Lake Superior was formed by forces of fire and ice, beginning about 1.2 billion years ago when volcanic lava poured from a gigantic rift in the continent and later shaped during the last Ice Age when glaciers carved the basin from layers of lava, then melted and retreated just 12,000 years ago. Its cold water, with an average annual temperature of 40° F (7° C), limits the growth of algae and aquatic plants, and the lake rarely freezes over completely in winter. Its immense scale defines both shoreline and distance, with the Circle Tour spanning 1,300 miles (2,092 kilometers), the lakeshore stretching 1,826 miles (2,938 kilometers) if straightened, and the lake covering 31,700 square miles (82,170 square kilometers), about the size of Maine or of New Brunswick and Prince Edward Island combined. Ojibwe people have respected, navigated, and lived near the lake for centuries, calling it Gichi Gami, while the French name "Lac Superieur" appeared on a 1647 map because of its position above Lakes Huron and Michigan. As the world's largest freshwater lake by surface area and the world's third largest lake by volume after Lake Baikal and Lake Tanganyika, it holds three quadrillion gallons (11.4 quadrillion liters), about ten percent of the earth's fresh surface water. Because of its vast size and continental position, Lake Superior shapes climate, habitats, landscapes, and human activity far beyond its shores, tempering nearby summer heat and winter cold and bringing deep lake effect snow to parts of the South Shore, where some places receive 200 inches annually.
