August 2, 2007
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Kilauea's newest fissure eruption forming perched lava ponds
If you've been keeping up with the on-line eruption updates from the Hawaiian Volcano Observatory, then you know that there's something peculiar that's been happening along the new July 21 fissure.
Since the fissure began erupting lava two weeks ago, perched, or elevated, ponds of lava have developed and overflowed to feed a couple of longer flows. These perched ponds are pools as large as 200 meters (650 feet) in diameter, and, in one case, almost 20 meters (65 feet) high-nearly as high as the elevation of nearby Pu`u Halulu!
So what exactly is a perched lava pond, and how does grow to such elevated heights?
In the simplest terms, a perched lava pond is a lava lake that is impounded by its own levees and therefore becomes "perched" above the surrounding ground. If you haven't seen one, you could visualize it as a giant above-ground swimming pool, or rain-catchment tank, with sides made of cooled basalt and an interior filled with molten lava.
Perched ponds form when slow-moving lava flows into relatively flat area and begins to spread outward. If the lava continues to spread, instead of forming a channel, it may begin to pond within the cooled outer edges of the flow. Pulses of lava may also roll slabs of the pond's crust over the sides.
Sooner or later, lava from the pond overflows its cooled edges. With each overflow, the sides of the pond are built up and outward.
As the edges of the pond grow higher, the pond's surface becomes elevated above the surrounding area.
The shape of the pond can be a nearly perfect circle if the incoming flow of lava is relatively rapid and constant. Irregular shapes of ponds suggest that the pond's flow was interrupted, or that the pond was breached and drained before it attained a circular shape.
Beyond being visually interesting, perched lava ponds are also useful features for scientific investigations. Geologists have discovered that the width and height of a perched lava pond is directly related to the slope of the ground it sits on and the rate at which lava flowed into it. Thus, if we know the slope of the ground and the dimensions of the pond, the lava supply rate can be calculated for old, cooled ponds.
Perched lava ponds are a unique feature that draws the attention of most dedicated volcano watchers. However, molten ponds are extremely dangerous. The levees that surround them could break at any time and inundate nearby areas. Thus, the area around the current eruption is closed to visitors.
Safer solidified perched ponds can be seen elsewhere in Hawai`i Volcanoes National Park. There are perched ponds, formed during the 1968 eruption, near Napa`u Crater and one that formed during the 1974 eruption of Mauna Ulu. Both of these ponds are about 150 meters (500 feet) in diameter.
Perched lava ponds can also be seen in other areas containing basaltic lava flows, for example, at Craters of the Moon National Monument and Preserve on the Eastern Snake River Plains in Idaho.
Far beyond the reach of visitors, solidified perched lava ponds have also been observed in submarine volcanic eruptions. The underwater region of Kilauea's east rift zone known as the Puna Ridge contains large, flat-topped benches that resemble perched lava ponds. These features are as wide as several kilometers and as high as a few hundred meters. Similar features have been seen elsewhere in Hawai`i, off the coast of Ni`ihau.
Perched lava ponds have been a prominent and important feature of the July 21 eruption. As lava issues from fissures and flows into adjacent areas, perched ponds are forming, breaching and feeding flows. HVO is continuing to closely watch and study these perched ponds in an effort to better understand volcanic processes.
On Saturday, July 21, after nearly three weeks of refilling, the floor of Pu`u `O`o subsided as lava erupted from a new fissure system on the east side of Pu`u `O`o cone. The four fissure segments, extending along a line nearly a mile long, began to pour lava onto the ground between Pu`u `O`o and Kupaianaha. The fissure segment closest to Pu`u `O`o, fissure A, had stopped erupting by mid-morning on Saturday, and fissure B, the next fissure to the east, was inactive by Tuesday, July 31. Fissures C and D, each progressively farther east, have erupted continuously for the last two weeks.
Early in the first week following the eruption, most of the lava has piled up fairly close to the vents, constructing large perched lava ponds up to about 50 feet above the surrounding lava surface. By mid-week, most of the perched lava ponds had broken through their walls, sending lava gushing out as thick `a`a flows, before promptly beginning to form new ponds.
On Friday, July 27, lava erupting from the eastern-most fissure, fissure D, began to supply lava to a broad `a`a flow heading slowly toward the northeast. This flow continued to move toward northeast through the past week and had reached about 2 miles from its source at fissure D by August 1. While the level of activity at fissure D has not noticeably changed during the eruption, fissure C has had only very minor activity for the last several days.
Six earthquakes beneath Hawai`i Island were reported felt within the past week on July 26 and 29. A magnitude-3.3 earthquake occurred at 1:24 p.m. H.s.t. on Thursday, July 26, and was located 10 km (6 miles) southwest of Kilauea's summit at a depth of 30 km (19 miles). On the same day a magnitude-2.2 earthquake occurred at 7:16 p.m., and was located 35 km (22 miles) west of Kawaihae at a depth of 8 km (5 miles). A third earthquake on July 26 was a magnitude-3.4 event at 10:06 p.m., and was located 19 km (12 miles) south-southwest of Kilauea's summit at a depth of 34 km (21 miles).
Three more earthquakes were reported felt on Sunday, July 29. A magnitude-2.2 earthquake occurred at 6:40 a.m. H.s.t., and was located 1 km (0.6 miles) north-northwest of Pu`ulena Crater in Leilani Estates at a depth of 3 km (1.8 miles). A second event was a magnitude-2.8 earthquake at 1:03 p.m., located 2 km (1.2 miles) east of Waimea at a depth of 17 km (10.5 miles). The third earthquake was a magnitude-2.5 event at 7:04 p.m., located 12 km (7 miles) southwest of Mauna Kea's summit at a depth of 19 km (12 miles).
Mauna Loa is not erupting. Two earthquakes were located beneath the volcano in the past week. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.
Updated: August 6, 2007 (pnf)