File:Amygdaloidal basalt (Two Harbors Basalts, North Shore Volcanic Series, Mesoproterozoic, 1097-1098 Ma; Burlington Bay, Two Harbors, Minnesota, USA) 1.jpg

English: Amygdaloidal quartz tholeiite basalt from the Precambrian of Minnesota, USA.

Along the northern shore of western Lake Superior are numerous exposures of a lava flow-dominated succession called the North Shore Volcanic Series. This is equivalent to and the same age as the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (www.flickr.com/photos/jsjgeology/albums/72157632266738191). The North Shore and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: www.kgs.ku.edu/Publications/Bulletins/237/Woelk1/gifs/fig...). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes.

The sample seen here is from shoreline of Burlington Bay at the town of Two Harbors, Minnesota. The rocks at the site are quartz tholeiite basalts - they are part of the Two Harbors Basalts succession. Three lava flows are exposed, designated flow A, flow B, and flow C in the literature. The basalt has numerous gas vesicles that have since been filled with minerals, resulting in amygdaloidal basalt.

Stratigraphy: Two Harbors Basalts, Upper Southwest Sequence, North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1097-1098 Ma

Locality: loose piece from shoreline exposure on the western side of Burlington Bay, eastern side of the town of Two Harbors, northeastern Minnesota, USA