Rock popups and earthquakes in Southern Ontario

Thursday, May 24, 1990

Paul Karrow

Rock popups are small upheavals a metre or two high in bedrock. They are known from several dozen sites in the Paleozoic carbonates and shales of southern Ontario, including Manitoulin Island, Bruce Peninsula, Mississauga, near Lake Simcoe, near Peterborough, and in Prince Edward County. They can be seen only where the rock is near the surface and exposed in outcrops or in excavations such as quarries, canals, and bridge footings. Some can be seen as low ridges on the ground surface, commonly first noticed on air photos, which extend for up to several hundred metres. Since they show no evidence of disturbance in glaciated areas they are believed in most cases to have formed since the last glaciation. Some occur on the former floors of glacial lakes and similarly post-date the lakes. These occurrences must be less than 10,000 to 12,000 years old. Others have been found disturbing recent sediments on the floor of Lake Ontario between Hamilton and Toronto.

Similar features are well known to occur as abrupt rock ruptures in the floors of quarries. As rock is removed by quarrying, the removal of the confining load allows pent-up stresses in the rock to be relieved. These features together with other studies show that the rock near the surface is in a state of high stress.

Both in quarries and under natural conditions there are eyewitness accounts that describe these ruptures as taking a short time – from instantaneous to a matter of minutes. One occurred overnight in 1949 along Spencer Creek northwest of Hamilton. Another was seen happening in the last century in Kentucky along a stream-bed.

Although generally regarded as a seismically quiet area, southern Ontario from time to time is shaken by small earthquakes. In fact at some time or other you can be jostled by an earthquake anywhere in the world. The recent strong earthquake in California reminded everyone of the continuing threat to property and human safety of the Pacific margin and increasing concern is being expressed about the very real risk in western British Columbia, particularly Vancouver. Meanwhile, most people are unaware that a zone of high earthquake risk extends along the Ottawa and St. Lawrence valleys, southwest across Lake Ontario, the Niagara Peninsula, and Lake Erie. Airborne geophysical surveys show the existence of a puzzling structural trend in the rocks of that area. While it is easy to relate the high risks of the west coast to the moving interactions of the large continent-sized plates of the earth’s crust, the origins of many other earthquakes, some very strong, well within the plates, remain poorly understood. Such are the recent earthquakes felt in southwestern Ontario originating in the Adirondack Mountains of New York, the Ottawa Valley, Burlington (Ontario), Cleveland (Ohio), and northern Kentucky. Lake floor sediment disturbances have been found near the sites of small earthquakes in the Great Lakes and the implications of the small faults and folds (popups) in the rocks at the surface in southern Ontario need more study. Damage to buildings is known from quarry floor heaves, so what may be the risks to buildings in Mississauga, where several natural popups have been mapped by geologists.

Is the accumulation of stress ongoing? Can more popups and/or small earthquakes be expected? The answer appears to be "yes". Aside from the tectonic stresses related to plate motions (and maybe others as yet unidentified) we know eastern Canada is being flexed by uplift following the removal of the last ice sheet. Ottawa, for example, has risen more than 250 metres in the last 12,000 years and Kingston is still rising relative to Hamilton about 30 cm per century. The earth’s crust on which we live is a dynamic thing, not the solid, everlasting, rigid mass we normally think of.