I have friends who grew up in Minnesota and periodically travel up to the North Shore for vacations and a little rock collecting. In Minnesota and Wisconsin the term “North Shore” affectionately refers to the north shore of Lake Superior stretching from Duluth northeast to Grand Portage on the International Border. Outside of these two states the North Shore is fairly unknown to most people; however, it is one of the treasurers of the nation. MN 61 follows the shoreline of the lake and spectacular views are available by simply pulling into an overlook or state park. The shoreline is dominated by steep cliffs intermixed with broad to narrow beaches filled with pebbles, cobbles and sand---a great place for rock hunting.
The entire North Shore is dominated by rocks associated with the Mid-Continent Rift System (MRS). This geological rift (think about the great East African Rift Zone) begin to form in the Precambrian (Proterozoic Era) perhaps 1.1 Ga splitting the stable part of the North American “continent” or plate (referred to by geologists as the craton). Geologists believe the rifting started when a large mantle plume (heat) developed under the area now known as Lake Superior. The Rift is nearly 1400 miles long extending from northeast Kansas to Lake Superior with an eastern arm curving around and heading toward Ohio (part of a Triple Junction, more geology jargon). Hugh amounts of lava erupted along faults while adjacent rivers from the up-lands dumped thousands of feet of sediments (later sedimentary sandstones and conglomerates) into the low lands of the Rift (the down-dropped basin of the Rift created by normal faults). In addition, some of the magma was "intruded" and crystallized as a coarse-grained gabbro (generally known as the Duluth Gabbro). For some reason, the rift “stopped splitting” (a failed rift in geological jargon), and the continent healed. The most likely culprit in keeping Duluth from being located on an ocean beach is another tectonic plate colliding with what is now the east coast of North America. This bumping probably put an end to the spreading.
Map of Midcontinent Rift Showing Ages of Surrounding Basement Rocks. Map from Middle Proterozoic and the Mid-continent Rift, Winona State University.
Most of the rocks in the rift are buried below the surface of the earth and are only known from geophysical studies and drill holes. In Minnesota the Rift first rocks appear along the Kettle River north of the Twin Cites at Banning State Park. In my home state of Kansas the Midcontinent Geophysical Anomaly (MGA) delineates the rift since the concentration of magnetite in the Rift rocks creates a magnetic “high” that is picked up by geophysical instrumentation. In Iowa the Rift rocks also are in the subsurface but drilling and geophysical studies have delineated a large uplift block (horst) in the center of the Rift that has shed off sediments to basins flanking the horst. This horst indicates a crustal shortening event probably related to the previously noted continental accretion and collision in the east.
Map (Iowa Department of Natural Resources) showing location of MRS with center horst. During the continent collisional event the crustal shortening reactivated the bounding faults as reverse faults as uplifted the horst.
Some of the really interesting aspects of the North Shore geology are the many instances of rivers coming off the highlands to the north and reaching the lake and spilling over waterfalls. The basalt of the Rift rocks creates the opportunity for the rapids and falls and today most are protected in state parks, places such as Gooseberry Falls State Park and Temperance River State Park. I have camped at many of these parks and they are spectacular.
One of the attractions for rockhounds at the North Shore is the chance to collect the famous Lake Superior agates that erode from the Rift basalt layers. Since the Rift rocks include substantial amounts of iron, the agates have some sort of a red or orange color---oxidized iron. Most likely the agates formed post-deposition of the basalt and are the result of percolating silica-rich groundwater filling the many vugs or vesicles in the basalt.
Falls of the Baptism River, Tettegouche State Park, Minnesota. Photo courtesy of www.superiortrails.com.
But back to my friends the Thompsons who vacation at the North Shore. Recently they brought home a piece of thomsonite claiming this mineral was named after some lost-lost northland relative! It was a very nice little nugget and they asked me about the composition---what was it?
Well, thomsonite [NaCa2Al5Si5O20-6H2O] is a zeolite mineral, actually a series of silicate zeolites with the monikers thomsonite-Ca (listed above) and thomsonite-Sr with the former being the most common but in some case strontium replaces the calcium. The zeolites are an interesting “group of hydrous silicates that show close similarities in composition, association, and mode of occurrence. They are framework aluminosilicates with Na, Ca, and K and highly variable amounts of H2O in the generally large voids of the framework…When a zeolite is heated, the water in the channelways is given off easily and continuously as the temperature rises, leaving the structure intact…This dehydrated zeolite structure can be completely rehydrated when it is immersed in water…and this allows it to be used as desiccants.” I find it interesting that the zeolite cations can easily exchange with “unwanted” cations in solutions such as the sodium in a zeolite replacing the calcium in “hard water.” Zeolites have an amazing number of industrial uses. (above from Klein, 2002). About three million metric tons are mined each year for industrial applications, and several thousand tons of synthetic zeolites are produced for their purity. There seem to be a few over 100 named zeolite minerals listed on www.mindat.org.
Most rockhounds collect zeolites because of their well displayed crystals such as those noted in mesolite, stilbite, scolecite, heulandite and numerous others. However, about the only zeolite considered to be a semi-gemstone is thomsonite (although natrolite has been facetted for collectors). The best know specimens of thomsonite are small pebbles that erode from the Rift basalts along the North Shore and then are available for observant rockhounds on the beach. I say observant because many of the pebbles really do not look like much of a find until then are opened. The specimen that I have (see photo) is very non-descript on the outside but has nice concentric colored rings and radiating fibers on the inside. Lapidaries like to collect and polish the small thomsonite pebbles that display a pinkish color on the outside.
So, if readers have travel plans for Minnesota (perhaps for my favorite winter sport--skinny skis on the trails, or for summer fishing and camping), consider the beautiful drive along the North Shore. The waterfalls are amazing and beaches are available for collecting those famous agates. However, keep your eyes open for these small, often non-descript, pebbles of thomsonite.
Klein, C. (after James D. Dana), 2002, The 22nd Edition of the Manual of Mineral Science: new York, John Wiley and Sons.
Miller, J. D. Jr., 2007, The Midcontinent Rift in the Lake Superior Region: Large Igneous Province of the Month, http://largeigneousprovinces.org/