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March 4, 1999

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

Gravity sees deep into the Big Island

Among the many tools earth scientists use, the measurement of gravity variations over the earth's surface is one of the most useful for studying Hawaiian volcanoes. You're surprised to hear that gravity is not a constant?

Gravity varies from location to location on the Big Island, and the gravity measurements can be easily corrected for the expected variations. After correction, any remaining variations in gravity must be caused by volcanic structures below the island's surface.

For example, we expect the acceleration of gravity to decrease away from the center of the Earth, so gravity is about 0.1% weaker at the top of Mauna Kea than it is in Hilo. For you weight-watchers, if you weighed 1,000 kg (1,000 pounds) in Hilo, then you would only weigh 999 kg (pounds) at the top of Mauna Kea. Gravity is even weaker at the top of Mt. Everest.

Getting back to Hawai`i, the purpose of making and correcting gravity measurements is to use them to study the insides of each of the island's volcanoes. On the Big Island, the values of corrected gravity are highest over the summits of each of our volcanoes and their rift zones. A high value of corrected gravity generally signifies that the deep rocks are denser than average lava flows. On the older islands of Maui, O`ahu, and Kaua`i, valleys eroded into those volcanoes show us that the summits and rift zones have lots of dense dike rock beneath. These dikes are the conduits by which magma is brought up to the surface and erupted. Variations in gravity can be a way of mapping the internal magma plumbing and storage chambers of each of the island's volcanoes, without waiting for erosion to expose them.

What has our most recent interpretation of gravity data told us about the geologic history of the Big Island? First, Kohala Volcano used to be much larger and longer, extending from its current summit in an east-southeast direction across the shoreline between Laupahoehoe and Hakalau. The submarine feature called the "Hilo Ridge" which extends east from Hakalau is also part of Kohala Volcano.

Mauna Kea appears to have a short rift zone extending west and a longer one extending southeast beneath Humu`ula Saddle. There is no surface evidence of the one beneath the Saddle, which is buried beneath Mauna Loa lava flows.

The summit and rift zones of Kilauea Volcano are easy to detect with gravity. The east rift zone is quite wide and extends offshore from Cape Kumukahi. The southwest rift zone is quite short and does not appear to extend beyond the coastline.

The northeast rift zone of Mauna Loa appears to curve around and connect to the summit of Kilauea. The southwest rift zone splits into two before joining back into one and extending offshore of Ka Lae (South Point). The apparent split is evidence of an older rift zone that once went directly south from Mauna Loa's summit. A large landslide, which extended from Kealakekua to South Point, sliced off the top of that rift zone and caused eruptions to migrate west, constructing a new rift zone. The old rift zone would have extended between Pahala and Na`alehu.

Finally, Hualalai used to have long rift zones extending northwest and directly southward. Its summit was over three miles southwest of its present summit. The same large landslide that disrupted the southern rift zone of Mauna Loa may have disrupted the entire magma plumbing system of Hualalai.

As we refine our measurements and interpretation methods, more fascinating details about the island's internal structure can be told.

Eruption Update

Lava continued to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the sea. No surface flows from breakouts of the tube system were observed on the coastal flats. Lava is entering the ocean near Kamokuna and forming a new bench. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

A resident of Ahualoa felt an earthquake at 2:41 p.m. on Saturday, February 27. The magnitude-2.5 earthquake was located 11 km (6.6 mi) southwest of Laupahoehoe at a depth of 25.4 km (15.2 mi).

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Updated: 18 Mar 1999