We hadn’t thought it through. Bring an 18-month old boy from Minnesota to Alaska for vacation, and he will 1) want to walk, and 2) not walk very far. The biggest state in the union, over 663,000 square miles of mountains, forests, vast wilderness, and he was perfectly happy with a small beach in Homer. There were eagles perched like overgrown pigeons on nearly every post and shed roof. There were boats passing by. Waves on the beach. What he liked best was right at his feet. Every. Single. Rock.
Fortunately, some things don’t change. The boy, now nearly 20, came home the other day with a handful of agates, each smaller than a fingernail, found near a friend’s north shore cabin. Whatever your age, it’s one of the great, simple pleasures of life to walk a beach or river bed and find these bits of beauty with their richly colored bands.
Even the smallest Lake Superior agate has a heft beyond what gravity can explain. When you hold an agate in your hand you are holding more than a billion years of history. You are holding a journey across half the globe.
Minnesota wasn’t anywhere near its current latitude (from 43 degrees 34’N to 49 degrees 23’N) during the event that immediately precipitated the formation of Lake Superior agates. About a billion years ago, today’s Great Lakes Region was part of a land mass geologists refer to as Laurentia. This early core of North America was located south of the equator and rotated 90 degrees clockwise from its current orientation. It was then that the continent threatened—but ultimately failed—to split down the middle. You may be familiar with this zone of near-separation, known as the Midcontinent or Keweenan rift.
Maps commonly depict the rift zone as a 1200-mile long feature, varying between roughly 50 to 100 miles in width, and extending generally from the base of what is now Michigan’s “thumb,” arcing up over Lake Superior, and down to Kansas. But recent paleo-magnetic and seismic studies have led some researchers to theorize that it may have extended as many as 1864 miles, reaching to Alabama in the east and “at least to Oklahoma” in the west [C.A. Stein, 2015].
Volcanic flows associated with this rift would literally pave the way for agates. While the rift is often described as a 1.1-billion-year-old feature, this reflects some rounding off of geologic time. According to research led by University of Ontario’s Don Davis, volcanic activity associated with the rift is estimated to have occurred over a period of 22 million years, in the period from 1,109 to 1,087 million years before present. During this time, as described by a marker erected by the Geological Society of Minnesota in Two Harbors, MN, basaltic lava repeatedly erupted and flowed out over the landscape.
The scale is mind-blowing. Dominated by basaltic lavas, the total volcanic output over the “life” of the rift has been estimated at 1.5 x 106 km3 [Hutchinson et al., 1990, Cannon, 1992]. In volume, this translates to about 123 Lake Superiors. A lot of lava.
Gas bubbles rising slowly through this hot lava were trapped in place as the surface of each successive flow cooled and hardened. These bubble-shaped cavities in the basalt became the spaces that agates would later fill—layer by layer by layer.
Agates are one of many types of so-called chalcedony—stones composed of silicon dioxide in the form of ultra-fine, microscopic crystals of quartz. In the case of Lake Superior agates, they also contain iron as one of the “impurities” that lend a distinctive red or orange color. As geologist Terry Boerboom of the Minnesota Geological Survey explains, the source of both the silica and the iron would have been the basalt itself, put into solution and carried into the cavities by groundwater circulating through seeps and cracks, under the prevailing conditions of heat and pressure.
Their formation, according to Boerboom, can be closely correlated to the period during and “shortly after” the basaltic flows, by which he means within a half million years or so. “In terms of geologic time, that’s essentially at the same time,” he notes. With that in mind, one can place the age of Lake Superior agates at roughly the age of the rift.
Still near the equator at the end of this period of volcanism, the agates rode along within the rift zone as the continent eventually made its way up the lines of latitude to its current position and configuration roughly 50 million years ago.
It turns out that agates (as quartz) are relatively tough. When the basalt was subjected to surface weathering that caused it to fracture and wear, many agates were freed from the encasing rock. Glaciers not only aided in this process but would later set them traveling again, at least regionally. The Superior lobe associated with the most recent ice advance 10,000-20,000 years ago is credited with transporting many agates from northern Minnesota south to the Twin Cities and beyond. Accordingly, the Lake Superior agate is as likely (some would argue, more likely) found in a plowed farm field, gravel mine or on a gravel road in central Minnesota as along the shores of its namesake lake.
Of course, Lake Superior agates formed all along the rift zone. But south of Minnesota, says Boerboom, the basalt flows that may contain agates are buried under a thick capping layer of Paleozoic rock, making them largely inaccessible, at least for the time being. As a Precambrian geologist, he spends a good part of each summer in the field, one of many researchers working under the auspices of the Minnesota Geological Survey to prepare geological maps. If he happens to spot an agate in the course of his work, does he pick it up? “Of course,” he says. “How can you not?”
There are agates around the world, but these are “ours,” expressing the unique identity of this land we call home. Most of us have picked up a Lake Superior agate, turned it over in our fingers, rubbed it with a wet thumb. It’s something we have in common. Like the agates, we all have our origins in far-away places, if we look back far enough.
Lapidarists and artists produce valuable works with these semi-precious gems, and its true that the biggest Lake Superior agates (at over 20 pounds) are impressive. But given the brilliant reach of time and the small space we are each afforded in which to build something beautiful of our lives—layer by layer by layer—these showpieces are not worth any more and maybe less than the scattering of little agates my son held in his upturned palm, or than the one you find and carry in your pocket for a while, taking it out now and then to admire.
Notes and Sources
Personal communications: Terry Boerboom, Minnesota Geological Survey, University of Minnesota, Mpls.; Carol L. Stein, University of Illinois, Chicago
Photographs: Thank you to Karen “Agate Lady” Brzys of the Gitche Gumee Agate and History Museum for providing our feature photo. Check out her website. Thanks also to contributing photographer Joe Gorka.
Cannon, W.F., 1992, The Midcontinent Rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics, v. 213, p. 41–48, doi:10.1016/0040-1951(92)90250-A.
Davis, D., and Sutcliffe, R., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geological Society of America Bulletin, v. 96, p. 1572–1579, doi:10.1130/0016-7606 (1985)962.0.CO;2. (and) Davis, D., and Paces, J., 1990, Time resolution of geologic events on the Keweenaw Peninsula and applications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54–64, doi:10.1016/0012-821X(90)90098-I.
Fairchild, Luke M., Nicholas L. Swanson-Hysell, Jahandar Ramezani, Courtney J. Sprain, and Samuel A. Bowring. “The end of Midcontinent Rift magmatism and the paleogeography of Laurentia.” Lithosphere 9, no. 1 (2017): 117-133.
Hutchinson, D., White, R., Cannon, W., and Schulz, K., 1990, Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift system: Journal of Geophysical Research–Solid Earth (1978–2012), v. 95, p. 10,869–10,884, doi:10.1029 /jb095ib07p10869.
Stein, C. A., J. Kley, S. Stein, D. Hindle, and G. R. Keller (2015), North America’s Midcontinent Rift: When rift met LIP, Geosphere, 11, 1607–1616. (in) Stein, S., et al. (2016), New insights into North America’s Midcontinent Rift, Eos, 97. Published on 04 August 2016.
Marker erected in Two Harbors MN by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation, the Minnesota Geological Survey, and the Minnesota Department of Natural Resources.