Scarborough Bluffs is on the move, a little bit at a time

Cracks at Scarborough Bluffs
A large section of the Scarborough Bluffs collapsed onto the beach below on Sunday, August 23, sending a cloud of dust into the air. (Photo: Nicholas Eyles)
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Nicholas Eyles

A little bit of Scarborough fell noisily into Lake Ontario last week, fortunately without any harm to any of its citizens but a reminder that we live in a world on the move. 

Years past, the cliffs at Scarborough Bluffs figured prominently on the letter head and flag of the once former city of Scarborough; an emblem of its origins when Elizabeth Gwillim the adventurous wife of Lieutenant Governor John Graves Simcoe, ventured out from then Fort York (now Toronto) and canoed along the foot of the cliffs in August 1793. She was struck by their resemblance to the tall chalk cliffs along the Scarborough coast of her native Yorkshire. The name stuck. Unlike the cliffs which collapsed in part last Sunday and were recorded on video; a loud crack, a noise like thunder as large blocks fell and a large dust cloud swirled over curious onlookers on the beach. 

Strangely enough, its all a result of what’s happening thousands of kilometres away to the east, in deep water along the centre line of the Atlantic Ocean. 

Elizabeth Simcoe was a keen observer of nature; her diary records the first record of an earthquake to strike the city on Sunday 27th 1795 when ‘a slight shock was felt at 5 am in the morning.’ She didn’t know it of course but these are common; we live and work on the North American plate, a large piece of the Earth’s crust, moving westward each year by some 4 cm or about the thickness of a piece of paper each day. Your house, our campus and our offices aren’t in the same place they were even yesterday but its not noticeable to us. 

The edge of the North American plate lies not along the east coast in Maritime Canada but way off in the middle of the Atlantic where a long chain of submarine volcanoes belches out new rocks that push the rest of the plate west (and the European plate eastward) widening the Atlantic Ocean and pushing North America further away from Europe. It’s what is called ‘seafloor spreading’ and can be seen at work on land in Iceland along the famous Reykjanes Ridge where tourists hike from the North American plate to its European counterpart before taking a soothing dip in sulphurous hot springs. When the volcanoes aren’t erupting that is.

But the same sea floor spreading processes that shove our plate westward, creates stresses in the rocks that lie below Southern Ontario triggering earthquakes, most at depths of about 15 km along large breaks (faults) that criss-cross our region; out of sight and out of mind until they suddenly move. The same forces that move the plate also create cracks in its rocks (‘joints’) and these are the culprits that triggered the collapse at the Bluffs last week. That loud crack was a large slab suddenly breaking off from the cliff; a process called ‘spalling.’ Lake Ontario is also changing; in the long term, over many thousands of years, its size and depth has increased; today it is at record highs created by short term swings from wetter to drier climate across the Great Lakes, all helping to undercut the cliffs and threaten cliff-top properties.

What happened along the Bluffs is a reminder that we live on a dynamic planet. Given the enormity of geologic time small changes add up. Did you know that Southern Ontario was at the equator 400 million years ago? 
But that’s a story for another day.

Nick Eyles is Professor of Geology at the University of Toronto Scarborough (UTSC) where he has taught for more than 33 years. His prime research interest is in glacial sedimentology and has over 30 years’ experience of field work on modern glaciers and ice sheets from Antarctica to the Arctic.