Why are the interior plains of canada flat?
Well, anytime the surface of the Earth is NOT flat, that means there is some geologic process at work. The Earth wants to have a smooth surface. Over time, gravity acts by erosion to pull down high points like mountains. So the answer to your question is that there are no geologic processes happening right now that are making mountains in the interior of Canada. The most important process that can make mountains is subduction. Subduction is where oceanic lithosphere sinks back down into the mantle. On the west coast of Canada, subduction has created and is still creating mountains. In the interior of Canada, subduction is not happening. Continental crust and lithosphere don’t ever subduct. Another process that can make hills/mountains is large-volume magmatism–the kind that has built up the islands of Hawaii from the seafloor. There is no large-volume magmatism going on right now in Canada. Rifting/tearing apart of lithosphere can make very small hills, but that isn’t happening either
