenetrable forests.  Although there is little fertile soil, the Shield does hold much of Canada's mineral wealth including gold, silver, uranium, iron, nickel, copper, and even diamonds.

The Shield is actually not a single entity but rather a combination of several small continental fragments (or protocontinents), each with its own geological history.  Collisions between these protocontinents welded them together and resulted in the formation of ancient mountain ranges; these have endured millions of years of erosion by water and glacial ice until finally their crustal roots were exposed.  These roots, then, are the Shield that we see today.

Between 2.0 an d1.9 billion years ago, the broad belt of igneous rock in northern Alberta, called the Slave Granite, was formed by the collision of a small continental fragment with a larger proto-continent.  Metamorphosed sedimentary rocks were caught up in this collision and at depths between 18 and 24 kilometres into the crust, they began to melt and form a granitic magma.  This magma eventually crystallized at 900-1000 degrees Celsius and formed the Slave Granite which still contained surviving blocks of the metamorphosed sedimentary rocks from which it had formed.

At Pelican Rapids, the glacially scoured peninsula projecting into the river gives the visitor a window to view the results of these processes which occurred deep within the earth.  You can see large blocks of metamorphosed rocks, called gneiss, that appear to float within the granite.  These dark-coloured gneisses were originally shales, and the light-coloured blocks were once sandstones.  Surprisingly, despite having been buried deep in the crust and surviving a continental collision and partial melting, the original sedimentary bedding in the sandstones can still be seen.

The granite at Pelican Rapids contains several high-temperature minerals such as garnet, cordierite, and hercynite, proving that it formed at 900-1000 degrees Celsius, rather than the usual 700 degrees Celsius for typical granites.

Reprinted from A Traveller's Guide to Geological Wonders in Alberta by Ron Mussieux and Marilyn Nelson with permission of the authors and the Provincial Museum of Alberta.

 


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