The high speed at which an impactor hits a target surface—a planet, moon, or other body—creates a shock wave and ejects material from the crater site. This produces a tremendous amount of heat that can melt or vaporize some material. The size and velocity of the impactor, the geology of the target surface, and the gravity field of the target body are all factors that influence the structure of an impact crater.
Impact craters typically have raised rims and floors that are lower than the surrounding ground. The debris of ejected material—called ejecta—falls back to the surface. This accounts for the piles of rock around the circular rim. Impact craters may also display a system of rays—bright, radial streaks of ejecta that can extend for great distances. Some large impact craters also have central peaks, uplifts, or terraces on their interior walls.
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Watch this video about the formation and evolution of the Moon.
After watching the video, explain how lunar maria formed and describe how more recent impactors have affected the Moon's surface. How do you think impact craters can be useful tools for establishing the relative ages of surfaces? Write your response in the notes box below.