Why did Michigan get split in half?

Answer 1

Michigan was never 'split in half.' At one point, Michigan and Ohio were having territorial issues around which of them had claim to the Port of Toledo. The US Government determined that Ohio would get Toledo and Michigan would be given the territory of the Upper Peninsula in compensation. Given the vast number of natural resources, Michigan got the better deal.

Answer 2

The short answer is "glacial erosion" ... the formation of the Great Lakes that separate the two parts of Michigan from each other (and from Canada) seems to have occurred in two stages:

1. Somewhat over a billion years ago, tectonic plates separated to produce fractures in the earth's crust that eventually became LaKe Superior, Northern Lakes Huron and Michigan, and finally the chain of Lakes Erie and Ontario.

2. This was followed by hundreds of millions of years of ice age activity, huge glaciers over a mile thick sliding down from the north and grinding the terrain, which was then washed away by the vast water runoff when the glaciers melted ... This happened over and over, each time deepening the Great Lakes and shaping the two Michigan peninsulas between them ... The last great glacier occurred ten thousand years ago, and the geology of Michigan has been pretty much fixed since then.

For what it's worth, the lake basin surrounding Lower Michigan is politically two lakes, Lake Michigan and Lake Huron ... but geologically it's all one great lake, Lake Huron-Michigan, all the way from Chicago to Sarnia and Georgian Bay.