December 26. The Great Lakes

from NASA

The origin of the Great Lakes goes back hundreds of millions of years, to the Silurian and Ordovician Periods of the Paleozoic and even longer ago. You may recall the resistant Silurian rocks that surround the Michigan Basin – but today, the important rocks are the underlying strata, the less resistant beds.

Lake Superior is underlain by the Mid-Continent Rift System, established more than a billion years ago (we talked about it January 26 and 27). There’s another fundamental break, called the St. Lawrence Rift, that formed about 570 million years ago in very late Precambrian time. It underlies the St. Lawrence River valley in Canada, where it is still seismically active, and it may extend as a weak zone southwest to where Lakes Ontario and Erie are today. And finally, around the Lower Peninsula of Michigan, alternating layers of high and low resistance were laid down over Paleozoic time, and warped into the bowl-shaped Michigan Basin.  All these things gave glaciers some paths to follow that were easier than others. 


The Pleistocene continental ice sheets had several points of origin. In eastern Canada the Laurentide ice sheet was centered in northern Quebec, but the ice flowed thousands of miles to the south and west. When it reached the area of today’s Great Lakes, it went into the low-lying areas underlain by weaker rocks, digging those areas into basins where ice thickness was somewhat greater. The ice covered everything around Michigan, including the resistant Silurian rocks, but it eroded more in the weaker zones. There’s one little area in Wisconsin where two of the main lobes of the ice sheet came together but left a small zone ice-free, but completely surrounded by ice.  

from US Army Corps of Engineers

Glaciers came and went multiple times. When you hear about glaciers “retreating,” don’t think of it as backing up. With minor exceptions, glaciers don’t back up. Glacial retreat just means that the rate of melting at the ice front exceeds the rate at which the ice is advancing. In the Great Lakes area, advances and retreats eventually made the old basins noticeably lower than the surrounding highlands, and those lowlands were filled with glacial meltwater.

The modern lakes are fed by precipitation including snowmelt, as well as by the rivers in the region. They are the largest volume of surface fresh water on earth, accounting for about 21% of the total.

While we’re on the topic of lakes, the Finger Lakes of New York also owe their origin to the Pleistocene glaciation. It’s basically the same thing as the Great Lakes, but on a smaller scale. Pre-existing river valleys were widened and sculpted by valley glaciers, and natural dams were built that created the Finger Lakes. Those natural dams are called moraines, and they represent a place where a glacier was neither advancing nor retreating, but in a continuous state of melting in place. The ice carried with it all sorts of debris – rocks and soil – which was deposited into a mound called a moraine. When that pile was at the terminus of a long valley glacier, it ended up serving as a dam to create a lake in the valley where the glacier used to be.

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Today’s anniversary is one that many of you probably recall – the earthquake and ensuing tsunami on December 26, 2004, which killed an estimated 230,000 people in 14 countries. The earthquake was on the subduction zone off the northern coast of Sumatra, in Indonesia, where the Indian-Australian Plate, which is oceanic in that area, is subducting beneath an extension of the Eurasian Plate into southeast Asia and Indonesia. Most of the western and southern islands of Indonesia are volcanic, the magmatic arc that forms above the subduction zone. It’s complicated somewhat by the larger islands, especially Sumatra, having elements of more rigid crust, possibly even some small continental blocks. The quake’s magnitude of about 9.2 is the third largest ever recorded. More than 1,600 km, 1,000 miles, of fault broke and moved an estimated 15 meters, or 50 feet. That’s incredibly huge, and that sub-sea movement is what generated the tsunami. The tsunami was the most devastating aspect of the quake, and it traveled more than 5,000 miles. It was still 5 feet high in South Africa. The greatest number of casualties was in Indonesia, with about 131,000 deaths, but the tsunami killed at least 35,000 in Sri Lanka, 12,000 in India, 5,000 in Thailand, and nearly 100 on the coast of Africa.

—Richard I. Gibson

Drawing Public domain (US Army Corps of Engineers) ; Satellite image from NASA