View Full Version : Circular Meteor Craters
2004-Feb-14, 06:05 PM
I've seen a lot of meteors in my life streaking across the sky, but have never seen one plunge to earth like a bolt of lightening. If most meteors come in and impact at a low angle, how do they produce circular impact craters? Meteor Crater Arizona is circular in shape and has almost the identical size and topographic expression as Hole-in-the-Ground, Oregon. The Oregon crater is a maar created by a large steam explosion triggered by volcanic activity. The Arizona crater has been determined to be a meteor crater by the finding of some material outside the crater that is believed to be from the meteorite. The large craters that we observe on Mercury, Mars and our Moon all seem to be circular in shape and have the appearance of being identical to large calderas. The large impact area in Sudsbury, Ontario Canada (the source of most of the world's nickel) is elongate, which I would expect to be the shape of a meteor impact crater. Can a circular impact crater be created in a laboratory by a projectile impacting the surface at any angle other than vertical?
2004-Feb-14, 07:05 PM
Certainly, it can and has been. The impact (kinetic force) of an extremely high velocity heavy projectile makes the ground act like water, "splashing" out from the impact, but the walls of the crater are mainly formed by the "wave" of soil and rock, while weathering causes the higher points (caused by the splash) to erode faster, resulting in a more even altitude of the debris field and crater walls between the "upstrike" and "downstrike" edges. The more recent craters on the moon are a good place to observe the amount of "splash" caused by meteor impact.
Observe that a rock thrown into a pond at an angle less obtuse than 15 degrees DOES cause a circular wave on the surface. A metor would strike at a fair angle due to being slowed by atmosphere.
2004-Feb-14, 11:37 PM
I don't agree that circular craters can also be caused by steep angled impacts. On Earth as you say, erosion can change the shape into a evenly shaped crater, but that's doesn't work on the Moon, Mercury and lots of moons in the solar system. The majority of craters are circular, but a significant portion should be elongated or show uneven characteristics, something we don't actually see.
2004-Feb-15, 01:14 AM
I believe that Chixculub is irregular.
I can see your point VanderL, perhaps as the surface was molten/semimolten and that is how the crater settled? But I think that may be stretching it.
2004-Feb-16, 03:49 AM
I contacted Dr. Wood to ask him your question JESMKS and, as usual, here is his prompt response:
Impact craters are not splash craters. They are explosion craters. That is impactors themselves don’t dig the holes that are called craters. Instead, an object coming in to a planet carries a kinetic energy equal to 0.5 * (object’s mass) * (relative impact velocity)^2. Upon impact all of the kinetic energy must be immediately converted into a different form (e.g, Law of Conservation of Energy). This energy is consequently released in enormous explosions that can be hundreds, thousands, millions, or even billions of times the size of the Hiroshima bomb. The reason that impact craters are circular is because they are explosion craters NOT splash craters. All explosion craters are circular unless the attack angle is so shallow (<5 degrees) that the explosion can be directed.
2004-Feb-16, 08:35 AM
Once again Dr. Wood has well and truely clarrified for me this phenomena. this make sense - where else would the explosion go!??!
And it is interesting to note that 5 degrees inclination seems to be a threshhold of some sort.
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