Volcano Watch - Rootless shields and hornitos

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As many of you die-hard eruption fans already know, lava is no longer entering the ocean, for the first time since last May. The tube leading to the East Kupapa`u entry, chief attraction of the County's lava viewing site, began a gradual decline in early December, carrying less and less lava until it stopped completely by January 22.

The demise of the Kamoamoa entry was more abrupt, with activity fluctuating for the last half of January, then coming to a halt at the end of the month.

So where's the lava now? If you've been out in Puna on a clear night lately, the bright glow from the upper flow field (between Pu`u `O`o and the top of Pulama pali) gives the story away. Persistent breakouts from the upper portion of the tube, within 3 km (1.9 mi) of the Pu`u `O`o cone, began in early December, probably in response to constrictions forming within the lower extremities of the tube system.

The breakouts fed short flows that fanned out in all directions from the breakout point, forming broad low mounds of lava flows known as "rootless shields." These features are "rootless," because they form over a lava tube--a shallow, sub-horizontal tunnel--rather than over a vertical conduit feeding magma to the surface from deep within the volcano.

The rootless shields are forming in part because the tube is completely filled with lava due to the reduced carrying capacity of the lower end of the system. As the amount of lava traveling down the tube leading to East Kupapa`u declined, the lava level in the upper tube began to rise, and the shields began to grow. By last week, there were six rootless shields, three of which were active. The shields are merging together to form a broad, elongate ridge aligned along the tube system above the pali.

We've seen periods of shield-building before-most recently from September through November 1999. At that time, four shields formed at about the same elevation as the current crop and a little farther to the west. The largest of the 1999 shields was 20 m (66 ft) high, over 500 m (1640 ft) in diameter, and was topped by a flat ponded-lava surface 175 m (574 ft) across. The current shields haven't attained that size, but they're still growing.

In the last month, the upper flow field has also produced several hornitos--rootless spatter cones that form over lava tubes. The hornitos quickly attained heights as great as 8 meters (26 ft) and assumed fantastic shapes--tall, narrow spires and shrouded monk-like figures that provide a three-dimensional Rorschach test for bemused scientists.

Hornitos have been uncommon during this eruption, probably because they require both a full tube and lava that hasn't had much chance to lose gas as it travels down the tube. The latter requirement is rarely met. For many years, there was an active lava pond at Pu`u `O`o, where lava degassed as it reached the surface.

Pu`u `O`o hasn't had a persistent lava pond since 1999, but usually there are several skylights in the roof of the tube where gas can escape. By mid-January, however, all of the skylights in the upper tube were sealed by plugs of lava. Hornitos began to build a few days later. The gas-rich lava blew out blobs of spatter at weak points in the tube near the sites of the blocked skylights.

We can't say exactly how long the rootless shields and hornitos will remain active, but judging from the shields' predecessors in 1999, it probably won't be more than a month or two before a new tube forms to the coastal plain and robs the rootless shields of their supply. Before then, the Kamoamoa tube, which is still producing breakouts on the pali, may recapture its original share of the lava.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Glow from lava in the crater of Pu`u `O`o and the active ponds of the "rootless shields" frequently light the night sky. Overflows from the ponds feed multidirectional flows radial to the shields. Active flows are observed on Pulama pali, but none are seen in the coastal flats.

Four earthquakes were reported felt during the week ending on February 14. Residents of Captain Cook and Honalo felt an earthquake at 4:28 p.m. on February 9. The magnitude-2.4 earthquake was located 7 km (4.2 mi) north of Captain Cook at a depth of 28.54 km (17.1 mi). On February 11 at 3:37 p.m., a resident of Captain Cook felt an earthquake. The magnitude-2.0 temblor was located 5 km (3 mi) east of Captain Cook at a depth of 9.8 km (5.9 mi). Two earthquakes were felt by residents of Volcano and Glenwood on February 12. The first earthquake at 6:47 a.m. and the second at 9:52 a.m. had magnitudes of 2.5 and 2.8, respectively. Both earthquakes were located 8 km (4.8 mi) southeast of the summit of Kilauea Volcano at a depth of 14 km (8.3 mi).