Aggregates include sand, gravel, clay, earth and bedrock, each with its own unique properties. Used to construct essential infrastructure and in the manufacture of many products, aggregates are the building blocks of our modern society.
Not all aggregates are created equal. They vary greatly in quality, depending on the bedrock or landscape feature they are extracted from.
Where do aggregates come from?
Aggregates come from the bedrock, gravel and sand deposits that form the Ontario landscape.
Underlying the whole province are ancient igneous and metamorphic rocks, commonly known as the Canadian Shield, that are more than a billion years old.
In southern Ontario and the Far North, these ancient rocks are covered by up to 1,400 metres of younger sedimentary rocks such as limestone, dolostone, shale, and sandstone. These younger rocks formed from sediments in ancient seas that covered Ontario between 540 and 350 million years ago. Over millions of years these sediments hardened into bedrock. In the James Bay and Hudson Bay lowlands of the Far North, there are also sedimentary rocks that were deposited up to about 100 million years ago.
Between 1.8 million and 10,000 years ago, vast masses of ice called glaciers advanced and retreated across Ontario, scraping and grinding the bedrock. The glaciers molded Ontario’s landscape and created glacial features such as moraines and drumlins, made up of loose sediments such as clay, sand and gravel. These features may contain rich sources of aggregate. The Oak Ridges Moraine in southern Ontario was formed about 13,000 years ago during the retreat of the glaciers
Rocks are divided into three main types:
Sedimentary rocks form from small grains, or sediments, that include fragments of broken rock or remains of ancient sea creatures. Deposited by wind, water or ice, these grains accumulate and turn into rock over long periods of time. Limestone is a type of sedimentary rock.
Igneous rocks form from molten rock deep within the Earth called magma. As the magma cools, it becomes rock that has a crystal structure. If the magma cools slowly, it becomes rock such as granite, consisting of larger crystals. If the magma comes to the surface of the Earth through a volcano, it cools quickly and becomes rock such as basalt, consisting of very small crystals.
Metamorphic rocks form when either sedimentary or igneous rocks are heated or compressed. The rocks partially melt and the chemicals within them may be rearranged, creating a different kind of rock. Marble is a metamorphic rock formed from limestone.
All of these types of rock can form bedrock, which is the solid rock that makes up the Earth’s crust.
What factors make aggregates more difficult to find and extract?
In southern Ontario and the James Bay and Hudson Bay Lowlands in the Far North, the bedrock is mainly sedimentary. These rocks formed in flat layers over large areas, so the location and type of bedrock is fairly predictable and consistent, which makes it easier to find and extract. However, each layer of sedimentary bedrock has unique qualities. Some layers are less predictable and need to be tested carefully to ensure the rock is suitable for its intended use. As well, some areas of bedrock are buried deep beneath the surface under loose sediments, making it more difficult and expensive to extract the aggregate.
While the sedimentary bedrock in the Far North is similar to that of southern Ontario, extracting aggregates in the Far North is difficult because the bedrock lies underneath vast bogs and wetlands.
In central and Northern Ontario, the bedrock is mainly igneous and metamorphic rock. The quality of the rock can vary greatly over small areas, and it must be tested to determine if it is suitable for the intended use. Large areas of this type of bedrock may contain minerals that make it unsuitable for use.
Like bedrock, the quality of sand and gravel deposits can vary greatly. Some types of glacial features are more likely than others to contain suitable aggregates. Sand and gravel must be tested to determine if it is suitable for specific uses.
What are high-quality aggregates?
High-quality aggregates are those that meet stringent standards for uses such as making concrete, paving roads and manufacturing industrial products. High-quality aggregates are used to build bridges and tall buildings like the CN Tower that need to be extremely strong to withstand the weight and stress they bear.
The quality of aggregates does not depend on whether they come from sedimentary, igneous or metamorphic rock, or how old they are. Their quality is determined by the size and shape of their individual grains, the minerals they are composed of, and other physical and chemical properties. These properties determine their strength and durability. Some aggregates contain materials that limit or prevent their use for some or all products.
