My, what a big hole that is!
When things happen slowly, people joke that it’s happening at a turtle’s pace. Geoscientists prefer to joke that it’s happening in “geologic time” because scientific research has proven that the Earth typically changes very slowly, over hundreds of millions of years. For example, it may take millions of years for a mountain chain to level into flat land. Other changes take place over the lifetime of humans, like the new land forming along the Hudson Bay coast because the land is rising due to a process called isostatic rebound. Changes over this time scale generally present no safety risk to people. But, sometimes the land changes overnight and, in some cases, creates changes that can be dangerous.
The type of rock in southern Ontario and along the James Bay and Hudson Bay lowlands is called dolomite and limestone. Under the right conditions, rain and groundwater can dissolve limestone rocks. As rain falls through the air and passes through the soil, it dissolves small amounts of carbon dioxide gas from the air or from plant roots. This mixes with water to form a weak acid, like vinegar, which is able to dissolve limestone. The acidic water seeps into cracks in the rock and gradually widens them. Indeed, water can eat rock.
As the acidic water sinks further into the rock, it may flow along widening natural rock layers to form cave systems. The result is a landscape called karst. There are many types of karst that range in size from dime-shaped depressions in the rock, to linear openings several centimetres wide that extend for many meters, to large cave networks. The karst caves in the Bruce Peninsula, in southwestern Ontario, and the Bonnechere area near Pembroke, are spectacular karst examples that attract tourists eager to explore the caves. The formation of karst takes thousands of years – it does not happen overnight.
Karst formations may be dangerous if they are located close to the surface in our communities. That’s because they can turn into sinkholes. Sinkholes, which can happen quickly, are sometimes large enough to cause trouble, known to swallow up unstable surface land that covers open cracks in the earth. That is one of the reasons the work of the Ontario Geological Survey (OGS) includes mapping the Far North. Documenting Ontario’s geology can help us spot risks like these.
Recent work by the OGS shows that some caves in southern Ontario are important sources of groundwater. These groundwater and cave systems are connected to the surface of the Earth. The air in these caves moves in and out as weather systems change on the Earth’s surface. It is as though they are breathing. If the cave air “exhaled” contains no oxygen, a gas that we need to live, the breathing caves may be dangerous to humans and animals. This is another area that the OGS is charged with exploring, and we continue our important work to protect the safety of Ontarians.
Clay deposits may also be a natural geological hazard. If you boat along the great northern rivers, like Severn, Winisk, Attawapiskat, and Albany, you have seen those geological hazards – landslides. The same types of landslides also occur in the Ottawa River Valley area. In these areas, vast areas of unstable clay lie hidden below the surface of the Earth, waiting for the right conditions to form a landslide. In fact, the town of Lemieux, located southeast of Ottawa, was relocated between 1989 and 1991 because geological work showed that the town was built on unstable clay. The decision to relocate the people was wise because on June 20, 1993, two years after relocation, a landslide destroyed the former town site. This is a remarkable example of the practical work done by geological surveyors.
These are examples of the natural, but complex relationship between people and the geology of Ontario. MNDM’s Ontario Geological Survey is responsible for documenting Ontario’s geology and has published geological maps that show the general geological features of Ontario. The maps, including a map of groundwater and karst in southern Ontario, are found at OGSEarth.
For communities undertaking land use planning, a combination of traditional knowledge with geological knowledge produces a more holistic understanding of the “Ontario beneath our feet.” This helps ensure that geological hazards are considered, avoids threats to groundwater source areas, and considers the mineral, energy, and groundwater development options. So, remember to consider the remarkable geology beneath you. Geology is not only cool – it is essential.
Andy Fyon is the director of MNDM’s Ontario Geological Survey. For more information about the geology of Ontario: http://www.mndm.gov.on.ca/en/mines-and-minerals/geology.
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