The Yarrabubba impact crater is the oldest known impact crater in the world, and it was hard to find, hidden by over 2 billion years of overburden and erosion. Found it was, though. Those interested in the details can find them in this article in Nature Communications.
This certainly qualifies as a major disaster. The crater is estimated at 70 km in diameter, corresponding to an impactor about 7 km in diameter. This assumes close to the minimum velocity for an impactor, Earth’s escape velocity, according to the Earth Impact Calculator. There are a huge number of trade-offs between size and velocity which could produce a crater of the requisite size, but they don’t change the effects much, for the purposes of this post.
The timing here is the interesting bit. This impact happened right at the end of the Huronian Glaciation, a period when the entire Earth was a ball of ice. The timing raises the question of whether the impact caused the end of the glaciation, warming Earth again and paving the way for a resurgence of life from whatever hidden pockets it had retreated to for survival.
The secondary effects would have been global, far beyond the rim of the crater.
The impact woke up the neighbours in several different ways.
- It vapourised as much as 200 km3 of water and melted another 5400 km3.
- It could have produced a planetary class tsunami tens of metres high.
- How high is uncertain. The devil is in the details. Land impact vs. shallow water impact vs. deep water impact.
The word of the day in this discussion is albedo, the ratio of energy reflected to energy absorbed. Ice, and even more ice covered with snow, have a high albedo, reflecting as much as 80% of the sunlight which falls on them. That is, in fact, about the maximum for naturally occurring surfaces.
Open water, on the other hand, has a low albedo, approximately 10%. If you want to thaw out a planet, change a very wide area, as in a large percentage of the entire area of the planet, from an albedo of 70-80% to an albedo of 10% or so.
A tsunami would do this over a sizeable area where the water was shallow, less deep than the height of the wave. It would shatter the ice from underneath, probably throwing it aside and creating a large area of open water.
Exactly how large would depend on where the impactor hit. An impact right along a coastline, near areas of shallow water, would break ice and expose water all along the coastal area. Some of the coastal land area would probably be exposed, too.
A rough estimate, using Google Earth on the modern Western Australian coastline, gives an estimated area of about 20,000 km2. Considerable, to be sure, but still pretty small on a planetary scale.
There is a feedback loop here. My own rough calculations indicate that such a patch of open water would absorb enough solar energy to recede at about 100 metres per month. It would take thousands of years to become significant on a planetary scale, and the feedback loop could be interrupted by other events.
There is, however, another factor here which might have led to a much quicker and farther reaching climate change, with much faster results. There is the likely scenario that the strike happened on land, and indeed the present day crater is on land. No tsunami of note on a planetary scale. The impact does equate to a Richter 9 earthquake. That doesn’t mean no effects.
As stated above, the impact would have thrown out, melted and vapourized, about 7500 km3 of water. That’s a volume about one third of the Greenland ice cap on present day Earth, which equates to a sea level rise of about 2.5 metres, worldwide.
The key point here is that all that water would fall down and rain down on top of the ice.
It won’t be as neat and tidy as that. The water will form pools, and the shallower ones could just refreeze again. Still, even a third of Earth’s ocean surface covered in water seven or eight metres deep could reduce the albedo of that entire area from 80% to 10%.
Such an effect would melt a lot of ice, over a time frame that would make a noticeable difference even on human time scales. It would be more robust, too, and harder to interrupt.
Detailed analysis here is well beyond my expertise and resources. Any real scientists who happen by here are welcome to laugh at me and rip my figures apart, or take the idea and run with it if they think there is any use in it.
As to why I’m spending time on this? Well, the Master Blasters are contemplating taking a Contract. We’ll see if they do.
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