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August 12, 2004

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

When Hualalai Turned Viscous

As you make the drive from Waimea to Kona, you may notice a peculiar grassy knoll and an associated hummocky ridge on the northern slope of Hualalai, directly mauka of Kiholo Bay. The old Mamalahoa Highway, the mountain road to Kona, takes you up and over the Pu`u Anahulu Ridge and beside the base of Pu`u Wa`awa`a, Hawaiian for "many-furrowed hill."

The cone stands 372 m (1,220 feet) tall and measures more than 1.6 km (1 mile) in diameter. The ridge runs nearly 9 km (5.5 miles) down Hualalai's slope and rises 275 m (900 feet) from the landscape.

These prominent features seem foreign to Hawaiian topography, because they are composed of rock with an unusual chemical composition that is rare in Hawaii and found nowhere else on the surface of the Big Island. Trachyte, pronounced tra- (as in trash) kite, a very viscous type of lava, flows much slower than what we witness at Kilauea and forms a more exaggerated topography.

If we equate Kilauea's flowing basalt with the viscosity of shoyu (soy sauce), we could liken Hualalai's trachyte to cold honey, slowly oozing downslope. This thousand-fold increase in viscosity is a result of trachyte's characteristic high silicon-dioxide (silica) content. Silica molecules are structured like jacks, so they tend to get tangled with one another, thickening the lava and inhibiting it from flowing. The Pu`u Wa`awa`a trachyte has 62 percent silica compared to 50 percent found in average basalt.

Trachyte is also characterized by magma that is relatively rich in sodium and potassium, which are very large elements that do not flow easily and add to the lava's viscosity. This explains why the cone and flow are so much more massive than their basalt counterparts.

Geologists hypothesize that Pu`u Wa`awa`a was formed during a fountain eruption of pumice a little over 100,000 years ago. During the next few tens of thousands of years, at least three separate, thick trachyte flows emerged from the vent and built the Pu`u Anahulu ridge. Later lava flows from Hualalai and Mauna Loa covered some of the trachyte, but the rest remains as Hualalai's oldest exposed rocks.

Hawaiian volcanoes behave in a very systematic manner. They begin building on the ocean floor, like Lo`ihi, and then enter into a shield stage, when they grow large and develop broad, gentle slopes like those of Kilauea and Mauna Loa. As they grow older, they enter a post-shield stage in which the magma becomes more viscous and the eruption style changes. Eruptions become infrequent as magma in a shallow reservoir solidifies. Eruptions are fed by magma from a deeper reservoir that has more sodium and potassium.

Hualalai is believed to have entered the post-shield stage about 100,000 years ago, and Pu`u Wa`awa`a may represent this transition. As magma in the shallow reservoir beneath Hualalai was cooling, certain minerals crystallized and changed the composition of the remaining liquid magma. Since the crystallizing minerals had little sodium, potassium and silica, the liquid magma became enriched in these elements.

When this last bit of shallow magma erupted out of Hualalai's north flank, gas pressure shot it high into the air, creating the impressive mountain of pumice that still stands as Pu`u Wa`awa`a. When the gas pressure was relieved, the magma then slowly squeezed out like toothpaste, and, over time, created Pu`u Anahulu.

Why does trachyte seem to be so rare in Hawaii? First of all, the transitional period between stages is relatively short. The formation of trachyte magma requires a delicate balance of magma reservoir size and supply rates for crystallization to change the liquid composition.

Secondly, trachyte has been found in numerous drill holes on Hualalai. Hydrologists discovered a trachyte flow as much as 100 m (325 feet) thick beneath younger lava flows on the mountain's northwest rift zone. Another buried flow up to 60 m (200 feet) thick may radiate southwestward from Hualalai's summit.

This suggests that trachyte is actually not uncommon, at least on Hualalai. Many trachyte exposures also occur in the West Maui mountains and on the East Moloka`i shield, perhaps formed when those mountains made the transition from the shield to the post-shield stage.

Activity Update

Eruptive activity at Pu`u `O`o continues. Lava in the Banana flow, which breaks out of the Mother's Day lava tube a short distance above Pulama pali, has been visible between the pali and Paliuli in the past week. Also visible on the pali was the distal end of the PKK (Kuhio) flow, which originates on the south side of Pu`u `O`o; the flow front stagnated on August 11. Lava has not entered the ocean since August 5. Eruptive activity in Pu`u `O`o's crater is weak, with sporadic minor spattering.

Three earthquakes were reported felt on the island during the week ending August 11. A magnitude 2.9 earthquake at 9:53 a.m. on August 4 was felt in Pahala. The earthquake was located 7 km (4 miles) northwest of Pahala. Later in the day at 5:09 p.m. a magnitude 3.4 earthquake struck 34 km (21 miles) east of South point. This event was felt in Hawaiian Ocean View Estates. On August 11, an earthquake at 5:14 a.m. was felt in Leilani Estates. This earthquake was located 2 km (1 mile) east-southeast of Pu`ulena Crater at a depth of 1 km (1 mile).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity was notably high for the third week in a row, with 33 earthquakes recorded in the summit area. Most of the earthquakes are the long-period type and located deep, about 40 km (23 miles) or more.

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Updated: August 24, 2004 (pnf)