Geologists call it “gas pistoning.”

That’s the cyclical rise and fall of the surface in a lava pond, driven by the buildup and release of gases.

And it’s what’s happening in the pond at the northeast spatter cone of Kilauea’s Puu Oo vent.

The release of the gas triggers vigorous lava spattering, as shown in photos and a video of the 30-foot-wide pond released Friday by the Hawaiian Volcano Observatory.

That’s only part of the ongoing action at Kilauea, in continuous eruption since January 1983.

Fed by Puu Oo, the Kahaualea 2 flow remains active, with the lava front moving through thick forest about 5 miles northeast of the vent. That is the farthest advance of the flow since mid-January, when the flow extended 4.6 miles northeast.

 In photos taken Friday, ephemeral fumes rising from the burning trees and acres of blacked terrain attested to the persistence of Pele.

The observatory notes that the Kahaualea 2 flow has been relatively slow and erratic due to disruptions caused by strong ground-deflation events that interrupt the lava supply.

At Kilauea’s summit caldera, the lava lake in the Halemaumau firepit continued its elevatorlike rise and fall and was about 140 feet below the floor of the crater on Saturday morning.

So-called tilt events — ground swelling and deflation — are believed to be linked to changes in magma supply to a storage reservoir just east of Halemaumau. From Friday to Saturday, instruments recorded weak inflationary tilt, just as they had during the previous 24-hour period. 

The summit lava lake lies within a nearly cylindrical vent cavity within the east wall and floor of Halemaumau Crater. The vent has been active since it opened with a small explosive event on March 19, 2008.

Meanwhile, 19 small earthquakes rattled Kilauea on Friday and Saturday: two beneath the summit caldera, five within the upper east rift zone, and 12 scattered on the south flank faults.