# Volcano Watch — What in the world is the Hilo Ridge?

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Okay, all you Big Island residents, where is the Hilo Ridge? It stands nearly 3,000 m (10,000 ft) above its surroundings, is more than 50 km (30 mi) long, and is one of the most prominent landmarks in the State. Its crest is only 30 km (20 mi) from Hilo--but northeast of town.

Okay, all you Big Island residents, where is the Hilo Ridge? It stands nearly 3,000 m (10,000 ft) above its surroundings, is more than 50 km (30 mi) long, and is one of the most prominent landmarks in the State. Its crest is only 30 km (20 mi) from Hilo--but northeast of town. Yes, the Hilo Ridge is a feature of the seafloor, out of sight under the waves but nonetheless an important piece in the puzzle of how the Big Island was made.

The Hilo Ridge starts at the east edge of a gentle shelf or platform about 400 m (1,300 ft) deep off the Hamakua coast, about 20 km (12 mi) east of Honomu. From the platform, the ridge extends eastward in a slightly sinuous trace, finally ending at a depth of about 5,000 m (16,000 ft) some 65 km (37 mi) east-northeast of Hilo.

Until recently, earth scientists thought that the Hilo Ridge was built by eruptions along a rift zone of Mauna Kea, much like ridges along the rift zones that radiate from the summits of Kīlauea and Mauna Loa. Just as Kīlauea's east rift zone extends far beyond the shoreline as the Puna Ridge, so the east rift zone of Mauna Kea was thought to have reached far out to sea as the Hilo Ridge. The on-land part of Mauna Kea's east rift zone today is barely recognizable, because younger flows have covered those erupted along the rift zone. These younger flows, however, did not reach far eastward beyond the coast, and so the submarine rift zone remained a prominent topographic ridge.

In a startling development at a meeting of the American Geophysical Union last month, researchers from the University of Washington (including a former HVO staff member, Robin Holcomb) proposed a new hypothesis for the Hilo Ridge. They suggest that the ridge was built along a rift zone of the older Kohala Volcano, not of Mauna Kea. In their view, Mauna Kea buries and obscures the Kohala rift zone on the Big Island itself but does not reach far enough eastward to cover the Hilo Ridge. What is the evidence for such a radical departure from past interpretations?

The University of Washington group determined the chemical compositions of rocks recently dredged from depths of 5,000 m (16,000 ft), 1,100 m (3,600 ft), and 400 m (1,300 ft) along the Hilo Ridge. They compared their results with those obtained by previous workers on dredged samples from depths of 1,600-3,300 m (5,200-10,800 ft). The results show a pattern that is inconsistent with the known chemical characteristics for Mauna Kea, particularly in terms of the abundance of certain isotopes of the elements strontium, neodymium, and lead. The data are, however, entirely consistent with the known chemical compositions of Kohala, an older volcano that stopped erupting some 120,000 years ago.

Kohala is a poorly understood volcano. One of the problems is that a large part of the volcano, south of a line between Kukuihaele, Waimea, and Kawaihae, is covered by younger lava flows from Mauna Kea. The Kohala Mountains themselves may be only the remnant of a northwest rift zone of Kohala Volcano that sprouts from a buried caldera southeast of Waimea. It is not difficult to imagine that Kohala's southeast rift zone curves gently southward from the buried caldera and eventually becomes the Hilo Ridge, just as Mauna Loa's southwest rift zone and Kīlauea's east rift zone bend away from their respective calderas.

Will this stimulating new view of the Hilo Ridge stand the test of time? As with any new idea in science, it will be closely scrutinized over the next few years as new information is obtained and discussed. The new idea certainly passes the laugh test, but will it pass the fact test?

### Volcano Activity Update

During the past week, there was constant effusion of lava from the vent within Puu Oo. Southerly winds carried the gas emissions from the volcano as far north as Honolulu. The concentration of sulfur dioxide measured in the parking lot of HVO at times exceeded 5 parts per million (ppm), and several visitor facilities within Hawaii Volcanoes National Park were closed.

Lava continued to flow through a network of tubes down to the seacoast where it entered the ocean at two locations - Wahaula and Kamokuna. Several minor breakouts from the tube system produced surface flows. 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 5:35 p.m. on December 29 was reported felt by a resident of Pahala. The magnitude 3.8 earthquake was located 11.5 km (7 mi) southeast of the summit of Kīlauea Volcano at a depth of 7 km (4 mi).