Geology

The Geology of Swan’s Island

by

David Bailey, Ph.D., Associate Professor of Geosciences
Hamilton College, NY

Swan’s Island is an island just off the coast of Maine near Bar Harbor and Acadia National Park. The rocks on Swans Island are part of the Avalon Terrane, crust accreted onto North America during the Silurian and Devonian (~438 to 410 million years ago). The eastern part of Newfoundland is also part of the Avalon Terrane.

Many rocks on Swans Island exhibit bimodal magmatism. Similar pictures of bimodal magmatism from Mount Desert Island can be found in many geologic textbooks.

The granitic and mafic rocks are Late Silurian (~424 million years old) based on zircon dating. Partial melting of the mantle produced mafic magma. As the hot, mafic magma moved upwards, it melted the overlying crust (which is granitic in composition), producing granitic magma. This is an example of magma mingling, when two different magmas form and interact at the same time.

The mingling of the granitic and mafic magmas in Maine produced many unusual and varied outcrops. In some outcrops, the mafic magma intruded into the granite when the granite was still liquid. However, the granitic magma was cooler than the mafic magma. As a result, the mafic magma crystallized quickly into ellipsoid-shaped rocks called pillows (see picture below). Basaltic magma erupting into water also cools and solidifies into pillows, which can be seen on the ocean floor.

In other places, the mafic magma intruded into the granite after it had cooled and crystallized. The mafic magma intruded into fractures in the crystalline granite forming mafic dikes (see picture below). The mafic magma cooled more quickly on the edges of the dike where the mafic magma was in contact with the colder rock forming a chilled margin. As a result, the dike is much more fine-grained on the edges and more coarser grained towards the middle of the dike where the mafic magma cooled more slowly.

Also found on Swans Island are outcrops of mafic rock with granitic dikes intruding into the mafic material. Differential cooling occurred on the two magmas because mafic magma solidifies at a higher temperature than granitic magma. Also, the middle of both types of magma cools more slowly than at the margins. This differential cooling produces a bizarre mix of outcrops. The outcrops with basaltic pillows indicate that the mafic magma was much hotter than the granite. But because pillows form when magmas erupt into liquid, the granite had not fully solidified. The mafic dikes form when the mafic magma intruded into granite that had already cooled and crystallized. Outcrops of granite intruding into mafic rocks occur where the mafic material had cooled and crystallized prior to the intrusion of granitic magma.

Erosion at work

Unlike the sandy beaches found in the southern parts of the eastern United States, most beaches in Maine are rocky. This is an example of a fine sand pocket beach in Maine surrounded by outcrops of granite.

The sand on the beach is derived from the granite bordering it. Erosion and weathering of the granite produces fine-grained sand. Close inspection of the sand shows that tiny grains of quartz and feldspar are mixed with some biotite. Quartz, feldspar, and bioite are three of the major minerals in granite.

The outcrops of granite on either side of the beach allow for deposition of sediments. The outcrops form a small cove around the beach. The water in the cove has a lower velocity than the water in the open ocean so more deposition of sediments occurs than erosion, forming a sandy beach. On a warm, sunny day, the beach is a fun place to spend a couple of hours!

More igneous rocks

In addition to the granite and basalt, Swans Island has some outcrops of diorite, a rock with a chemical composition between granite and basalt. Diorite is also exposed in many other places along the Maine coast.

The diorite is predominantly composed of hornblende and plagioclase. There are two hypotheses concerning the formation of the diorite on Swans Island. The diorite could be derived from magma mixing between the granitic and basaltic magmas. Granite and basalt are on opposite ends of the spectrum in terms of mineral and chemical composition. When the two types of magma mix, they would form a magma of more intermediate or diorite-like composition. However, the diorite could also be derived from fractional crystallization of the basaltic magma in the magma chamber, which would push the composition of the magma closer to granitic composition.