I spent yesterday afternoon shoveling my garage roof from more than a foot of snow that had accumulated since the Christmas Eve storm. Usually I’m on top of snow removal but had a bad cold at the time and just couldn’t muster the energy. I would have waited for spring to melt it away but the garage door started to jam against the ceiling joists after the last snowfall. It dawned on me that the weight of the snow had pushed the the roof and its supporting structures down just enough into the door opening to cause the jam.
It was hard work removing the compacted layers of old snow but I completed half the roof. The joists rebounded just enough to allow the door to move freely. It occured to me while I was chopping away up there that examples of rebound abound in nature. One is right on our doorstep here in Duluth: Lake Superior.
Satellite photo of Lake Superior taken yesterday just before 11 a.m. Like a tilting bathtub, water is slowly moving from the east end of the lake (right) to the west due to the crust rebounding since the glaciers departed some 10,000 years ago. Credit: NOAA/Space Science and Engineering Center, U. of Wis-Madison
In a recently published book by Dick Ojakangas titled Roadside Geology of Minnesota, I learned that the tremendous weight of the glaciers that once overlayed the lake depressed the crust. Since they began to recede some 10,000 years ago, the crust has been slowly rising back up to its pre-glacial level. But because the ice was thicker and lasted longer in the northeastern part of the lake basin, that part is experiencing a stronger rebound that the southwestern end of the lake. This makes the lake tilt toward the southwest causing water to rise at the Duluth end at the rate of a foot a century. In 500 years the grounds of the Duluth Entertainment Convention Center and new hockey arena will be sitting in five feet of water.
The crater Moltke is 4.3 miles across and a great example of a simple, bowl-shaped crater. The bumpy apron of material around the crater is impact ejecta — deposits of rock that were excavated during the impact. Credit: Apollo 10 / NASA
Anyone who’s ever looked at the moon through a telescope is familiar with craters. Almost all of them were formed by meteorites, comets and asteroids slamming into the moon’s surface at a speed of around 12 miles per second. At that speed, an impactor will blast out a crater 10 to 20 times larger than itself. Craters under 10 miles across look like simple bowls while larger craters will show additional features like terraced walls and central uplifts including mountain peaks. These peaks are some of the most dramatic features you can see through a small telescope. When the sun is just rising over the crater and its inner mountain peak catches the first rays of sunshine, you’re guaranteed to let out a wow.
One of the ways a crater’s central peak forms is through rebound of the crust. An asteroid impact depresses part of the lunar crust like the snow did to my garage roof except in a far more cataclysmic fashion. After rock is vaporized and blasted out to form a new crater, the crust rebounds into an elevated floor with one or more mountain peaks. A good example of a central peak crater is 52-mile wide Tycho (at right).
Craters larger than about 110 miles in diameter have ring-shaped uplifted zones in addition to peaks. The very largest craters are called impact basins and consist of muliple rings of ruptured crust. Lava from beneath the crust often wells up through the fractured rock and floods the impact’s center creating a bullseye-shaped pattern. The familiar dark spots that form the face of the man in the moon are great examples of impact basins.
This image, taken by NASA’s Clementine spacecraft, consists of color-coded topography overlaid on a shaded relief map of the Moon. Purples and blues are low, and orange and reds are high. Younger craters are superimposed on the basin. Credit: Clementine Science Group, Lunar and Planetary Institute
One of the largest impact basins in the solar system can be found just 238,000 miles away on the moon. Called the South Pole-Aitken basin, it measures a whopping 1500 miles across and 13 miles deep in some places. It’s located among countless craters on the lunar farside. Below are two more examples of large impact basins, one on Mars and the other on Jupiter’s moon Callisto.
The Hellas basin in Mars southern hemisphere is 1,300 miles across and up to six miles deep.The dark ring is some of the material thrown out from the impact. Credit: NASA
Valhalla on Callisto has more rings than any other impact basin known. They extend some 2500 miles across the moon which is just 3000 miles across. Credit: NASA
The next time you find yourself on the rebound, consider that you have plenty of company out there.