Lake Superior-Pukaskwa National Park
July 26, 2004
Dikes are vertical wall-like structures that form as a result of magma being injected into the fractures of rocks. Dikes are discordant features. This means that they cut through existing layers of rocks. Being intrusive, dikes crystallize below the ground. When the surrounding rock is eroded, dikes are exposed and often appear as dark walls of rock. The accompanying image shows 4 such dikes in one short stretch of shoreline in Canada's Pukaskwa National Park (eastern shore of Lake Superior, Ontario). These features are hard to miss, but the unique aspect of this location is that it is best seen from the water looking toward the shore of the Park. The rugged Canadian Shield bedrock is a perfect backdrop for this geological feature, as is the cold deep blue waters of Lake Superior. The waters off shore on this date were only about 40 degrees F (4 degrees C).