Penokean (Island Arc Collision)

The Early Proterozoic


1,900,000,000 - 1,800,000,000 years ago

(1.9 - 1.8 billion years ago)

Island Arc Collision

    The large bright-green and dark-green sections on the geologic map of Wisconsin represent volcanic rocks that were part of a volcanic island arc 1.9-1.8 billion years ago. A volcanic island arc is a series of volcanoes that forms along a subduction zone. Japan is an example of a modern day volcanic island arc. Rocks that formed in this island arc include: olivine and pyroxene, basalt and pillow basalt, andesite, rhyolite, welded tuff, and some greywacke and granite. The volcanic rocks on this table were part of this volcanic island arc 1.8 billion years ago.

    This island arc slammed into the northern Archean terrane deforming both the northern terrane and the sedimentary rocks in the Animikie Basin. When this collision occurred, large mountains formed and the rocks were deformed by high temperatures and pressures and were subject to thrusting, folding, and faulting. The mountains have long since been eroded flat, but evidence of this collision still exists in the rocks of the Lake Superior region, including parts of Wisconsin, Michigan, Minnesota and Ontario.