Shining Big Sea Water: The Story of Lake Superior

Chapter One OF BEDROCK AND ICE

3 Chapter One OF BEDROCK AND ICE Lake Superior, the largest freshwater lake in the world, displays its geological past to good effect. Its irregular shoreline of bays, inlets, and peninsulas; the colorful palisades of rock on Ontario’s north shore; and Michigan’s dramatic Au Sable Dunes are footprints from the march of time. Billions of years ago, the earth was a mixture of water and rocky landmasses that were proto-continents or what geologists call cratons. The cratons were rooted in the earth’s mantle (the superheated rock between the crust and the molten core), and parts of them periodically sank into themantletobemetamorphosedwiththeadditionofnewelements,such as nickel, cobalt, and magnesium. One of these cratons, formed about 2.5 billion years ago, extended from present-day Greenland, across eastern Canada, and as far as central Michigan, Wisconsin, and Minnesota. Known as the Canadian Shield, this mass of granitic bedrock can be seen today in the cliffs along Lake Superior’s north shore or where it has been scraped bare by the glaciers of the past two million years. Rooted in the unstable magma, the cratons moved around over millions of years due to plate tectonics. Shortly after the Canadian Shield was formed, it collided with other cratons, creating a “supercontinent” that included northern Asia and much of today’s Europe. About 2.1 billion years ago the North American craton fragmented along a line that roughly parallels the south shore of present-day Lake Superior and the north shores of lakes Michigan and Huron. The seas swept in, and for several million years Wisconsin and Michigan lay under water. The 4 Shining Big Sea Water lands around this arm of the sea—still without vegetation—eroded, and sand was washed into the shallow water, accumulating in some places to a depth of several hundred feet. Under heat and pressure it hardened into sandstone; if the folds of the earth brought it closer to the hot magma, it solidified into quartzite. When the land rose out of the water (due to another craton collision), the newly fashioned sedimentary rock formations emerged. The sandstone can be seen today in the red cliffs at the tip of the Bayfield Peninsula, and it is the basic rock of the Apostle Islands. The quartzite hills around Marquette, Michigan, are remnants of the metamorphosed sandstone. Thesedimentaryrocksthatmaterializedfromtheshallowseainnorthern Michigan, Wisconsin, and Minnesota contained large deposits of iron ore. The origin of this ore remains somewhat a mystery. Ore-­ bearing sediments accumulated around the world at the same time, about two billion years ago. There are no iron ore deposits from an earlier or a later period. The best explanation for this phenomenon is the presence, for the first time, of oxygen in the atmosphere. Single-celled plants had appeared almost as soon as the oceans were formed, and these bluegreen algae were capable of photosynthesis, the process of converting the atmosphere’s carbon dioxide into free oxygen. The oxygen joined nitrogen in the atmosphere and was dissolved into the ocean. Iron is quite soluble in oxygen-deficient waters and would have been present in large quantities in the earliest oceans. With the oxygen it formed ore compounds—hematite, pyrite—that are insoluble. The precipitates collected in depressions at the bottom of the sea and later emerged as iron ranges in Michigan, Wisconsin, and Minnesota. About 1.8 billion years ago a craton drifted up from the south (possibly after splitting off from Africa) and collided with the Canadian Shield craton, forming an arc from southern Ontario across northern Michigan and Wisconsin and southwest to the Minnesota-Iowa border. The intruding craton slipped under the Canadian Shield, creating an enormous volcanic uplift which, after eons of weathering and ­ erosion, can be seen today in Iron Mountain, Michigan, and the Northern Highland The picturesque markings of Michigan’s Pictured Rocks are created by mineral-rich water seeping through cracks in the sandstone cliffs. Wave action has bored caves in the softer sandstone. The layers were created by the interaction of land and water billions of years ago. 5 6 Shining Big Sea Water of Wisconsin. Geologists believe this ancient mountain range might, at one time, have been as high as the present Rockies. The final chapter in the geological formation of the basin that would become Lake Superior was a cataclysmic event that began about 1.1 billion years ago and ended a hundred million years later. It...