It is the second youngest product of the Hawaiian hotspot and the current eruptive center of the Hawaiian-Emperor seamount chain. Because it lacks topographic prominence and its activities historically coincided with those of Mauna Loa, Kīlauea was once thought to be a satellite of its much larger neighbor. Structurally, Kīlauea has a large, fairly recently formed caldera at its summit and two active rift zones, one extending east and the other west, as an active fault of unknown depth moving vertically an average of per year.