Geology of Good Harbor Bay - Minnesota Historical Markers on Waymarking.com

The volcanism occurred 1.1 billion years ago when the North American continent began to rupture along a great rift valley which extended from the Lake Superior region SW to Kansas. As this rift valley opened, basaltic lavas erupted into it intermittently for about 20 million years accumulating to a thickness of up to 20 kilometers in the Lake Superior region.

With each eruption, red hot lavas fountained from kilometer long fissures for up to decades at a time flooding over large areas of barren landscape. Flood basalt eruptions typically followed on another in geologically rapid succession but at times there were significant intervals (thousands to millions of years) without volcanic activity. During such intervals, streams and rivers flowing over and eroding the volcanice terrain would depoist sediments into lakes in low lying areas. when volcanic activity resumed, these sediments could in turn be buried, heated and compacted by lava flows and transformed into sedimentary rocks

And example of such a geological cycle of eruption, sedimentation and renewed volcanism appears in the cliff face across the highway from this marker. Beneath a dark gray basalt flow is a reddish thinly bedded siltstone, sandstone and shale formation. Beneath these sedimentary rocks is another lava flow which is exposed in the creek bed of cut face creek just north and down the hill from the road cut. the full thickness of this sedimentary rock formation is about 40 meters. This thickness indicates a prolonged lull in volcanic activity perhaps lasting several million years. The broken up and mineralized character of the basalt at the left side of the cliff face resembles features observed when lavas explosively encounter standing water. This and the fine sediments beneath the lava suggest that a shallow lake may have existed in the area at the time of renewed volcanism.

Marker Type:: Roadside

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