There would be seismic events triggered all over the world, and there are faults and volcanoes in the U.S. that could be triggered in this way. Impactors this large would incinerate an entire continent, and cause violent shockwaves in the atmosphere like those caused by nuclear weapons detonating, only far more powerful. If the event is large and close enough to the U.S., vast areas would be laid waste the same way Hiroshima and Nagasaki were by nuclear weapons. The crater formed would be some 40 or 50 miles across if it's an asteroid, and it would be much bigger if a comet struck because of the much greater impact velocity. Everything for hundreds of miles around the crater would either vaporize or ignite. Massive amounts of melted rock and shards of debris would be thrown into space which will travel in long ballistic tracjectories. These would fly out all over the globe before falling back into the atmosphere. Here we would see what looks at first like the mother of all meteor showers, but this would quickly result in everyone caught outside burning to death who didn't die in the secondary impacts from large chunks of debris forming secondary craters on the Earth. The infalling debris would not only strike the ground and set fires everywhere around the world, it would heat the very air to a temperature of at least 700 or 800 degrees here. The debris would also destroy every satellite and spacecraft orbiting the Earth, paralying government's attempts to cope with the disaster. If the impact occurs on a shoreline, mega-tsnamis initially more than a mile in height would result, and they would spread out across the ocean at supersonic speeds. As they spread out, they lose height but nevertheless they would rear up again to monsters hundreds of feet tall that will sweep inland for up to tens of miles, killing everyone in their path all around the entire Pacific basin. The smoke from massive firestorms started plus the pulverized rock from the impactor and the target rock it stuck would fill the atmosphere and cut out the sunlight, creating a nuclear winter worldwide. Even during daytime and the sky would be as black as the darkest night. The temperatures would drop until the Earth turns into a dark and frozen wasteland. Plants would die out, animals would soon follow. Everyone who didn't die in the impact would then face starvation and freezing to death because agriculture would be impossible. Fuel and electricity would be impossible to come by, at least enough to keep all of the survivors alive. The nuclear winter could last for months, years or even longer, nobody knows. A three mile wide impactor is quite enough to exterminate the human race and all higher plants and animals, and wiping the slate clean of all but the most hardy of lifeforms. They would survive the catastrophre and in time come to dominate the Earth. We exist today because a comet or asteroid eliminated the dinosaurs and at least 75 percent of all the other species living at that time. Since then our ancestrors took over and paved the way for us. It is possible we could go the way of the dinsosaur and some form of life alive right now could in time take our place at the top of the food chain.