Where does gravel come from?
Gravel/pebbles is rock that has been broken down by weathering and/or erosion. They are similar geological processes, but they are not the same thing.
Weathering degrades a rock without changing its location. Erosion, on the other hand, causes rocks, or particles of rock, to be carried away from their original locations and deposited elsewhere.
For example, water getting into a rock and splitting it when it freezes is weathering because the rock stays in place. Water carrying pebbles and sand down a river is erosion because the location of the pebbles and sand changes.
Gravel starts as bedrock. Over time, bedrock fractures and breaks due to physical and chemical weathering.
An example of physical weathering is, as mentioned above, water getting into a rock, freezing, expanding and acting as a wedge that splits the rock. Another example is rocks that are subjected to significant changes in temperature, such as in desert environments. They can expand in the heat and contract when cooled and this can cause them to weaken and break apart.
Chemical weathering is when a chemical reaction causes rock to break down. For example, rocks containing iron often oxidize (rust), which weakens materials in the stone. Water is also often a cause of chemical weathering, such as when water interacts with materials in rock and causes the materials to transform into new minerals that do not bind the rock together as well.
Fragments of bedrock are transported by wind, water and ice, which causes them to break down further. Rocks colliding with each other become smoother and more rounded as smaller pieces are broken off.
Dirt and sand are the eventual results of all this weathering and erosion as rock breaks down to particles.
Wind, water and ice eventually deposit gravel in places such as beaches and calmer areas of rivers or lakes, where these forces no longer have enough strength to carry the gravel further.
Glaciers often play an important role in creating gravel deposits. Glaciers pick up rocks as they move, or push them ahead of the glacier, and grind the rocks down, a form of physical erosion. As a glacier melts, it often deposits the gravel in concentrations that make it easy to extract, usually just by picking it up with an excavator.
When looking at gravel, see whether it looks like the nearby bedrock. If it doesn’t, the gravel was likely transported by glaciers.
Gravel and crushed stone are both aggregates and are used in construction. (Gravel and crushed stone are both small rocks. The difference is that gravel is broken down naturally, while crushed stone is broken down by a machine, i.e. a rock crusher.)
Aggregate is the most-mined material in the world because it is used in virtually all infrastructure, including homes, roads, schools and hospitals. Aggregate makes up about 80% of concrete and 94% of asphalt.
Building one kilometre of highway takes about 18,000 tons of aggregate - enough rock to fill four Olympic-sized swimming pools! Nova Scotia needs 10-15 million tonnes of new aggregate each year to build and maintain our infrastructure.
