Great Duck Island
Photo taken by Mikey Cornish
Found 12 miles south offshore of Mount Desert Island, Great Duck Is. is home to the Alice Eno Research Station, one of the two island research stations of College of the Atlantic. The research station is focused on Pelagic seabird populations including those of the Herring Gull and Leach’s Storm Petrel. College of the Atlantic shares the island with the Nature Conservancy and a private landowner.
The geology of Great Duck tells an interesting story in relation to Mount Desert Island and geologic research on the island is just beginning. In structure Great Duck reflects the geologic history of Mount Desert with intrusive and extrusive rocks right next to each other. This suggests that the island was part of a magma chamber that was uplifted, tilted, and eroded. The island can be divided into two general sections. The Granitic section and the Rhyolitic section. The southern half of the island is predominantly Granite. Blocks of Rhyolite are found within the granite becoming more prominent up until the the middle of the island. In contrast, the Rhyolitic section of the island has granite blocks within the rhyolite also becoming more predominant towards the middle of the island. From north to south the island represents a gradual transition between the extrusive and intrusive rocks.
Figure 1: Quartz phenocryst in Great Duck Island Granite
Rocks
Figure 3: The granite of Great Duck Island. Taken facing east, here it is clearly showing East/West trending fractures.
Figure 2: Pegmatic felsic dike cutting basaltic dike (270 degrees) cutting rhyolite on Great Duck Island
Granite - Phaneritic. Medium grained. Ranges from light to dark pink/red. Lots of K-Spar and quartz. 60% K-Spar, 20% Quartz, 10% Biotite. Often quartz phenocrysts up to 6 inches in diameter. Pegmatic regions. Large blocks of rhyolite dispersed within the bedrock.
Basaltic - Basaltic dykes frequently cutting both rhyolite and granite. Fine grained, aphanitic, mafic. Frequently associated with pegmatite regions.
Rhyolite/Felsites - Aphanitic. Very fine grained. light gray. Often showing flow. Some pyrite and meta-sedimentary xenoliths showing epidote as a weathering product. Often interspersed with large granite blocks.
Figure 4: Granite hand sample. Shown here with basaltic dike.
Figure 5: Rhyolite/Felsites hand sample.
Figure 6: Stitched panorama and 36 relief of basaltic dike cutting rhyolite
Minerals
Figure 7: Sedimentary xenolith within rhyolite flow showing epidote
Figure 8: Rhyolite/Felsites hand sample showing pyrite crystals