Asteroid impact. How big does it have to be to globally affect plate tectonics?

I think we've established that this monstrous object is going to have to be pretty darn big. You suggested that it could be 500 kilometers in diameter; let's do the calculations to figure out just what the effects would be.

From Wikipedia, the median velocity of an oncoming object hitting Earth is about 22.5 km/s. From here, we can assume hat the asteroid (because this thing is going to have to be big) has a density of about 2 grams per cubic centimeter. Let's also say that this thing hits the Earth perpendicularly - i.e. it is a direct hit. We know that the plates are about 100 kilometers thick; therefore, we need the crater to be that deep if we want to throw everything into total chaos.

Using this handy-dandy calculator, we find that an impact with those parameters would create a crater 529 kilometers deep and 2,115 kilometers wide. Now that is what I call big. Earth's average radius is about 6,370 kilometers, so while this might not split the planet in two, it would give it a devastating blow.

We can adjust the values of the body to accommodate the results you want. If the crater will be about 100 km deep, we only need an asteroid 80 km wide, creating a crater with a diameter of about 407 km. The material ejected by the impact would be spread out over 874 kilometers, creating a sizable dent in a continent.

That's all just accounting for the crater. I suspect you would need a larger asteroid to do the kind of geological damage you're suggesting. But an asteroid even only 80 km wide would most likely throw the Earth into an impact winter longer than it has ever seen before. The object that killed the dinosaurs was probably only about 10 km in diameter. I'm a bit scared about what could happen if an asteroid 80 km wide hit Earth. Most likely, as you hypothesized, all life would be wiped away.

posted over 9 years ago

HDE 226868‭

586 reputation 61 463 77 49

I'm going to talk about Earth below, but since you say "an Earth-like planet", consider that just an space-saving measure.

Take an Earth-like planet, about the same size, composition and gravity.

...

Huge (say 500 kilometer diameter) asteroid strikes.

Well, as it's said in the movie, oh shit - there goes the planet.

An impact crater is:

an approximately circular depression in the surface of a planet, moon or other solid body in the Solar System, formed by the hypervelocity impact of a smaller body with the surface.

The "approximately circular" part means that by definition, for an impact crater to form you need a more or less head-on collision. You can't simply "scrape" the target body, even if that counts as an impact, because that will create a very much elongenated shear rather than a circular depression which thus is not an impact crater but rather something else. This would appear to rule out an event similar to that described by the Moon's origin Giant Impact hypothesis, since that body didn't hit the Earth directly but rather scraped along the surface. Borrowing from the former linked page:

Computer simulations show a need for a glancing blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: trillions of tons of material would have been vaporized and melted. In parts of the Earth the temperature would have risen to 10,000 °C (18,000 °F).

In our solar system as of today, a reasonable point of comparison to what you are proposing would appear to be Saturn's moon Mimas. Mimas has a diameter of just under 400 km and an impact crater (Herschel) 130-140 km across. Quoting the Wikipedia page on Mimas:

If there were a crater of an equivalent scale on Earth it would be over 4,000 kilometres (2,500 mi) in diameter, wider than Australia.

Also:

[The Herschel crater] is so large that astronomers have expressed surprise that Mimas was not shattered by the impact that caused it.

I don't see any estimates on the size of the impact body, but it's probably a safe bet that it was a lot smaller than the resultant crater. If we play nice and say that the impact body was half the size of the resultant crater (it probably was quite a bit smaller than that), that makes the impact body approximately 70 km diameter. What you are proposing is an impact of a body on the order of ten times that size.

This is Mimas, showing Herschel to the Cassini probe (image courtesy NASA, photo ID PIA12570):

An impact on Earth by such an asteroid, under that assumption, would result in a crater a thousand kilometers across. A more reasonable guess is probably a factor of ten times, which means that if the Earth survived, the crater would be on the order of 5000 km across.

The diameter of the Earth is a little less than 13000 km.

Under those assumptions, the crater would represent more than a third of Earth's diameter, quite comparable to Herschel in comparison to Mimas. If the size amplification of the crater compared to the impact body is even larger, the crater caused by a 500 km body impact becomes larger than that. I think it stands to reason that a crater around a third of the diameter of the body it appears on is as large as it can become, based on the fact that to my knowledge, we are not aware of any larger craters anywhere in the solar system.

If the Earth would survive such an impact, I imagine that scientists would be equally surprised as they are about the Herschel crater. (And of course, such an impact would wreak complete havoc with the environment, but you are asking about plate tectonics.) Or said in another way: I doubt the planet would survive the impact, so plate tectonics don't enter into the picture.

posted over 9 years ago

Canina‭

569 reputation 35 72 69 39