A black basalt intrusion into surrounding pink granite at Acadia National Park. An abundance of potassium feldspar renders the granite pink.
Photo: Maine Geological Service, Henry N. Berry
Every Outcropping of Granite Tells a Story
By Kevin McKeon, Maine Master Naturalist
Rocks are classified into three types, igneous, sedimentary, and metamorphic, and are formed in different ways. Igneous rock forms from magma — a mix of rock, gasses, and minerals under the Earth’s crust, rendered into a fluid or molten state by extreme pressure and heat. As pressure increases, magma sometimes erupts dramatically from fissures and volcanoes where it becomes lava and quickly cools. About 75% of all these volcanic eruptions occur in the ocean at over one mile deep. Lava cools to form igneous rock types called volcanic or extrusive rock. Other times, magma forcefully pushes or intrudes into the upper layer of Earth’s crust without surfacing, where it slowly cools. This magma forms another type of igneous rock called intrusive, or plutonic rock.
Other common rock types — built through the effects of time, heat, and pressure — are sedimentary and metamorphic. Sedimentary rock forms from layers of settling grains of dust and sand, pollen, and decaying organic materials like plankton, seashells and fish bones. Metamorphic rock forms from rock being subjected to high heat and pressure, or from chemical reactions that alter the texture and mineral make-up without melting it, thereby changing the original rock’s structure.
When magma intrudes into Earth’s crust and cools, different rocks are formed. Magma cooling relatively quickly, like under the oceans, tends to form basalt — the most common rock on Earth and in our solar system. Magma cooling more slowly allows the formation of relatively large mineral crystals, which is the basis of granite. These magma intrusions usually happen one to 30 miles under our feet into weaknesses in the Earth’s crust. These weaknesses result from constant tectonic movements generated by Earth’s interior heat, like volcanic eruptions, earthquakes, mountain formation actions, or oceanic trench-building between continents. A trench-forming rift in Iceland called the “Silfra Crack” offers scuba divers the ability to touch two continents at the same time.
So, like basalt, granite is an igneous rock, formed by the cooling of magma. “Granite” is Latin for “granum,” or “grain,” a reference to the rock’s hard, tightly fitting granular structure. Slower cooling allows the growth of larger crystals in granite. Granite crystals are always made from the two most common minerals in Earth’s crust, quartz and feldspar, minerals harder than steel and which give granite a white to pink color. Very slowly cooling magma results in a type of granite called pegmatite, Maine’s State Rock, which has even larger crystals. The larger, white or milky quartz crystals we see along our trail walks are evidence of the slow cooling. The common darker minerals in granite are mica and hornblende and give granite its famous “salt & pepper” look. Most of these crystals are visible to the naked eye. Various types and amounts of minerals give granite its different colors.
Some fussy geologists will tell us that true granite contains between 20% and 60% quartz, and more alkali feldspar (sodium and potassium) than plagioclase feldspar (aluminum-based silicates of sodium and calcium); otherwise it’s a type of granite called granitoid. Even fussier students of deeper, granite-specific studies assert that our rock is classified by its evolutionary origin: I-type, igneous; S-type, sedimentary; M-type, mantle; or A-type, anorogenic.
Another group of granite definers are the folks dealing in the business of granite sales — the stone dealers. Since this rock resists acids and weathering, and polishes to a fine sheen, these dealers offer granite for countertops, gravestones, ornaments, sculptures, and buildings. Dealers also offer black gabbro, dark-green peridotite or streaky gneiss — not technically granites, but often sold as “commercial granite”. Regardless, many folks like the innate properties of both the granite and non-granite stuff, and it serves their purposes.
Granite in New England will erode at about one inch per 10,000 years, and doing so exemplifies geological processes of continental change, maintenance, and evolution. As it erodes and breaks down into its tiny mineral pieces of sand and clay, it all ends up in the ocean, where it filters down to form the sea floor. As layers accumulate over the centuries, heat and pressure caused by its own growing weight form sedimentary rock. Tectonic plate movements then move this layer of rock under and over each other at the Earth’s rift and subduction zones — eventually entering Earth’s high-heat zone under the crust where magma is formed with other rocks, then finally erupting or intruding to form new rock— completing Earth’s rock cycle.
Silfa Crack: https://youtu.be/tliU5A8m7Lg
Pegmatite: https://sanfordspringvalenews.com/out-in-the-woods-55/
Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net
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