Ganymede is composed of roughly equal amounts of silicate rock and water ice. Its body has an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans together. Its surface has dark regions, with impact craters dated to four billion years ago. This covers about a third of the satellite. Lighter regions, crosscut by grooves and ridges and only slightly less ancient, cover the rest. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity caused by tidal heating.