# Volcano Watch — Loihi smaller and younger than Kīlauea

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Despite its notoriety as Hawaii's youngest volcano, Loihi remains a submarine mystery for most of us. This is because fieldwork there is limited to manned or remotely operated vehicles. At its shallowest depth, Loihi is still 980 m (3,200 ft) below sea level.

Loihi has had earthquake swarms almost every year since 1980. The most intense seismic activity ever recorded by HVO began in 1996, when 4,377 earthquakes shook the summit of Loihi from mid-July to mid-August. Over 100 of these temblors were larger than magnitude 4. Subsequent submersible dives have discovered a newly formed small crater and active thermal springs.

Loihi's summit is a plateau about 3 km (2 mi) wide and 5 km (3 km) long, lying 40 km (25 mi) due south of Halape. If going by kayak, put in at Punaluu, a landfall only 34 km (21 mi) northwest of Loihi's summit.

The volcano stands about 3.2 km (10,500 ft) above its base atop the sloping submarine flank of Mauna Loa and Kīlauea. Some subsidence of the sea floor has resulted from the mass imposed by Loihi, so the volcano's total thickness is about 3.5 km (11,480 ft).

Loihi, which means "long" in Hawaiian, is a narrow ridge 18 km at greatest breadth and 32 km in length. A useful Big Island comparison is to imagine the subaerial outline of Kohala volcano, which reaches from Waimea to Hawi. Height is another matter, however; Kohala's summit is only about 1,600 m (5,250 ft), half as high as submarine Loihi if the volcanoes were placed side by side on a flat base. Another way to envision Loihi's dimensions is to imagine a volcano stretching from Kīlauea's summit to Keaau, but standing 3,350 m high (11,000 ft).

Landsliding is rampant at Loihi, affecting more than three-quarters of the volcano's flanks. Indeed, the most startling aspect of Loihi's shape is its knifelike form. Slopes of 35-40 degrees are common. To find slopes this steep, drive along the Chain of Craters road where it plunges over the Hilina Pali escarpments. The precipitous slopes of Oahu's Koolau Range, viewed from Kaneohe, could also serve as a reminder about Loihi's steep sides.

Where unaffected by landsliding, Loihi's slopes are about 10-14 degrees. Such slopes are characteristic of the submarine parts of Hawaiian shield volcanoes. A similar average slope is found for Kīlauea and Mauna Loa at their 500-m water depth. In contrast, the subaerial parts of shield-stage volcanoes are commonly much gentler, about 4 degrees.

Volumetrically, Loihi occupies about 715 cubic kilometers (172 cu mi), making it fourth-smallest of the volcanoes in the Hawaiian-Emperor seamount chain. (Smaller are three unnamed seamounts near the bend in the Hawaiian-Emperor chain.) For local comparison, Kīlauea has a volume ranging from 12,000 to 20,000 cubic kilometers (2,880-4,800 cu mi), or 15-30 times more voluminous. (Kīlauean estimates vary greatly because the volcano's base is poorly defined.) Of course, Loihi is still growing.

Loihi is undated, so its age remains a matter of speculation. Most commonly cited is an age of 100,000-150,000 years, an estimate based on rates of accumulation (meters of upbuilding per year) known from other volcanoes. Hawaiian volcanoes are thought to breach the sea surface about 300,000 years after their birth, give or take about 100,000 years. If these assumptions hold true, then Loihi is about halfway to islandhood, which must await another 150,000 years, despite having grown nearly three-quarters of the way from ocean floor to sea surface.

Loihi is sufficiently far from the centers of Mauna Loa and Kīlauea that it will emerge through sea level and become its own little island before those two volcanoes can spread their arid flanks southward. But continued Loihi growth and the expansion of Kīlauea and Mauna Loa will create an isthmus between them at some later date, when Loihi becomes part of the Big Island.

### Volcano Activity Update

Eruptive activity at the Puu Oo vent of Kīlauea Volcano continued unabated during the past week. Fresh lava from the southwestern vents within Puu Oo coated a quarter of the crater floor. Blebs of molten material were seen being tossed from the West Gap Pit vents, located immediately outside the crater. Lava from the rootless shield complex at the top of the Mother's Day Flow expanded the margin of the flow field another 400 meters (yards) westward. No active flows are on Pulama pali or the coastal flat below Paliuli. No lava is entering the ocean.

One earthquake was reported felt in the week ending on December 18. Residents of Waimea, Kamuela and Kapaau were gently rocked by a magnitude-2.6 earthquake at 5:25 a.m. on Sunday, December 14. The temblor was located 7 km (4.2 mi) northeast of Waimea at a depth of 12 km (7.2 mi).

Mauna Loa is not erupting. The summit region continues to inflate. Seismic activity remains very low, with no earthquakes located in the summit area during the last seven days. Visit our website (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time earthquake information.