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Volcanowatch

February 16, 2006

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


Origin of the Big Island's Great Valleys Revealed in Hawaiian chant

 View of Kohala Volcano
Kohala Volcano taken from the slopes of Mauna Kea.

An old Hawaiian chant tells of a mysterious, powerful place in North Kohala, where two streams begin side by side but run in opposite directions and form some of the Big Island's largest valleys.

Just north of Kohala's swampy summit, a small ridge separates the streams by only a quarter of a mile. The ridge dictates whether the falling rain will go southeast and plunge into Waipi`o Valley or northwest, down into Honokane Valley. The sacred area owes its peculiarity to the work of Madam Pele.

The ridge between the twin streams is part of the northwest- and southeast-trending rift zones of Kohala Volcano. When Kohala was active hundreds of thousands of years ago, vertical sheets of magma, known as dikes, made their way up from the magma reservoir and intruded into the rift zones (just as a dike has intruded beneath Pu`u `O`o and feeds the eruption on Kilauea's East Rift Zone). As these dikes force their way up, they pry the rift zone apart and often cause fractures and faults to form parallel to the rift zone.

When Kohala was active, the extension caused by the dikes produced a series of faults along the rift zone, forming horsts and grabens. Here is a simple example of how these geologic features formed. If you took five hardcover books off the shelf, squeezed them together between your hands spine side up, and then slowly reduced your pressure, the books in the middle would drop down to form a trough, or graben, while the outer books would remain higher, representing horsts. The release of pressure would simulate the prying action generated by an intruding dike.

In the area north of Kohala's summit, the horst and graben structure prevents rainwater on the surface from naturally flowing northeast, down the mountain slope. Instead, the grabens act as culverts, causing rainwater to flow laterally before it finds its way down to the ocean. One graben diverts rainwater southeast, to the back of Waipi`o, while the other diverts rainwater to the northwest, to the back of Honokane.

In addition to forming horsts and grabens, the Kohala dike complex plays another important role in the development of the Big Island's large valleys - that of creating and maintaining a water table. Because Hawaii is made up of lava flows full of voids, vesicles, cracks, and lava tubes, the rock is very permeable and porous. Rainwater seeps easily into, and saturates, the rock, creating a large lens of fresh water beneath the island. In most places, the top of this water table is just a few feet above sea level.

Unlike porous lava flows, however, dikes cool underground into dense rock with few cracks and vesicles. The dikes act as impermeable walls through which groundwater cannot flow. Rainwater seeping into a rift zone gets trapped in these dike "reservoirs," well above sea level. The groundwater is confined to flow along the dikes, until it finds a route where it can escape its captor.

In Kohala, just as the grabens divert surface water, the numerous dikes near the summit inhibit groundwater from seeping downslope to the northeast, where it naturally wants to go. Rather, the Kohala dike complex impounds the water and guides it northwest or southeast, down the axis of the rift zones, before the water finds a way out.

Therefore, most of North Kohala's groundwater ends up in either the Waipi`o/Waimanu drainage or in the Honokane/Pololu drainage. The enormous amount of water routed through these areas causes the valley walls to frequently collapse, accelerating valley development.

The numerous, shallow stream valleys between Waimanu and Honokane (Honokane Iki, `Awini, Honopue, Waikapu, `Apua, and Laupahoehoe), on the other hand, are deprived of groundwater by the orientation of the rift zone and its dikes. Meanwhile, the fault grabens starve the streams of rainwater from the summit; thus, the streams can do relatively little to carve the landscape.

Activity update

Activity at the summit of Kilauea Volcano has remained at a moderate level. Frequent earthquakes continue beneath the upper east rift zone of Kilauea, between Lua Manu and Pauahi Craters. Inflation of the summit caldera continues at the accelerated rate started on January 12.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with occasional surface flows breaking out of the tube. In the past week, however, surface flow activity has been very low, and no flows were observed on the pali.

As of February 16, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The active lava bench continues to grow following the major collapse of November 28 and is now approximately 800 m long by 200 m wide. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were two earthquakes beneath Hawai`i Island reported felt within the past week. A magnitude-1.4 earthquake occurred at 7:51 a.m. on Wednesday, February 8, and was located 8 km (5 miles) northwest of Ka`ena Point at a depth of 6 km (4 miles). A magnitude-2.0 earthquake occurred at 5:26 a.m. on Friday, February 10, and was located 5 km (3 miles) south of Volcano at a depth of 4 km (2 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit. Inflation continues, but at a rate that has slowed since early October 2005.

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Updated: February 22, 2006 (pnf)