GEOLOGIC EXPOSURES OF ISLE ROYALE NATIONAL PARK FOR THE GEOLOGIST AND THE PUBLIC

Known for its wolf-moose dynamics and boreal vistas, Isle Royale National Park is also renowned for biological diversity and physical isolation. However, it also has globally significant geological aspects. Isle Royale provides excellent exposures of Proterozoic Mid-continent rift rocks, including some of the Earth’s largest lava flows, many of which demonstrate complex solidification features. In addition, the park displays Holocene and Pleistocene glacial features.

Visitors quickly learn the geomorphology of the island; as hikers experience perpendicular ridges and valleys and relatively flat E-W topography. Once explained, visitors can easily appreciate and relate to the south dipping lava flows that make up the island, even actively seeking out these “triangles” of dipping basalts and conglomerates.

Easily accessible sites include Monument Rock, Suzy’s Cave and Scoville Point. All three locations provide excellent exposures of the glacial features from the shorelines of Lake Minong (10.5 ka) and Lake Nippissing (5 ka).

The Portage Lake Volcanics include twelve named flows with unique lithologies and many distinctive interbedded sedimentary units. Lava flows provide near complete views of lava internal anatomy and solidification features. Blake Point has spectacular 5m columnar jointing, and Raspberry Island exhibits excellent exposures of upper/lower ophitic zones, pegmatitic zones and segregation cylinders.

Amygdule minerals of Isle Royale are of interest to both geologists and the public. In addition to native copper, prehnite, quartz, calcite, and pumpellyite (largely unique to the island), can all be found as secondary minerals. Likely formed during regional metamorphism associated with the rift, they are often found as veins or amygdules within the lava flows, but can also occur within the conglomerate layers.

Isle Royale provides a wealth of geologic exposures accessible by both the scientist and the public. By emphasizing the geology of the island, visitors can appreciate how rift formation and later glaciation influenced the modern lake.