#47 - Impact Craters

Amelia Creek Crater (ACC) plays a significant role in Miranda Chase #15, Wedgetail. But how big an impact did it have when it hit a half-billion years ago? Well, the answer was more interesting than I thought and sent me down a deep rabbit hole for most of a day. Here are a few highlights of what I've found:

ACC is approximately 20 kilometers across, placing it around 50th on the list of the 190 known impact craters. Curiously, they're finding new ones all the time. How? A vastly improved understanding of geology since I studied it in the 1970s. That was when Walter and Luis Walter Alvarez put forth their astonishing discovery of the vast Chicxulub Crater that led to the demise of the dinosaurs. The impactor is estimated to be ten kilometers across and made a crater 200 km wide--in addition to wiping out a huge percentage of terrestrial life.

Once you wander past planetary formation, the possible collision that may have broken off the moon, and other such cluttered early-solar-system issues, Chicxulub is about as big as it gets. 

But then I got curious about the various sizes, their craters, and the wider range of impact effects.

A few clarifying terms:

• Comet: A dirty snowball often moving around fifty kilometers per second
• Asteroid: A rock that won't form a tail, typically lazing along in the fifteen km/s range
• Meteors: A Meteroid is probably a small bit broken off bit of comet or asteroid. A Meteor is when it hits the atmosphere, burns up, and creates a shooting star. A Meteorite is that rare Meteor that survives to reach the planet's surface.

So, first I wander over to the Purdue/Imperial College of London Earth Impact Effects site. From there I learn that stony asteroids (kinda the middle ground between porous rock and iron) weigh in at about 3,000 kg/cubic meter (that's 6,600 pounds for a block a little over a yard square). It will likely be trucking along at 15-20 km/sec (10 more so miles per second, so 36,000 mph), and typically strike at a 45-degree angle. 

So, stony, 17 km/s, 45 degrees.

And I started fooling around with the side setting. Anything under a hundred meters across is probably going to explode in mid-air. Like the 1908 Tunguska Event in eastern Siberia: a stony asteroid probably 50 meters wide (half a football field or so), exploded 5-10 km in the air and leveled 2,000 sq km of forest. It never made it to the ground at all.

So, what made the Amelia Creek Crater?

I wandered over to https://neal.fun/asteroid-launcher/ and I began to play. 

At a 1.5 km across, with a stony asteroid hitting sedimentary rock at 20 km/s, I get a very nice crater at 21 km across. It's the other impacts that get really interesting.

• The fireball is 40 km across.
• Every tree with three hundred kilometers would catch fire. That's an area the size of France or Colorado.
• All homes and most buildings in an area that size would collapse.
• Trees out to 400 km would be leveled by the shock wave that would feel like an F5 tornado out to 200 km.
• Oh, and don't forget the accompanying 8.0 Earthquake

Australia's big. Roughly the size of the continental US. But the Amelia Creek event punched a 600 km diameter hole in the middle of it. 

Did I mention that ACC was only the 8th largest known impact site in Australia? Or that the Earth is 70% water so the majority of the big impacts were into the ocean? Boggled my mind a bit, which led me to one last place.

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