Скачать или смотреть Basalt Outcrop (hiking guide stop #2)

Straight Lake State Park, Polk County, Wisconsin
Geology Hiking Guide for the Ice Age Trail

Ian F. Freeman, Rebecca J. Moore,
and Kent M. Syverson

University of Wisconsin-Eau Claire Department of Geology

Captions:

A basalt outcrop is located along the trail 0.2 miles north of 280th Ave. The vegetation-covered, dark grey basalt outcrop juts toward the trail. Outcrops are up to 20 m in diameter and were sculpted by glaciers. The basalt contains amygdules, which are small holes representing former air pockets in a lava flow. These holes later filled with red and white minerals.
The earliest event in the area is represented by the 1.1 billion-year-old Midcontinent Rift. Volcanic basalt oozed from the rift as North America split apart, stretching from Lake Superior to Oklahoma. As the rift expanded, a large trough formed containing tens of thousands of feet of basaltic lava flows. If the rifting event had continued, an ocean would have formed between the two halves. However, the rifting stopped, and the closing rift pushed up the basaltic rocks seen here along the trail.