HILO, Hawai'i — Larger cracks are appearing along a 30-acre lava delta formed by Kilauea Volcano at the edge of the ocean at Hawai'i Volcanoes National Park, and scientists continue to monitor the outcropping, waiting to see if it will finally collapse.

Experts with the U.S. Geological Survey's Hawaiian Volcano Observatory surveyed the delta by air and ground yesterday, and "it just keeps getting bigger," said geologist Christina Heliker.

"We haven't had a monster collapse yet, but there's really a significant hazard there because there's cracks that run all the way across the bench, parallel to shore," she said. "If the thing finally slumped along those fractures, we would have a pretty catastrophic event."

Hawai'i Volcanoes National Park Superintendent Cindy Orlando announced in late June that the area around the outcropping was closed to visitors because of the hazard. That closure is still in effect, and parks officials warned the public not to hike past ropes or warning signs.

The bench is at the lava ocean entry point known as East Lae'apuki, and is about 750 yards long and 250 yards deep.

There have been "small collapses" of perhaps two acres each that sheered off at the apex of the bench over the last few weeks, but lava flowing from the tube system leading from the volcano quickly replaces the lost land, Heliker said.

The largest bench collapse took place in the same area in 1996, she said.

On Dec. 2, 1996, a helicopter pilot saw the beginning of the collapse, and then left the scene. When he returned about two hours later, he saw a 25-acre bench was gone.

There were no injuries.

One smaller, earlier collapse was fatal. In April 1993, Kona photographer Prem Nagar disappeared when the half-acre lava shelf on which he was standing collapsed into the sea.

The collapses happen because the lava benches build up over unstable, underwater piles of rubble. Shifting or landslides in the rubble below erode the support for the surface outcropping, and finally the lava deltas collapse.

The collapse abruptly exposes hot lava in the tube system to the surf, which triggers steam explosions that fling rocks and hot lava into the ocean and back on the shore.

The delta cracks are a few feet wide in places, which Heliker said is "a good warning sign."

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.