Earth shattering events at Mount Desert Island!

Map 1

Back in the Silurian Period around 420 million years ago, the area that we now call Mount Desert Island was an explosive place that was best avoided! At depth in the crust, molten magma moved upward and differentiated as it cooled into several distinctive rock types. The silica-poor portions of the magma became the dark colored igneous rocks diorite and gabbro. The silica-rich magma became the light colored igneous rock granite. Through complex processes, the magmas explosively intruded into the crust to form a bowl-shaped intrusion of diorite and gabbro on the rim and granite in the center. The granite makes up a large portion of the island and is well exposed on Cadillac Mountain. The intrusion was so forceful that it violently broke the surrounding rock into which it intruded, forming an unusual feature called the Shatter Zone. This zone nearly completely surrounds the Cadillac Granite and is over a mile wide in places. The rocks in this zone are composed of great fragments of the surrounding country rocks frozen in a matrix of granite. It is well exposed at a number of localities on Mount Desert Island, and quite spectacular at Little Hunters Beach on the south side of the island. Here the Shatter Zone incorporates chunks of the Bar Harbor Formation - well-layered tan and lavender siltstone and sandstone.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Further reading

The Geology of Mount Desert Island, by Richard Gilman, Carleton Chapman, Thomas Lowell, and Harold Borns, 1988, Maine Geological Survey.

---

Text and photos by R. Marvinney

Originally published on the web as the October 2010 Site of the Month.

Last updated on October 29, 2010

Maine.gov | Department Home | Bureau Home | Site Policies