The formation of "perched lava ponds" at Kilauea's newest fissure is the latest phenomenon to intrigue scientists studying the long-running eruption.

The ponds appeared July 21 at the fissure on the northeast side of the Pu'u 'O'o cone. It's the first time lava has surfaced in that area since 1999, said Lopaka Lee, a geophysicist at the U.S. Geological Survey's Hawaiian Volcano Observatory.

Since the fissure began, at least three "perched" or elevated pools of lava have developed and overflowed to feed longer flows. The largest is 650 feet in diameter and almost 65 feet high, said Lee, who was tramping around the ponds conducting research yesterday despite the constant threat of a breach.

Perched lava ponds are somewhat uncommon because they require a fairly constant flow of lava and the right terrain, Lee said.

"It's pretty impressive. It's like turning on a spigot and the lava flows down like a river," he said.

The ponds form when slow-moving lava flows into a relatively flat area and spreads outward into ponds instead of forming a channel. As the outer edges cool, levees are created, capturing newer lava. Sooner or later, the ponds overflow, and with each episode, the levees thicken and grow taller, Lee said.

He compared the ponds to a giant above-ground swimming pool, with sides made of cooled basalt and an interior filled with molten lava.

The shape of the pond can be a nearly perfect circle if the incoming flow of lava is relatively rapid and constant, according to Lee. Irregular shapes suggest the pond's flow was interrupted, or that the pond was breached and drained before it attained a circular shape.

The pools can collapse, drain or harden, creating a flat formation that resembles a frozen water pond.

Lee said geologists have discovered that the width and height of a perched lava pond is related to the ground slope on which it sits and the rate at which lava flowed into it.

"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," he said. "And by knowing how fast lava is filling the pond, you can calculate how fast it might flow into adjacent areas."

Lee warned that the ponds are extremely dangerous since the edges can break anytime and inundate surrounding areas. The eruption area is closed to visitors.

Solidified perched ponds safe for viewing can be seen elsewhere in Hawai'i Volcanoes National Park. One that formed during the 1968 eruption can be found near Napau Crater and another formed during the 1974 eruption at Mauna Ulu. Both are about 500 feet in diameter, Lee said.

Kilauea has been continuously erupting since Jan. 3, 1983.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.