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Granite Island is a 2.5-acre (1.0 ha) island in Lake Superior located about 12 miles (19 km) northwest of Marquette in the Upper Peninsula of the U.S. state of Michigan. Built upon it is the Granite Island Lighthouse , also known as Granite Island Light Station, [ 1 ] "one of the oldest surviving lighthouses on Lake Superior". [ 2 ]
The biotites in the exterior aureole become more iron- and titanium-rich as they near contact with the Lilesville Granite. In the inner part of the exterior aureole, potassium-rich feldspar begins to appear as it gets closer to the granite. This part of the exterior aureole is a darker gray matrix composed of quartz, feldspar, and biotite.
Monzogranite – Biotite granite rocks that are considered to be the final fractionation product of magma – A silica-undersaturated granite with <5% normative quartz; Monzonite – Igneous intrusive rock with low quartz and equal plagioclase and alkali feldspar – a plutonic rock with <5% normative quartz
Two splash-form tektites, molten terrestrial ejecta from a meteorite impact. Tektites (from Ancient Greek τηκτός (tēktós) 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts.
Unakite is a metamorphic rock that is altered granite composed of pink orthoclase feldspar, green epidote, and generally colorless quartz. Discovery
Granite Island and the lighthouse were purchased by Scott and Martine Holman in 1999 from the U.S. Coast Guard. The facilities underwent a three-year restoration process. [ 17 ] The lighthouse was listed on the National Register of Historic Places in 1983.
Breccia is composed of coarse rock fragments held together by cement or a fine-grained matrix. [5] Like conglomerate, breccia contains at least 30 percent of gravel-sized particles (particles over 2mm in size), but it is distinguished from conglomerate because the rock fragments have sharp edges that have not been worn down. [6]
The Stannard Rock Reef is located off Keweenaw Peninsula about 24 miles (39 km) south of Manitou Island and 44 miles (71 km) north of Marquette, Michigan. [1] [10] In 1835, Captain Charles C. Stannard of the vessel John Jacob Astor first discovered this underwater mountain that extends for 0.25 miles (0.40 km) with depths as shallow as 4 feet (1.2 m) and averaging 16 feet (4.9 m).