A Brief Review of the Geology of Monhegan Island

Among the outermost islands of Maine's central coast, there are few that rival Monhegan for remoteness and scenic wonder. About 65 people live there year-round, most making their living from the sea or catering to the annual invasion of visitors. The unique charm of the village and ruggedness of the coastline has made Monhegan a summer haven for artists and other visitors for more than 100 years. The geologic history of the island, however, extends much farther back in time - to more than 400 million years ago.

Monhegan Island, Manana Island, and nearby smaller islands are composed of igneous rock - that is, rock that solidified from a molten magma. The molten magma was probably generated those many hundreds of millions of years ago when massive plates that make up the crust of the earth collided. This magma moved upward through the crust, but cooled and solidified to form a massive body of gabbro before reaching the surface. Millions of years of erosion have now exposed it at the surface. Gabbro is a dark colored, coarse grained igneous rock consisting primarily of the minerals plagioclase feldspar, olivine, pyroxene and hornblende. These are all from the class of minerals called silicates (those with significant silicon content). Plagioclase is a silicate mineral with variable sodium and calcium contents. The other minerals all have variable amounts of calcium, iron, and magnesium. Gabbros may be further divided into subgroups such as olivine norite (lots of olivine, Ca-poor pyroxene) and hornblende gabbro (abundant hornblende) based on the relative abundance of these minerals.

Figure 1

The best geologic map of Monhegan was produced in 1900 by Edwin C.E. Lord of the United States National Museum (Figure 1). He mapped most of the rocks on the southern 2/3 of the island as olivine norite, with gabbro and hornblende gabbro on the north end. He notes that there are gradations among all these rock types and that the transitions from one variety to another are gradual.

The island is traversed by nearly 12 miles of hiking trails, which afford the visitor exceptional access to the bold cliffs on the east side of the island and a variety of geologic features.

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You can find more information on Monhegan Island at A Visitor's Guide to Monhegan Island

A Brief Review of the Geology of Monhegan Island

Further reading