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Basalt Dikes at Schoodic Point, Acadia National Park
The most striking bedrock feature at Schoodic Point is the array of black, basalt dikes which cut through the pale pink granite. Basalt is a smooth, dark colored rock that forms from rapid cooling of molten rock. In places where the molten rock erupts on the earth's surface, as in Hawaii or Iceland, basalt is a volcanic rock (lava). At Schoodic Point, however, the basalt solidified in vertical cracks that were below the surface, perhaps as feeders to volcanos. The vertical sheets of basalt are called dikes. The rocks at Schoodic Point formed at depth; the overlying rock having been gradually removed by erosion and uplift. The dikes break up somewhat more easily than the granite, so the modern wave activity emphasizes the dikes.
Flow of Molten Rock
Subtle features in the dikes demonstrate that the basalt was once molten and flowed into place. In some places there are streaks in the basalt parallel to the edge of the dike that formed as it was flowing, analagous to streaks in a marble cake. Fragments of granite enclosed by basalt also indicate that the basalt was once molten, so it could engulf pieces of granite.
Opening of Fractures
In order for the molten basalt to flow into the granite, the granite had to pull apart along fractures. Comparing the opposite sides of a dike, it is easy to see that they originally fit together and are now separated by the dike filling. Look at these pictures to see where the granite broke along cracks as the basalt flowed upward.
Web page authored by Henry N. Berry, Maine Geological Survey
Originally published on the web as the November 2004 Site of the Month.
Last updated on April 19, 2012
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