# Volcano Watch — Lava Flows of Hilo

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Hilo is situated on lava flows from two of the five volcanoes that form the Island of Hawaii. However, most of the surface flows one drives by (and upon) every day are flows from Mauna Loa.

Hilo is situated on lava flows from two of the five volcanoes that form the Island of Hawaii. However, most of the surface flows one drives by (and upon) every day are flows from Mauna Loa. In the northern part of Hilo near the Wailuku River, Mauna Loa flows overlie much older ash deposits and flows from Mauna Kea. The Wailuku forms the approximate boundary between Mauna Kea and Mauna Loa Volcanoes. Driving southeast along the Volcano Highway, one reaches the southern boundary of the Mauna Loa flows about a mile southeast of Keaau, where flows from Kīlauea can be seen.

In order to piece together an eruptive history of the Mauna Loa flows that reached Hilo, the ages of as many flows as possible must be determined. Carbonized roots and other plant material were collected from beneath lava flows, and this material was analyzed using radiocarbon-dating techniques. The ages obtained from this method have a margin of error; thus the ages given are approximate.

All together, 27 Mauna Loa flows have been identified in and near Hilo, but only the larger flows are described in this article. The youngest flow is from the historic Mauna Loa eruption of 1880-81, and the oldest flow yet found lies near Hoaka Road, with an age of more than 24,000 years.

Mauna Loa lava flows consist of both pahoehoe and aa. Both types of flows can be "picritic," which means that a bright green mineral called "olivine" is present in abundance. Most flows have been named after the cultural features they cross.

Some of the oldest Mauna Loa flows exposed in the Hilo area lie in, and north (Hamakua) of, Waipahoehoe Stream, the stream that is crossed by Kaumana Drive near Chong Street. Two of the oldest flows (nearly 24,000 years old and older) are exposed only within the stream.

The pahoehoe flow of Punahoa ahupuaa (approximately 3,100 years old) is a wide flow lying between the stream and the Wailuku River; it is overlain at its southern edge by younger flows described below. To the north along Waianuenue Avenue, two older flows underlie the Punahoa pahoehoe. Hilo Hospital is built on the 14,000-year-old pahoehoe flow of Waianuenue Avenue. The younger aa flow of Rainbow Falls (the Anuenue flow about 10,500 years old) overlies the Waianuenue Avenue flow. It can be seen within the Wailuku River, where it forms Wailuku Falls; here the Anuenue flow overlies not only the 14,000-year-old pahoehoe, but also much older flows from Mauna Kea.

South (Puna) of Waipahoehoe Stream, the younger flows lying atop the Punahoa pahoehoe are the pahoehoe flow of Kukuau ahupuaa (about 1,300 years) and the historic 1880-81 pahoehoe flow (home to Kaumana Cave). The younger flow has a glassy, fresh appearance that distinguishes it from the older one. Both these flows trend in a northeastward direction and can be seen on Komohana Street south of the intersection with Mohouli Street.

Two broad aa flows cover much of the rest of Hilo southeast of Waiakea Stream. The University of Hawaii campus is built on one of these aa flows, the picritic aa of Ainaola Drive (about 9,000 years old). The flow is broadest along its makai edge, where it is overlain by the other, younger aa flow. Many of the north-south roads mauka of, and including, Kīlauea Avenue have been bulldozed atop the Ainaola Drive flow. Along its southeast edge, mauka of Kanoelehua Avenue, this aa flow overlies three older flows from Mauna Loa. The uppermost is the pahoehoe flow of Waiakea Homesteads (9,500 years old). It overlies the pahoehoe flow of Laula Road (nearly 10,000 years old), which, in turn, lies atop the picritic aa flow near Kīlauea Avenue. Driving mauka along Haihai Street, one crosses three of these four flows: first, the picritic aa flow near Kīlauea Avenue, which lies between Kanoelehua Avenue and Kinoole Street, then the pahoehoe flow of Laula Road, upon which the southern part of Hilo Golf Course is built, and finally the Ainaola Drive picritic aa. Ainalako Road crosses the pahoehoe flow of Waiakea Homesteads. Near Wilder Road, the Ainaola Road aa is overlain by the young pahoehoe flow of Wilder Road (about 4,800 years old).

The second broad aa flow lies makai of Kīlauea Avenue and is several miles wide. By far the largest flow in the Hilo area (and on Mauna Loa), this olivine-bearing aa flow of Panaewa Forest Reserve (about 1,500 years old) forms the entire beach front eastward from Waiakea Pond (Wailoa State Park) to the area beyond Leleiwi Point all the way to Haena below Keaau. Hilo Airport is built on this flow. This flow crosses Highway 11 (along the entire stretch of road divided by trees) near Keaau until Haihai Street. This huge flow probably represents a single, prolonged eruptive episode. If this flow had not been emplaced, Hilo Bay would not exist.

The Halai Hills are old spatter-and-cinder cones of Mauna Loa that formed when eruptive activity became restricted to a few closely spaced centers. These cones last erupted more than 10,000 years ago.

### Volcano Activity Update

Eruptive activity from Puu Oo continues with spattering up to 20 m (65 ft) from the 51 vent area visible from Puu Huluhulu. Vents on the west and south sides of Puu Oo feed aa flows directed to the west and southeast, respectively. These flows override earlier flows and extend only 1.5 km (1 mi) from the base of the cone.

There were no earthquakes reported felt during the past week.