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Home > Explore! > Bedrock Geology > Field Localities > Schoodic Point

Basalt Dikes at Schoodic Point, Acadia National Park

view from Schoodic Point toward Cadillac Mountain
Click on photo for enlarged view

Among the several far-flung treasures of Acadia National Park is Schoodic Point, a remote, quiet bedrock headland across the bay east of Mount Desert Island (location map).

Basalt Dikes

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.

basalt dikes in granite
Figure 1		 nearly parallel basalt dikes
Figure 2		 basalt dike showing preferential erosion
Figure 3		 wide basalt dike
Figure 4		 dikes following fractures in granite
Figure 5

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.


Flow Lines in Basalt		 Engulfed
Granite Fragment
curved flow lines at edge of dike
Figure 6		 closeup of curved flow lines
Figure 7
fragment of granite within basalt
Figure 8

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.

basalt flow into fractures
Figure 9		 dike in which walls would fit together
Figure 10		 detached granite fragment in dike
Figure 11

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