HILO, Hawai'i — Two more pieces of a coastal delta where lava from Kilauea volcano enters the ocean have slumped off and collapsed in the past week or so, and national park officials are still warning visitors to stay away from the area.

Scientists surveying the east Lae'apuki area by air on Friday noticed that up to 20 yards of the leading edge of the western half of the delta was gone. Observers reported that another piece had sheered off on Sunday. Another collapse was reported June 27.

Experts with the U.S. Geological Survey have warned that a much larger 25-acre portion of the coastline appears ripe for collapse. Observers from the air can see sizable cracks paralleling the coastline, said Jim Kauahikaua, scientist-in-charge for the USGS Hawaiian Volcano Observatory.

Lava benches or deltas form when lava flows into the sea, creating unstable land jutting into the ocean. They can collapse without warning, blasting boulders, steam and scalding water back to shore.

On July 1, Hawai'i Volcanoes National Park closed trails leading to sites where lava is flowing into the sea. Park Ranger Mardie Lane said the closure means hikers along the coastline can get to only within about a mile of the nearest lava ocean entry.

Hikers in shape for a 5-mile trek in daytime heat can reach the lava by walking toward the higher ground of Pulama Pali. Lane warned that the hike is difficult in the dark.

Parks officials advise hikers to talk to the rangers at the visitor's area at the end of Chain of Craters Road. Rangers are on duty there from 9 a.m. to 9 p.m.