Science for Everyone

Don't Take this Place for Granite

Post by Bailey Mortiz, Scallop Research Intern

This may or may not be a crescentic gauge carved out by an active glacier.

Flipping through the igneous rock section of my geology textbook, theres a striking photo of white and black rocks swirled together, a prime example of two different magmas mixing in a chamber hundreds of thousands of years ago. The photos caption cites none other than our neighboring island, Vinalhaven, which has been studied by geologists the world over for its fascinating and unique rock features. Every geologic formation and the rocks that build it have a story to tell about the locations history. Hurricane Island is no exception, and as is true almost anywhere on the Maine coast, theres a lot of interesting geology to be observed underfoot.

 xenolith- Magma of different composition mixed with the granite to form these ubiquitous enclaves around the island

Granite makes up the island and granite is responsible for much of its human history, giving rise to the booming quarry town that once was. Granite consists primarily of 3 minerals; quartz, feldspar, and biotite. Different ratios of these minerals give rise to different colors and textures of granite. The granite of Hurricane Island is tannish in color and has notably high potassium feldspar content. Reddish granite slabs pop up here and there around the island, and these were brought in from other places with other mineral compositions to be carved by the skilled quarrymen of Hurricane. Architects and builders could peruse catalogs that described granite by its color and grain size quality in order to decide where to purchase the material from. Potassium rich granite from Hurricane Island was selected for the Washington Monument and streets of Havana, Cuba.

As these granite bodies cooled within the earth, another type of magma, one that was darker in color and therefore more rich in iron content, intruded in and caused pockets of different material, called mafic enclaves, to appear throughout. Next time you visit Hurricane Island, see if you can spot some on the exposed rock, ranging anywhere from several inches to several feet across. But these odd bubbles within the granite were not always visible. About 13,000 years ago, the last of the glaciers that once covered this place in deep ice finally retreated. Although the granite has been well weathered, there are still some potential places where we can find evidence of past glacial movement. Striations are a set of parallel scrapes that a glacier literally carves into the bedrock as it slides. Crescentic gauges are chips out of the bedrock that occur as heavy ice puts pressure on the surface below it. Examples of both can be seen around here.

Beautiful colored garnets peeking out of a pegmatite boulder.

On a stroll to the north end of the island, we came across a magnificent while boulder with large grain sizes of feldspar. Even though it did not originate here, it is my favorite rock on Hurricane. Embedded amongst the white feldspar minerals is another glistening surprise; beautiful, perfect maroon garnets! Garnets are the birthstone of January and a common gemstone for jewelry, but it’s even cooler to find them in their natural state. This rock is a pegmatite, which is an igneous magma that cools really slowly, allowing enough time for larger and more unique minerals to form. But a phenomenon like this would not be found on Hurricane. Perhaps a glacier carried it along, or maybe it was brought by human hands for trade. Either way, it adds a nice touch to the natural geology museum that exists all around us.

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