<P> Kīlauea has a large summit caldera, measuring 4 by 3.2 km (2 by 2 mi) with walls up to 120 m (400 ft) high, breached by lava flows on the southwestern side . It is unknown if the caldera was always there or if it is a relatively recent feature, and it is possible that it has come and gone throughout Kīlauea's eruptive history; what is known is that it likely formed over several centuries, with its construction estimated to have begun about 500 years ago, and that its present form was finalized by a particularly powerful eruption in 1790 . A major feature within the caldera is Halema ʻumaʻu Crater, a large pit crater and one of Kīlauea's most historically active eruption centers . The crater is approximately 920 m (3,018 ft) in diameter and 85 m (279 ft) deep, but its form has varied widely through its eruptive history; the floor of the Halema ʻumaʻu Crater is now mostly covered by flows from its most recent eruption, in 1974 . </P> <P> Kīlauea has two rift zones radiating from its summit, one leading 125 km (78 mi) out to the east, the other 35 km (22 mi) long and trending towards the southwest . Tectonic extension along both rift zones is causing Kīlauea's bulk to slowly slide seaward off its southern flank at a rate of about 6 to 10 cm (2 to 4 in) per year, centered on a basal décollement fault 7 to 9 km (4 to 6 mi) beneath the volcano's surface . 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 . Non-localized eruptions, typical of rift zone activity, have produced a series of low - lying ridges down the majority of the east rift zone's length . Its upper segment is the most presently active section of the volcano, and is additionally the site of a number of large pit craters; its lower extremity reaches down Kīlauea's submerged flank to a depth of more than 5,000 m (16,400 ft). By contrast, the much smaller southwestern rift has been quiet since a rifting episode in 1974, and to date has not been involved in the current eruptive cycle at all . 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 . </P> <P> A prominent structure on Kīlauea's southern flank is the Hilina fault system, a highly active fault moving vertically an average of 2 to 20 mm (0.1 to 0.8 in) per year along the system . Its physiographic province is 500 m (1,640 ft) deep, but it is unknown if it is a shallow listric fault or if it penetrates to the very base of the volcano . A number of cinder cones, satellite shields, lava tubes, and other eruptive structures also dot the volcano, evidence of its recent activity . Kīlauea has some interactions with Mauna Loa, its larger neighbor and only other recently active volcano on the island; interspersed lava flows and ash deposits belonging to its neighbor have been found on its flanks, and some of Mauna Loa's flows are, in turn, blanketed in Kīlauea tephra . In particular, the saddle between the two volcanoes is currently depressed, and is likely to fill over in the future . </P> <P> All historical eruptions at Kīlauea have occurred at one of three places: its summit caldera, its eastern rift zone, or its southwestern rift zone . Half of Kīlauea's historical eruptions have occurred at or near Kīlauea's summit caldera . Activity there was nearly continuous for much of the 19th century, capped by a massive explosive eruption in 1924 before petering out by 1934 . 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 . </P>

What type of activity is indicating that mauna loa may be entering a new period of volcanic activity
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