Impact craters generally have a rim with ejecta around them, in contrast volcanic craters usually do not have a rim or ejecta deposits. As craters get larger (greater than 10 km in diameter) they usually have a central peak.[2] The peak is caused by a rebound of the crater floor following the impact.[3]
Porter Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Gullies are visible in the upper left.
Gullies in Porter Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous photo.