Both of them are roughly circular craters more than a kilometer in diameter. Both are in the western United States. They are relatively near in age, only a few tens of thousands of years old. Each appears to have been cut through a relatively hard layer of rock near the surface, with some weaker rock underneath (note the undercutting visible on the far side of both craters). Yet this is where the similarities end.
The first picture is of Meteor Crater, which I wrote about a few months ago. It was formed by this:
Perhaps it would be more accurate to say that's what's left of what formed it, a 150 foot wide piece of iron ore that struck Earth 20,000 to 50,000 years ago.
The second photo is of a crater called Hole In The Ground, which is in eastern Oregon. It was formed between 13,500 and 18,000 years ago. Just looking at its similarity to Meteor Crater, you might suspect that it too, was formed by a big rock hitting the Earth. You'd be wrong, though. It was formed by something like this:
That sign shows how Mt. Mazama was turned into what became Crater Lake, but it's the same idea. Big, giant, kabloowie, forming a crater.
In fact, the first geologists to encounter Meteor Crater thought that it, too, was a volcano. It wasn't until they explored under it that they realized this wasn't the case. At the time I was visiting it, I was wondering how anyone could think this hadn't been done by a meteor, because it looked so much like a huge bomb crater.
Then someone showed me Hole In The Ground.
What can geology teach us here? It teaches us that things that look the same aren't necessarily the same. Even when you think about it really hard, you can still be wrong. It's only through forming a hypothesis, and then testing it against what that hypothesis means that you can figure out if you're right or not.
That's how science works. It's why we know that these similar-looking things actually have very different stories.
A recent stupid conversation with someone at another blog brought this example to mind again, so I figured it was time to put this up here. The nature of the argument isn't important - in fact, I'm willing to bet that the individual I was having it with will never figure this out. Still, it's one of those lessons that everyone should try to remember when they're telling me what a naive or stupid person I am for not believing what they've concluded because what they've seen is sorta, kinda like something else they sorta, kinda know about.
UPDATE/Afterword (Mar. 19, 2013): For anyone interested in visiting Hole In The Ground, Lockwood Dewitt provides a reference:
The reason I chose this photo as an introduction to Hole-in-the-Ground can be seen above that little tree past and to the left of the central playa, on the opposite wall. The offset in that layer of basalt marks a fault which bisects this crater, and it's thought that this was the weak feature that allowed the basaltic magma to migrate upwards at this spot. More detailed information on HIG can be found in USGS Circular 838, at mile 25.1 in this road log.
Geo 365: March 19, Day 78: Hole-In-The-Ground
I reckon that Lockwood will be writing more about this crater in the days ahead.
It's a bit out of the way, to say the least, but at least it's not a tourist trap.
UPDATE 2 (Mar. 19, 2013): At the previous link, Lockwood Dewitt provides this correction:
The mode of formation is different from Crater Lake- that is a collapse feature, a caldera, while this is an explosive feature, a true crater. (Though looking at the cross-section in that road log, it definitely looks like subsidence was involved.)
Geo 365: March 19, Day 78: Hole-In-The-Ground: Comment by Lockwood
I've corrected that part of the narrative about Hole In The Ground's formation to no longer say " followed by a collapse".
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