Watch this slow motion video of a rock crashing into water. Take a minute to think what that might look like if you scaled up the size of the rock and the body of water. Then, take yourself further down the rabbit hole and imagine what’s actually taking place when an impact crater forms after a meteor strikes the Earth.
How Impact Craters Form
Unlike the rock in the video above, craters produced by the collision of a meteorite with the Earth (or another planet or moon) are called impact craters. The high-speed impact of a large meteorite compresses, or forces downward, a wide area of rock. The pressure pulverizes and compresses the rock. Almost immediately after the strike, however, the compressed rock rebounds. The rebound effect forces enormous amounts of shattered material to jet upwards, while a wide, circular crater forms where the rock once lay. Most of the material falls around the rim of the newly formed crater.
It is convenient to divide the impact process conceptually into three distinct stages: (1) initial contact and compression, (2) excavation, (3) modification and collapse. Usually, there is overlap between the three processes with, for example, the excavation of the crater continuing in some regions while modification and collapse is already underway in others.
The animation below illustrates the different stages of impact crater formation.
One of the most striking impact craters in North America is Meteor Crater, in Arizona. It’s roughly 1,200 m (3,900 ft) in diameter, 170 m deep (570 ft), and is surrounded by a rim that rises 45 m (148 ft) above the surrounding plains. The center of the crater is filled with 210–240 m (690–790 ft) of rubble lying above crater bedrock. The crater was created about 50,000 years ago during the Pleistocene epoch, and the impactor’s diameter was approximately 50m (160 ft).
So an object of 50 m produced an impact crater roughly 24 times it’s diameter. Is it the size of the meteor that impacts the size of the crater, or is there more to the story?
What determines the size and shape of an impact crater?
The size and shape of the crater, and the amount of material excavated, depends on factors such as the velocity and mass of the impacting body and the geology of the surface. The faster the incoming impactor, the larger the crater. Typically, materials from space hit Earth at about 20 kilometers (slightly more than 12 miles) per second. Such a high-speed impact produces a crater that is approximately 20 times larger in diameter than the impacting object – as we have seen with Meteor Crater. In the case of smaller planets which have less gravitational “pull” than larger planets, impactors will strike at lower speeds. Additionally, the greater the mass of the impactor, the greater the size of crater.
A projectile (like the tiny plastic sphere in the image below) produced the crater in a laboratory hypervelocity impact experiment. The impact velocity was 1,250 m/s, and the target was flour. Note the remarkable difference to the crater-projectile ratio.
What would happen if an obscenely large impact occurred?
What happens when you have a really, really, really large impact? Complete devastation for everything on Earth, with the possibility of moon formation.
It’s possible the Earth’s moon formed in such a manner. In the giant impact scenario, the Moon forms from debris ejected into an Earth-orbiting disk by the collision of a smaller proto-planet with the early Earth. Earlier models found that most or much of the disk material would have originated from the Mars-sized impacting body, whose composition likely would have differed substantially from that of Earth.