Tycho (at center) is one of the most prominent craters on the moon. Measuring about 50 miles across, it has a distinctive central peak. Photo: William Wiethoff
Two or our readers and regular contributors, William Wiethoff and Jim Schaff, sent along a few photos they took recently of the moon through their telescopes. When I saw Will’s photo of Tycho and its central peak, I was reminded of a crater in our very own neighborhood. Ever hear of the Slate Islands? They’re up in northern Lake Superior about 100 miles east of Thunder Bay, Ontario, and a favorite place for kayakers and paddlers.
The Slate Islands are located in northern Lake Superior in Ontario, Canada. Credit: NASA Landsat image
Scientists studying the rock formations on the islands discovered something amazing. It appears they’re actually the remains of an asteroid impact 450 million years ago. The islands are at the center of a 20-mile-diameter crater, the eroded remains of which are hidden beneath the restless lake.
Take a look at Tycho again (at right). See that peak in the middle? That mountain rose upward after the impact that made the crater. When a planet’s crust gets hit by a sizeable space rock, the force of impact not only excavates a huge hole (crater) but compresses and shocks the rock beneath. Over time that rock relaxes and rebounds upward, forming a central mountain peak. Lots of mid-sized lunar craters sport mountains, and that’s exactly what happened with the Slate Islands — they’re crustal rocks that rebounded after being struck by an asteroid.
One of the key ways scientists identify whether or not something is a crater on Earth is by examining thin slices of quartz and other rocks from the site under a microscope. Only the enormous pressure from a cosmic impact can change the microscopic crystal structure from its normal pattern to a distinctive cross-hatching of intersecting parallel lines. Shocked quartz was discovered in nuclear bomb tests in the 1940s, and later used to identify impacts, including the islands. A slice of it is pictured in the NASA photo above.
The view out the Space Shuttle window looking east over Lake Superior. The ring around the Slate Islands shows the outline of the original 20-mile-wide crater. Credit: NASA
Other signs of the impact include shattercones, created when high speed shockwaves from the asteroid or meteorite impact, shatter the bedrock into a series of grooved, nested cones.
The conical rock along the shoreline of one of the islands
is a 30-foot tall shattercone, one of the largest in the world.
Credit: Wiki Commons/Zatoichi26
Even though all evidence of the actual asteroid are long gone, it left "tracks" in the rocks and rock formations that can by read by careful analysis. The Slate Islands are one of 59 impacts uncovered in North America. Learn more here.
The next time you take a trip around Lake Superior, you may want to consider stopping over at Slate Islands Provincial Park . Its now-tranquil setting masks the cataclysm of that long-ago day when Earth plowed headlong into an intruder from space.
McGreevy Harbour in the Slate Islands today. Credit: Wiki Commons/Zatoichi26