Where Are All the Impact Craters on Earth?

The Moon is pockmarked with impact craters. Everywhere you look, you can see a crater. On Earth, however, impact craters are few and far between, with barely any visible. Why do we see so many craters on the Moon but so few on Earth?

The Moon and Earth are in the same place in the solar system (i.e., same distance from the Sun and other planets). This means that both planetary bodies should have a similar density of craters on their surface, but only if their surfaces are the same age. The difference in the number of impact craters on the surface of the Moon (over 2 million craters) and Earth (~190 confirmed impact craters) means that their surface ages are very different. Said another way, the number of craters on a planetary surface is related to the average age of its crust. The higher number of craters visible on the Moon means that the surface of the Moon is older than the surface of Earth. The small number of craters on the Earth’s surface also means that it is a geologically active planet.

Two major processes control the removal of impact craters from Earth: (1) plate tectonics and (2) weathering. Plate tectonics is the major reason that we do not see many craters on Earth’s surface. Earth is broken up into tectonic plates, made up of continental and oceanic crust, that move across the surface like a conveyer belt. The continent portion of tectonic plates remains at the surface and the oceanic part of tectonic plates is recycled into the planet. As a result, the oldest continental crust is 4 billion years old, while the oldest oceanic crust is only 200 million years old. The continental crust has a much longer memory of Earth’s history compared with the oceanic crust.

The map of the distribution of impact craters on Earth shows that almost all the 190 craters are located on continental crust (can you find Tenoumer crater)? Like Earth, the Moon is 4.6 billion years old. Without plate tectonics, the ancient surface of the Moon is still mostly intact. The oldest terrain on the Moon is known as the lunar highlands. Looking up at a full Moon, the lighter material you see is the lunar highlands. This is also where most of the large craters on the Moon are located. Because the highlands are the oldest materials on the surface they have the longest memory of lunar history, and, therefore, the highest number of craters. Fun fact, only one Apollo mission directly explored the extensive lunar highlands: Apollo 16 (see the Apollo 16 landing site image above).

The second process responsible for the limited number of craters on Earth’s surface is weathering. Weathering is the process of water, in some form, breaking down rock. This process obscures the memory of Earth’s crust by eroding and moving material across the surface of the planet. Most of the impact craters on Earth are difficult to see and can only be identified by studying the nearby rocks to see through the weathering process. The Moon does not experience this kind of efficient weathering because it does not have an atmosphere or water flowing across its surface.

Because most craters are heavily weathered on Earth, it has taken a long time to recognize certain structures as impact craters. The town of Nördingen in Bavaria, Germany that dates to the 9^th century is centrally located in the Ries crater. It was not until the 1960s that the crater was recognized to be the result of an impact event, rather than it being a volcanic crater. Weathering on Earth has also downplayed the importance of impact structure to humans. Another impact crater, the Sudbury impact structure in Ontario Canada, is home to some of the world’s largest deposits of copper and nickel. Other valuable ore minerals, like the platinum group elements, are used in everything from jewelry to dentistry to vehicle exhaust catalysts.

Chicxulub crater is another important impact in the evolution of life on Earth, but we’ll save the story of the discovery of that crater and its impact (no pun intended) for another time.