Craters are the footprints of a dramatic geologic event involving eruption, impact, or collapse. Broadly speaking, a crater is a depression in the Earth, but the word most readily evokes the interior of the steep-sided bowl produced in explosive volcanic eruptions. This classic shape (Mount Fuji is the prototype) can be seen today atop several active Alaskan volcanoes, including Shishaldin on the Aleutian island of Unimak. Shield volcanoes (such as Kilauea Volcano in Hawai‘i), which eject a more fluid basalt than composite volcanoes, form craters after erupted magma drains away and, over time, through the breakdown of shield walls. Shield volcanoes may also develop pit craters, if magma intrudes just below the surface and then withdraws, leaving an unsupported terrain prone to collapse. Until late in the nineteenth century it was believed that the moon’s craters were caused by volcanic activity. A later analysis of lunar craters, spurred by accumulating scientific evidence, suggested, for example, that a mysterious mile-wide depression near Flagstaff, Arizona, now called Barringer Meteorite Crater, was formed by the impact of a meteoroid. Over 150 large impact craters have now been identified on Earth, the largest being the Sudbury Crater in Ontario, Canada, around 140 miles in diameter.

Pamela Frierson