From the Meteor Crater Website:

Approximately 50,000 years ago, on a continuous plain extending for miles in the high desert plateau of Northern Arizona, out of the northeastern sky, a pinpoint of light grew rapidly into a brilliant fireball. This body was probably broken off from an asteroid during an ancient collision in the main asteroid belt (between the planets, Mars and Jupiter) some half billion years ago. Hurtling about 40,000 miles per hour, it was on a rendezvous course with earth. In seconds, it passed through the earth's atmosphere with little loss of velocity or mass.

 In a blinding flash, a huge iron-nickel meteorite or dense cluster of meteorites, estimated to have been about 150 feet across and weighing several hundred thousand tons, struck the rocky plain with an explosive force greater than twenty million tons of TNT. Traveling at supersonic speed, this impact generated immensely powerful shock waves in the meteorite, the rock and the surrounding atmosphere. In the air, shock waves swept across the level plain devastating all in the meteor's path for a radius of several miles. In the ground, as the meteorite penetrated the rocky plain, pressures rose to over twenty million pounds per square inch, and both iron and rock experienced limited vaporization and extensive melting. Beyond the affected region, an enormous volume of rock underwent complete fragmentation and ejection.

 The result of these violent conditions was the excavation of a giant bowl shaped cavity. In less than a few seconds, a crater was carved into this once flat rocky plain. During its formation, over 175 million tons of limestone and sandstone were abruptly thrown out to form a continuous blanket of debris surrounding the crater for a distance of over a mile. Large blocks of limestone, the size of small houses were heaved onto the rim. Flat lying beds of rock in the crater walls were overturned in fractions of a second and uplifted permanently 150 feet. Fragments of rock and iron-nickel, some as large as a few feet across, were thrown several miles away. In some of the shocked meteorites, the intense pressures transformed small concentrations of graphite into microscopic sized diamonds. A dense hot cloud rose high above the crater carrying with it droplets of molten iron-nickel, pieces of molten rock, and abundant rock debris. This material rained down as fallout until the cloud drifted away and dissipated to the surrounding area. Meteorite fragments that separated early from the main mass during its passage through the atmosphere continued to fall at lower velocities on the crater and surrounding area during and immediately after the impact.