Simple craters are bowl-shaped with a raised outer rim.
Complex craters have a raised central peak surrounded by a trough and a fractured rim. Many of the largest craters are formed by the impact of meteorites . Two major impact events have occurred in the 20th cent. In 1908 at Tunguska near Lake Baykal one occurred that caused vast destruction of timber from its blast, and the other in 1947 at Sikhote-Alin also caused great damage. Craters that have been obliterated by erosion over thousands of years, leaving only a circular scar on the earth's surface, are called astroblemes .