December 18, 2003
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Lo`ihi smaller and younger than Kilauea
Despite its notoriety as Hawai`i's youngest volcano, Lo`ihi 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, Lo`ihi is still 980 m (3,200 ft) below sea level.
Lo`ihi 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 Lo`ihi 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.
Lo`ihi'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 Punalu`u, a landfall only 34 km (21 mi) northwest of Lo`ihi's summit.
The volcano stands about 3.2 km (10,500 ft) above its base atop the sloping submarine flank of Mauna Loa and Kilauea. Some subsidence of the sea floor has resulted from the mass imposed by Lo`ihi, so the volcano's total thickness is about 3.5 km (11,480 ft).
Lo`ihi, 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 Lo`ihi if the volcanoes were placed side by side on a flat base. Another way to envision Lo`ihi's dimensions is to imagine a volcano stretching from Kilauea's summit to Kea`au, but standing 3,350 m high (11,000 ft).
Landsliding is rampant at Lo`ihi, affecting more than three-quarters of the volcano's flanks. Indeed, the most startling aspect of Lo`ihi'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 O`ahu's Ko`olau Range, viewed from Kaneohe, could also serve as a reminder about Lo`ihi's steep sides.
Where unaffected by landsliding, Lo`ihi'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 Kilauea 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, Lo`ihi 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, Kilauea has a volume ranging from 12,000 to 20,000 cubic kilometers (2,880-4,800 cu mi), or 15-30 times more voluminous. (Kilauean estimates vary greatly because the volcano's base is poorly defined.) Of course, Lo`ihi is still growing.
Lo`ihi 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 Lo`ihi 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.
Lo`ihi is sufficiently far from the centers of Mauna Loa and Kilauea 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 Lo`ihi growth and the expansion of Kilauea and Mauna Loa will create an isthmus between them at some later date, when Lo`ihi becomes part of the Big Island.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Fresh lava from the southwestern vents within Pu`u `O`o 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 Kapa`au 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.
Updated: December 21, 2003 (srb)