Minnesota Thomsonite

Thomsonite is a distinctive, beautiful, and relatively rare zeolite mineral found along a small stretch of Minnesota’s North Shore of Lake Superior. Minnesota Thomsonite is highly prized as a gemstone with artists worldwide. The finest specimens are round nodules with striking combinations of colors arranged in concentric and radiating patterns, often forming “eyes”. Our family owned the Alley Agate Shop in Grand Marais, Minnesota for many years. As part of our business we also owned and operated the Thomsonite Mine near Good Harbor Bay, 6 miles southwest of Grand Marais, from about 1977 – 1985. Much of the finest Thomsonite ever found came from our mine. The old mine is long-since abandoned and so overgrown that there is scarce little evidence left of the bustling operation it once was.

The Thomsonite story begins with the advent of the Midcontinent Rift, a major geologic event that occurred about 1.1 billion years ago in the Lake Superior region. At that time, eastern and western North America began rifting (splitting apart from each other), forming huge cracks in the Earth’s crust. As this happened, enormous quantities of basaltic lava erupted onto the surface and accumulated in a sequence of lava flows that became known as the North Shore Volcanic Group (NSVG). Over millions of years these lavas piled up to about 32,000 feet in depth – that’s over six miles deep. Eventually, for reasons unknown, after about 25 million years the rifting stopped and the eruptions ceased.

The particular lava which hosts the Thomsonite is called the Terrace Point Basalt – named for the point of land that protects Good Harbor Bay. If you travel along the Minnesota North Shore Drive (Hwy 61) you’ll see continuous outcrops of the NSVG all the way from Duluth to the Canadian border. The Terrace Point Basalt is particularly apparent at the tall roadcut on Hwy 61 as it bends on the southwest side of Good Harbor Bay, just a quarter mile from the Cut Face Creek rest area.

Much of the volume of erupting lava is steam. As it cools, steam bubbles within the magma become trapped. As the layers piled up, the Terrace Point Basalt became deeply buried. Groundwater, coursing through the layers above, picked up elements that it then deposited as minerals inside the vacant steam bubbles. Over ensuing millenia the vesicles eventually became filled with hydrated calcium-sodium-aluminum-silicate – that is, Thomsonite. A billion years later the Ice Age came along and glaciers carved away the layers over top, exposing the once-buried Terrace Point Basalt and, with it the Thomsonite.