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December 16, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


A simple question leads to a big lava flow

An employee of the Hilo Hawaiian hotel recently called the U.S. Geological Survey's Hawaiian Volcano Observatory and asked about the origin of Coconut Island. This is a 1.3-hectare (3.1-acre) island, also known as Mokuola, located in Hilo Bay, just offshore of the hotel on Waiakea peninsula.

The answer was easy to find because nearly all of Hilo east of Kilauea Avenue and the Wailoa river is formed by the vast Pana`ewa flow. The Pana`ewa flow is one of the largest flows issued by Mauna Loa Volcano. From a vent in the Kulani Correctional Facility, more than 1 cubic kilometer (0.24 cubic mile) of lava flowed to the northeast in a fan-shaped pattern.

The southeastern boundary of the flow crosses Highway 11 on the north side of the new Kea`au bypass road intersection. The gentle rise in the roadway is the flow boundary. The flow enters the ocean at Ha`ena, near the old Shipman Kea`au ranch house.

The northwestern boundary of the flow crosses Highway 11 at the four-mile bridge and runs parallel to the roadway on the east. The shopping mall is built on the Pana`ewa flow, and the steep slope facing the highway north of Puainako Street is the boundary of the flow. The edge of the flow crosses Highway 11 before Maka`ala Street and becomes the eastern levee of the Kawili stream drainage. The flow boundary crosses Kekuanaoa Street near Leo's Rubbish Service and constitutes the eastern bank of the Waiakea pond and the Wailoa River.

The entire coastal shoreline from Suisan at the mouth of the Wailoa river to the ponds at Ha`ena is formed by the Pana`ewa flow. All the land mauka (upslope) of those boundaries toward Kulani is covered by the Pana`ewa flow. The surface area of the flow totals more than 173 square kilometers (67 square miles).

The total volume of the eruption is unknown because the undersea portion of the flow is not mapped. The Pana`ewa flow created the reef on which the Hilo Bay breakwater wall is built. Thickness of the Pana`ewa flow near the shoreline was measured at 31 m (100 feet) by core from the scientific drill hole in Keaukaha.

The Pana`ewa lava is characterized by abundant green and white minerals. The green mineral is olivine, an early-forming mineral in basaltic magmas, and the white mineral is feldspar, a late-forming mineral. The presence of feldspar indicates that the Pana`ewa eruption tapped a shallow magma body that cooled for a considerable period of time.

Exposed sections of the Pana`ewa flow are characterized by a thick, dense interior. The dense nature of the rock makes it ideal for construction aggregate, and all of the major rock quarries in Hilo mine the abundant Pana`ewa flow. The thick and dense characteristics of the unit suggest that the flow was a massive one that was probably erupted over a short period of time. In contrast, the current Pu`u `O`o eruption has produced a somewhat larger volume (1.8 cubic kilometers) (0.43 cubic mile) of lava by a series of flows over the past 17 years. The unit is characterized by thin laminar layers with spaces between the flows.

The Pana`ewa flow is dated around A.D. 610 (plus or minus 50 years) from radiocarbon analysis. This is about the age of the last lava that overflowed the summit caldera from places other than the rift zone gaps. From the presence of feldspar in the rocks, the thick and dense characteristics of the flow, and the near-coincidence of the age of the flow and the time when overflows from the summit crater ended, it can be postulated that the voluminous Pana`ewa eruption was related to the formation of Moku`aweoweo caldera.

A Hawaiian chant tells of a battle between Pele's sister, Hi`iaka, and Pana`ewa, a kupua or gnome with magical powers, in the forests on the outskirts of Hilo. From the translation of the story, it appears unlikely that the myth recounts the massive Mauna Loa eruption of A.D. 610, but, rather, it seems to describe the effects of a later, powerful hurricane.

Eruption Update

Lava finally reached the ocean again. Shortly after sunset on Monday, December 13, a small lava flow oozed into the water near the old Highcastle overlook. This is the first ocean entry of lava since September 12, 1999 when a 12-day pause in eruptive activity shut off the supply of lava to a well-established lava tube system from Pu`u `O`o to the coast near Kamokuna. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The three flows described last week continue to be active this week. A lobe from the westernmost flow entered the ocean.

Residents of Pahala felt two earthquakes on Tuesday morning, December 14. The first earthquake was at 10:41 a.m., and the second was 6 minutes later. Both temblors were located beneath Pahala at a shallow depth and had magnitudes of 2.9 and 1.9, respectively.

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Updated: 5 Jan 2000