While there is no new lava flowing into the sea, the Big Island is a little bigger than it was a few days ago. That's a result of being spread apart by a new dike that caused a brief eruption along Kilauea's East Rift Zone yesterday.

The rift zone is roughly 3 feet wider in the region of Makaopuhi Crater, and parts of the island's southeast coastline are an inch or so farther seaward as a result of the volcanic activity.

In recent years, the island has grown by dozens of acres from the flow of lava into the sea. But when molten rock moves underground without erupting to the surface, that can also expand the island.

Geologists have long known that the south flank of Kilauea is on the move — slipping gradually seaward due to a number of forces, said Asta Miklius, a geophysicist with the Hawaiian Volcano Observatory.

The latest of those forces is a vast slab of magma that formed over the weekend, extending several miles laterally from the region under Kilauea's summit out to the East Rift Zone.

This feature, called a dike, is envisioned by geologists as an immense underground structure that is vaguely the shape of a slice of sandwich cheese standing on edge. While it may be only a few feet thick, it can be one or two miles tall and several miles long. Molten rock fills a crack in existing stone, and the pressure of more rock pushing in from behind can cause continued cracking, allowing the dike to grow.

As the dike formed over the weekend in Kilauea's upper East Rift Zone, the cracking of rock was detected in the form of flurries of small earthquakes. They started under the Pauahi Crater and then moved down the East Rift Zone toward Makaopuhi Crater.

HELP FROM GPS

Yesterday morning's fissure eruption at Kane Nui O Hamo, just north of Makaopuhi, may simply have been a section of the dike cracking all the way to the surface. It spilled a little lava into the forest, then backed off.

Scientists measure the swelling of the rift zone with highly accurate satellite-based measurements. They have permanent and temporary Global Positioning System devices scattered across the volcano.

Among the early signs that something was happening underground was that the GPS units on the north and south sides of the rift were suddenly moving away from each other, even though they were both still anchored to the ground.

SPREAD ABOUT 40 INCHES

The instruments measured about 4 inches of widening during the day Sunday. Cracks appeared on the volcano's surface and on the Chain of Craters Road in response to the stretching.

By midnight, the spread had reached 8 inches, by Monday morning it was 15 inches, and by sundown it was up to 28 inches. By late yesterday it was nearly 40 inches.

The spread was larger than the 16 inches in 1997 and the 8 inches in 1999 in which, like the latest event, the eruption at Pu'u 'O'o stopped and a dike formed in the rift zone.

Scientists believe the widening of the surface was due to the crack being forced open by the continued flow of magma into the dike. It was like driving a wedge between a pair of bricks on a patio. Slowly the bricks move apart, just as the north and south sides of the rift zone were being driven apart.

Miklius said that the lateral movement of the island's south flank is much greater near the rift than away from it. A section of the coastline near the bottom of Chain of Craters Road has moved about an inch seaward, she said.

And not all the stress caused by the dike is being resolved by movement. Some of it is being stored, and may appear later as new earthquake activity, she said.

"The stresses being set up are at fairly shallow levels. We might expect to get some increased earthquake activity on the south flank" of Kilauea in the future, she said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.