Recent activity has mostly shifted to Kīlauea's eastern rift zone, the site of 24 historical eruptions, located mostly on its upper section; by contrast, the volcano's southwestern rift zone has been relatively quiet, and has only been the site of five events to date.Geologists have dated and documented dozens of major eruptions over the volcano's long history, bridging the long gap between Kīlauea's oldest known rock and only extremely recent written records and historical observation. Located along the southern shore of the island, the volcano is between 300,000 and 600,000 years old and emerged above sea level about 100,000 years ago.It is the second youngest product of the Hawaiian hotspot and the current eruptive center of the Hawaiian–Emperor seamount chain.Nonetheless, their proximity has led to a historical trend in which high activity at one volcano roughly coincides with low activity at the other.When Kīlauea lay dormant between 19, Mauna Loa became active, and when the latter remained quiet from 1952 to 1974, the reverse was true.Since then, the volcano's activity has likely been as it is now, a continual stream of effusive and explosive eruptions of roughly the same pattern as its activity in the last 200 or 300 years.
The eastern rift zone in particular is a dominant feature on the volcano; it is almost entirely covered in lava erupted in the last 400 years, and at its crest near the summit is 2 to 4 km (1 to 2 mi) wide.
The southwestern rift zone's extremity is also underwater, although its submarine length is more limited.
The southwestern rift zone also lacks a well-defined ridge line or a large number of pit craters, evidence that it is also geologically less active than the eastern rift zone.
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 125 km (78 mi) east and the other 35 km (22 mi) west, as an active fault of unknown depth moving vertically an average of 2 to 20 mm (0.1 to 0.8 in) per year.