Coming directly form the idea of Uniformitarianism is the fact the earth must be "very old". Processes that erode away mountain ranges and cut channels in the crust are not fast.

Uniformitarianism automatically gives rise to a very old age for the earth. We can measure the processes of geology today and then project backward to determine how long a particular transformation might take.

No, there is evidence that suggests there are catastrophic events from time to time. They are not necessarily one of a kind, rather they are infrequent, and thus harder to characterize.

The great meteor crater in Arizona was caused by one such event. A meteor (about 30 meters in diameter), striking the earth at about 15 meters per second velocity, could produce such a crater.

There is a theory that suggests a meteor hitting the earth about 66 million years ago may have hastened the extinction of the dinosaurs.

The theory is that the solar system and all of the other solar bodies were formed by the force of gravity causing atoms to coalesce. When the mass was great enough the atoms of helium and hydrogen under the influence of gravity created the sun. The heavier elements were also drawn together by gravity and formed more dense smaller bodies called planets.

Eventually most of the larger bodies coalesced into the planetary structures, but there are still "rogue" objects in the solar system (meteors, asteroids, and comets) From time to time two of these objects collide, the larger adds the mass of the smaller. This history is easily seen on the surface of the moon.