Gooseberry Falls State Park, North Shore of Lake Superior

December 15, 2015


Basalt insertPhotographer: Dale Hugo
Summary Author: Dale Hugo

The middle falls of the Gooseberry River at Gooseberry Falls State Park in Minnesota, features outcrops of basalt with noticeable columnar jointing. The falls itself drops from a vertical basalt wall, whereas a horizontal layer of basalt underlies the base of the falls, where the river water splashes.

Basalts are usually associated with only ocean bottom igneous rock. So what's it doing here in the middle of the North American Continent? About 1.1 billion years ago it's thought that a rift started to form an ocean with seafloor spreading, like that of the present day Atlantic Ocean. But the action stalled leaving a nascent ocean bottom, now exposed and scoured by glaciers of the last glacial epoch. Note the cave, partially shadowed, in the background rock face.

On the inset at left hexagonal and pentagonal fractures are visible along the surface of the basalt.

Photo Details: Top - Camera Maker: LG Electronics; Camera Model: LG-LS720; Focal Length: 3.49mm; Aperture: ƒ/2.4; Exposure Time: 0.0015 s (1/653); ISO equiv: 100; Software: Microsoft Windows Photo Viewer 6.1. Bottom - same except: Exposure Time: 0.0010 s (1/961).