how were the great lakes formed

The Great Lakes were formed primarily through geological processes involving ancient tectonic activity and more recently, glacial activity during the last Ice Age. Hundreds of millions of years ago, geological events such as the splitting of tectonic plates and volcanic activity created large basins and depressions in the earth's crust in the region that would become the Great Lakes. For example, the Midcontinent Rift caused a valley that is now Lake Superior, and other fault lines contributed to basins for Lakes Ontario and Erie. Additionally, a mantle hotspot beneath the region created a low topography that influenced lake formation long before the Ice Age. About 20,000 years ago, during the last glacial period, massive glaciers advanced and receded across North America. These glaciers carved out deep valleys and basins by eroding the land beneath. As the glaciers melted and retreated, meltwater filled these glacial-carved basins, forming the lakes. The weight of the ice had also compressed the earth's surface, which began to rebound once the glaciers melted, altering the lake shapes and water flow outlets further over thousands of years. Thus, the Great Lakes we see today are a result of ancient tectonic and volcanic activity creating the basins, followed by glacial carving and meltwater filling during and after the last Ice Age.