It turns out that “rocks running into other rocks” is a pretty complicated process. The most important aspect of the process is how much energy is involved. The two factors in finding the energy of an impact are (1) how much mass is in the impactor, and (2) how fast it is moving. The bigger and faster the object, the more energy is involved in the collision. This is no different from cars on a highway – faster moving cars, or bigger cars, will do more damage when they hit another car than those that are slower moving, or smaller. That’s because more energy is involved with collisions by the larger, faster cars.
In almost all cases, the energy of an impact event between two planetary bodies is very high, even if the impactor is relatively small. In spite of their size, the speeds between bodies in the solar system are incredibly fast. For example, impactors that strike the Moon are coming in at speeds like 10 miles per second (17 k/s), or 36,000 miles per hour.
That is so much energy that when the impactor hits the target, it does not simply break itself and the ground to pieces, it actually explodes. The best models of impact events are explosions. In this explosion the impactor and part of the target can be vaporized, turned into a gas, and the ground itself blows out into a huge depression in the ground. The material blown out is called “ejecta,” and some ejecta known as “rays” can end up a thousand miles away from the impact site, or more.
One of the key preconceptions people have about impact craters is that they are somehow “dug out” but another big rock. But as we see here, digging is not involved, and explosions are! That’s one of the reasons why impact craters are almost always circles. Even though the impactors are coming in from all directions, once they contact the ground the explosion starts and throws material out all around. Only those impactors that are coming in from the steepest angle (very low to the ground), create craters that have a lopsided shape and a strange ejecta pattern.