Volcano Watch — Lava flows of east Puna

Lava flows are the biggest volcanic hazard in east Puna. In the past two centuries, four eruptions from Kīlauea's east rift zone have produced widespread lava flows: in 1790 (more than 45 km² [16 mi²]), 1840 (22 km² [7.9 mi²]), 1955 (17 km² [6.1 mi²]), and 1960 (11.5 km² [4.1 mi²]). In addition, one or more rift eruptions in about 1750 covered an area of more than 42 km² (15 mi²) near and southeast of Heiheiahulu and Pu`u Honua`ula. Lava from one or more eruptions 300-400 years ago flowed from Kīlauea's summit into east Puna and overran more than 178 km² (64 mi²) of land north and northwest of Pahoa.

Kīlauea's east rift zone - the source of most lava flows in east Puna - forms a low but distinct drainage divide that largely governs where flows travel. For example, fissures for the 1840 eruption are along the north side of the ridge between Ka`ohe Homesteads and Nanawale, and flows consequently moved north and northeast from the rift zone. Nanawale subdivision, north of the rift zone, is built on an 1840 flow.

In contrast, fissures in 1955 cut the south side of the rift zone between Upper Kaimu Homesteads and Kapoho Cone, and flows moved southeastward rather than northward. Fissures formed in about 1750 also lie on the south side of the rift zone, so flows moved southeastward.

In 1960, Kapoho village, in a down-faulted trough along the crest of the rift zone, was destroyed as lava eventually filled the trough and flowed northeastward to the lighthouse. Fissures for the 1790 eruption were distributed across the rift zone between I`ilewa Cone and north of Halekamahina, so that some flows moved southward and others northeastward.

The summit flows erupted 300-400 years ago blanket a broad swath from the summit to the coast between Ha`ena and Honolulu Landing. Such residential developments as Hawaiian Beaches, Hawaiian Paradise Park, Orchid Land, and `Ainaloa are built mostly on these flows, as are the subdivisions southeast of Highway 11 (Hawaiian Acres, Crescent Acres, Eden Roc, Fern Forest, and Mauna Loa Estates).

Certain areas have not been covered by lava in the past 400 years. For example, flows older than 400 years underlie much of the area along the coast between Kalapana Seaview Estates and MacKenzie State Park and extend mauka to source vents at Pu`u Kali`u and I`ilewa cones. Similarly, the area along the rift zone from I`ilewa through the middle of Leilani Estates to the vicinity of Pu`ulena Crater has not been covered in the past 400 years. A large area north of the rift zone is on flows older than 400 years, reaching from Ka`ohe Homesteads to Nanawale Homesteads and even farther northeast.

Nonetheless, most of east Puna has been covered by lava at least once in the past 400 years. There is no reason to believe that the frequency or size of eruptions in the next 400 years will be any different from those of the past 400 years. Likewise, there is no reason to think that old flows are more likely to be covered before young flows (a "law of averages" argument) or that young flows are more likely to be covered before old flows (an "if it happened once here, it can happen again" argument).

Currently HVO has no evidence that Kīlauea is building toward an eruption in east Puna. Pu`u `O`o seems to be a plug in the rift zone, keeping magma from moving farther down rift. It is certain, however, that lava will eventually return to east Puna.

The above summary is based on two geologic maps, U.S. Geological Survey Map I-2225, published in 1991, and USGS Map I-2524, published in 1996.

Volcano Activity Update

During the past week, there was constant effusion of lava from the vent within Pu`u `O`o. For a brief period of time on September 28, lava topped the crater rim and flowed through the southeast spillway - the first time since August 6. Lava continued to flow through a network of tubes down to the seacoast where it entered the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

An earthquake at 21 minutes after midnight on September 29 was reported felt by a resident in Kona. The magnitude 3.2 earthquake was located 22 km (13.2 mi) west of Kailua-Kona at a depth of 40 km (24 mi).