Why Is Lake Superior? JOHN C. GREEN, Ph.D. The genesis of the lake goes back over a billion years. Today, it's stable — unless another glacier comes along. T i akes in general are fragile, temporary features on the earth s landscape. Typically, they appear and vanish within a lifetime of a few thousand years, though a few have persisted to make an important mark in the geologic record. Most lakes are formed by short-lived geologic processes, and, in turn, destroyed through filling by sediment or draining by their outlets. Lake Superior, though, is on a totally different scale. Unlike most lakes, it is a major feature 011 earth's surface and will be here for a long, long time. This largest and deepest of the Great Lakes owes its existence to two extraordinary and contrasting events: • A period of intense volcanic activity over a billion years ago that split North America apart, and •Glaciation during the Pleistocene Ice Age, ending just ten thousand years ago. Neither of these dramatic events alone could have given birth to the largest freshwater lake in the world. Together, however, they produced the magnificent result we enjoy today. 10 THE MINNESOTA VOLUNTEER Billion Years. Lake Superior didn't exist before the Ice Age. To see how it began, we must go back to Early Precambrian times. Lake Superior lies embedded in the Canadian Shield, an im- mense expanse of ancient rock that forms the basement of North America. Along most of the lake's north and east shores in Canada, and not far to the south and west in Michigan, Wisconsin, and Min- nesota, are rocks of Early Precambrian age. These rocks were produced by undersea volcanism, metamorphism, shearing, and deformation. The culminating act in their creation was the intru- sion, about 2.7 billion years ago, of molten granite. (Figure 1.) These rock-forming processes helped establish the thick conti- nental crust in this area. Well-known rock units north of the Mesabi Range, such as the Ely Greenstone, Knife Lake Slate, and the Vermilion, Giants Range, and Saganaga granites, belong to this group. After erosion had worn these rocks nearly flat, a shallow sea invaded the Lake Superior region. In this sea, sand, then a thick iron formation (now the commercial ores of Minnesota and Michigan), mud, silt, and more sand were deposited 2.0 to 1.9 Jagged rocks at the mouth of the Goose- berry River are mute test- imony to the awesome forces which created the Lake Su- perior basin. JULY-AUGUST 197.S 11 FIGURE 1 ONTARIO ONTARIO Thunder Bay; Gnd.-ci- Portage Keweenaw & Point \ Silver Bay .Marquette Ironwood MICHIGAN Bedrock Map of Lake Superior area. Horizontal lines: Igneous and metamorpliic rocks of Early and Middle Precambrian age, 2.7 to 1.8 billion years old. Dark stipple: 1.2 to 1.1 billion year old lava flows. Checks: Late Precambrian gabbros and diabase related to rifting. No pattern: Late Precambrian and younger sandstones. Dashed line with arrows shows axis of Lake Superior syncline. Late Precambrian rocks are tilted toward this line — a kind of trough in the bedrock —from opposite sides. billion years ago in Middle Precambrian times. During a mountain-building episode 1.8 billion years ago, these layered sedimentary rocks are gently tilted in the Mesabi Range-Canadian Lakeland area, but strongly folded and com- pressed in east-central Minnesota and Upper Michigan. More hot, plastic granite was injected between the rock layers to the southwest of today's lake. There followed another long period of erosion and relative stability. The stage was now set for the climatic event that made Lake Superior possible. Colossal Rift. About 1.2 billion years ago, the continent began to split apart along a great hook-shaped fracture. This rift ex- tended from eastern Kansas north through Iowa and the Twin 12 THE MINNESOTA VOLUNTEER Cities area, through what is now Lake Superior, and then south through Michigan's Lower Peninsula — possibly as far south as Kentucky. Along this rupture, molten rock welled from below the earth's crust and spilled onto the surface. Most of this lava was basalt — dark-colored and rich in iron, calcium, and magnesium — and very fluid, much like the basalts being erupted in modern times in Iceland and Hawaii. The flows, numbering in the hundreds, spread like thin pan- cakes across the barren landscape (land plants had not yet evolved). The flows ranged greatly in size, though many were only a few feet thick. Occasional streams, flowing across a cooled lava flow, deposited layers of sand and gravel. Some molten rock never reached the surface. Instead, it cooled and hardened beneath a thick cover of lavas. The resulting rock, much coarser-textured than the quickly-chilled lavas, formed the immense Duluth Gabbro Complex and other smaller intrusive gabbros and diabases. These basalt lavas and diabases are exposed today along the North Shore, especially at state parks and waysides such as Gooseberry Falls and the Temperance and Cascade Rivers. More basalts formed the backbone of Isle Royale and Keweenaw Point, Michigan, and contained the famous copper deposits of the Keweenaw. Some of the lavas were of a more light-colored silicon composition, such as the thyolite that formed Palisade Head. Similar, but geologically younger, rifts have split continents apart to form our ocean basins. The Atlantic basin, for example, started opening 190 million years ago and is still expanding as new oceanic crust forms from molten basalt flowing up from the earth's mantle along the Mid-Atlantic Ridge. Unlike the mid-Atlantic rift, however, the Precambrian mid- continental rift was played out in a few tens of millions of years. Only tens of kilometers separated the rift's two sides before the process stalled. Once the tension that opened the rift ceased, the new rock solidified in the fractures and welded the two sides together. As the molten rock from the earth's interior emerged, how- ever, the surface — undercut of its support — began to subside. The weight of these tens of thousands of feet of heavy rock con- tinued to cause the surface to sink slowly for millions of years JULY-AUGUST 197.S 13 along the direction of the old rift. In tlie Lake Superior region, this sinking tilted the hardened lava toward the center and created a lowland. Streams, eroding the now upturned edges of the lavas and the older rocks beyond, washed pebbles, sand, and silt into the sag — the Lake Superior Syncline — and built up deposits thousands of feet thick as it continued to subside. Mostly sandstone, these rocks today show cross-bedding, rip- ple marks, and mud cracks, and other evidence of their watery origin. Major remnants of these deposits can be seen in east- central Minnesota (Hinckley Sandstone), at Fond du Lac near Duluth, the Bayfield Peninsula and Apostle Islands of Wisconsin, and along the north shore of Michigan's Upper Peninsula. Finally, about 600 million years ago, the crust stopped sinking, the accumulation of sediment ceased, and the Lake Superior area became relatively stable. The sandstones in the Lake Superior Syncline — softer than the older igneous and metamorphic rocks — were slowly worn down by weathering and stream erosion. At the onset of the Pleistocene Ice Age — about a million years ago — the region was probably a broad plain surrounded by gentle uplands. A river system may have drained the plain toward the east. Now the stage was set for the final drama whose finale would be the creation of Lake Superior. Final Act. Just when the first ice sheets spread south from Canada across the Lake Superior plain isn't known, but several hundred thousand years ago these massive ice sheets had reached south to Illinois, Nebraska, and Kansas. Many different glacial advances were separated by periods of melting and mild climate. Only the last glacier, at the close of the Pleistocene, is recorded in the rock and land forms of northeastern Minnesota. The evidence shows that the ice flowed southwest following low- lands where the softer sandstones lay along the old continental rift. The glacier could have excavated this soft sandstone and shale down below the level eroded by rivers before the lee Age. Today, the lake bed is made up of glacial deposits and lake sediments that in places are hundreds of feet thick. One drilling attempt, made from a ship in 938 feet of water, penetrated 670 feet of glacial and lake deposits but didn't touch bedrock. This 14 THE MINNESOTA VOLUNTEER Patterns from the past. A rock outcrop near the mouth of the Lester River wears these ancient glacial scars. indicated that, in some places, the glaciers had eroded the basin down to as least 1,000 feet below sea level. The last glacier to fill the basin, at the close of the Pleistocene, began to melt back about 11,500 years ago. As the glacier re- ceded, streams and meltwater became ponded against the ice at the end of the basin in the vicinity of Duluth-Superior. Water in this new lake, called Glacial Lake Duluth. rose and spilled over the southern rim at Moose Lake in Minnesota, and at JULY-AUGUST 197.S 15 FIGURE 2 Striped area shows the probable extent of Glacial Lake Duluth which existed about 11,500 years ago as the continental glacier was melting back to the northeast. the Brule/St. Croix divide in Wisconsin, then drained into the Mississippi River (Figure 2). Within the basin, the glacier continued to melt gradually to the northeast. Eventually, it uncovered outlets to the east at lower elevations around the rim of the basin.