January 24, 2008
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Rootless Shields are not a Gang of Nomadic Warriors
Until last November, the current eruption was characterized by a perched lava channel. The fissure system that opened up just east of Pu`u `O`o on July 21, 2007 issued lava into an open channel that frequently overflowed. These lava overflows, over time, built confining levees that elevated, or perched, the lava stream more than 30 m (100 feet) above the pre-July 21 ground level.
The formation of this perched lava channel is unique in the 25-year history of the current eruption. Among Kilauea eruptions, this feature would be enough to distinguish the current activity. But not to be outdone, the eruption is producing a new class of geological features that is no less unique and visually stunning. These features are known as rootless shields.
Shield-like features are familiar formations of Hawaiian volcanoes, but rootless shields are less familiar. Indeed, Hawaiian volcanoes are generally referred to as shield volcanoes due to their broad, gently-sloping sides. The shield-like nature is the result of frequently erupting sheets of very fluid lava that, over time, cover extensive areas in a radial pattern. If you've been to the southern part of the Big Island on a clear day, you've seen the beautiful, sloping shield of Mauna Loa Volcano that completely dominates the landscape. Mauna Loa's shape was acquired in just such a way.
Rootless shields are smaller, shield-shaped mounds that form on active lava flows. But, in contrast to shield volcanoes like Mauna Loa, which have deep roots into magma chambers below, rootless shields are fed by shallow lava tubes that flow just below the surface. Thus, they are "rootless," because the source of their lava is not a deep conduit.
Once the Thanksgiving Eve breakout started, a shield began to form directly over the breakout point on Fissure D of the July 21st eruption-the ultimate source of lava flows since July 21, 2007. It was fed directly from fissure D and was, thus, not rootless. Within weeks, a second, third, and fourth shield formed, each at or near the downhill base of the previous shield. This resulted in a chain of shields that step downslope and trace the lava tubes formed by the Thanksgiving Eve breakout. Because these shields are built over a lava tube, and not a deep magma source, they are considered rootless shields. To date, there are six well-defined rootless shields-four on the main branch of the tube which extends towards the Royal Gardens subdivision, and two more on a tube that branches off and from the main tube.
The shields are low-lying and can be difficult to spot from the air, but are unmistakable from the ground. The largest is about 40 m (approximately 130 feet) high, and 400 m (approximately 1300 feet) wide. Amazingly, shields like this can be built in a week!
The rootless shields are not entirely stable. Like a dam, they may rupture or collapse, disgorging lava from their interiors. For example, it was the rupture of the southeastern-most rootless shield that sent lava flowing toward the Royal Gardens subdivision earlier in January.
Why rootless shields are forming in this area is a bit of a mystery. We suspect that their formation is controlled by a combination of land-slope and rate of lava supply. These factors are known to contribute to the formation of perched lava ponds, which often develop on the flanks or summit of shields.
Despite their impressive growth, an important implication of these rootless shields is that their formation prevents the flow from heading downhill. In other words, more lava is consumed in shield building and less in extending the flow. This is good news for communities downslope.
Kilauea summit and Pu`u `O`o continued to deflate. There have been two deflation-inflation tilt events in the past week. Seismic tremor levels at the summit are elevated to nearly moderate levels. Summit sulfur dioxide emissions have nearly tripled. Earthquakes were located mostly beneath the general summit area and the south flank faults.
On July 21, 2007, lava began erupting from a set of fissures on the east flank of Pu`u `O`o. Eruptive activity soon stabilized at fissure D, 2.3 kilometers (1.4 mi) northeast of Pu`u `O`o. For most of last fall, this lava was directed entirely into a perched lava channel, consisting of separate pools of molten lava separated by bridges of cooled lava. At dawn on November 21, lava began to erupt directly from fissure D, outside of the perched channel, creating the Thanksgiving Eve breakout (TEB) flow. From December 27 until last weekend, the lava supply to the original perched channel was completely redirected through the TEB outlet. By Monday, January 21, though, lava had reentered the perched channel and partially refilled pool 1. That lava has apparently stagnated, but could certainly resume again.
The TEB flow has continued to build itself vertically and laterally, as a series of low shields, over the last several weeks. Two weeks ago, the front of the southeast-most shield collapsed and a large volume of lava surged out to form a rapidly moving `a`a flow. The flow advanced about 3.4 km (2.1 miles) reaching to within 180 m (~590 ft) of the top of the Royal Gardens subdivision before stagnating. The eruption then resumed its construction of low shields within about 2 km (1.2 miles) of the TEB outlet. In the past week, a pahoehoe flow fed from the lower end of the southeast-most shield has been creeping slowly downslope.
Weak incandescence in Pu`u `O`o was observed at night by Webcam a few times in December-for the first time since August 31-but has otherwise been absent since. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially enroute to the erupting fissure.
Vent areas are hazardous. Access to the eruption site, in the Pu`u Kahauale`a Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).
One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-3.1 earthquake occurred at 5:25 p.m. H.s.t on Sunday, January 20, 2008, and was located 7 km (4 miles) northwest of Mauna Kea summit at a depth of 24 km (15 miles).
Mauna Loa is not erupting. Four earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates.
Visit our Web site (hvo.wr.usgs.gov) for daily Kilauea eruption updates and nearly real-time Hawai`i earthquake information. Kilauea daily update summaries are also available by phone at (808) 967-8862.
Updated: January 28, 2008 (pnf)