According to National geographic, an anniversary is coming. Not sure which anniversary, but it’s a biggie – the anniversary of the extinction of the dinosaurs. Well, at least the anniversary of the Chicxulub crater, which is about 180 km wide. This would be caused by an impactor of about 10 km diameter, which perhaps shows how fierce these impacts were. Of course, there are arguments over whether it was the asteroid that killed the dinosaurs, but it certainly would not have helped. It was about the same time that in India the Deccan traps formed, where huge amounts of basalt were extruded out from the mantle.
So, what happens when an asteroid strikes? We have some idea relating to smaller ones because hydrogen bomb tests have provided evidence of the consequences of the localised production of heat equivalent to the low hundreds of Mt of TNT. At the point of impact extremely intense pressure is generated, which is transmitted into each body by waves. As the smaller body enters the major body, all points of contact lead to the generation of pressure waves that radiate into both bodies. Waves from different point sources will lead to vibrations in different directions, the vibrations of the rock become too great and the rock simply pulverises, which leads to the absorption of energy. The next waves have to travel through pulverized material, wave interference results, and huge amounts of energy are absorbed. Modelling from the hydrogen bomb data leads to the following conclusions. At a velocity of 10 km/s, an impactor of diameter of 1 km will generate a crater 12.2 km diameter and 0.6 km deep; an impactor of diameter of 0.1 km would generate a crater of diameter of 2.6 km and depth of 0.55 km. As a final example, with an impact velocity of 5 km/s and an impact diameter of 2 km, the crater diameter would be 11.5 km and of depth 4 km. Once the distance is big enough that the impact acts as a point source, the shock wave continues through to the other side of the body, where it can be reflected, or disrupt the surface. It may be that the reason the Deccan traps occurred at about the same time may be because the asteroid caused the eruption. I think that on Mars the Tharsis volcanic field was caused by the Hellas impactor, and maybe Elysium by the Argyre impact. They are more or less on opposite sides of the planet.
Anyway, the asteroid vaporized just about everything above a layer of granite, and it dug an impressive hole in that too. There was not only the impact, but apparently the seafloor where it landed would have had considerable amounts of gypsum, and that would have vaporized and probably pyrolyzed. The net result was from the combination extreme acid rain, helped by the Deccan Traps, there was ocean acidification, and this led to a collapse of plant production, the base of the food chain. Anything large would die of starvation. The survivors tended to be the small, the rat-sized mammals and birds.
The time taken for extinctions is controversial because there are no continuous fossil beds for the period, and the probability that an animal will be fossilized is very small. Accordingly, there may have been small numbers of animals that lingered on. There was also a general decline before the impact, perhaps because of climate change, and many of the dinosaurs had got so big any minor change would prevent them getting enough food. Prior to the impact, the average temperatures rose rapidly by three to four degrees Centigrade, and that would greatly affect plant production. Plants cannot migrate other than through their seeds being transferred, so it is possible very large numbers of plants were too stressed to be productive. Alternatively, plants may have evolved to be less nutritious. When you are slow moving and need tonnes of food, any small adverse change can be deadly. Marine animals were particularly susceptible, and ichthyosaurs became extinct well before the impact.
We don’t know when the impact occurred to within a few tens of millions of years, so what was that about an anniversary? There is one site in North Dakota where there are fossilized bones of fish, and they perished just as they were speeding up a growth spurt, which would arise due to an increase in the food supply. Bone tissue made during a growth spurt is spongier. That suggests they died in the Northern spring. So we know when in the year, but not which year. Apparently sturgeon and paddlefish died with debris of tektites embedded in their gills. These tektites (which are glassy globs) were thrown up by the heat of the impact, but would start to come back down after about fifteen minutes and would soon stop. For these fish to have tektites embedded, they died almost immediately after the impact. Further, all the bodies face one way in one layer, and the other way in the next layer. There were huge tidal waves sloshing around all the way up to the Dakotas. (Note that this part of North America was a river valley.)
The Southern Hemisphere would have an advantage here, because going into autumn, life is getting ready for winter, it has fattened up, and it is ready to hunker down. That might give some advantages, nevertheless it did not seem to. The extermination of life here was just as severe. Yet oddly enough we have the tuatara in New Zealand, the only remaining species from the order Rhynchocephalia which originated in the Triassic, about 250 million years ago. It is a rather slow-moving animal, so maybe it low energy requirements got it through this disastrous period.