May 30, 1997

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

Dikes in the East Rift Zone

Great rift zones define the divergent boundaries in the mosaic of mobile tectonic plates that form the Earth's crust. From these divergent plate margins, new crust is continually being produced and, as if on a giant conveyor belt, continually moving away from the rift zones.

The Hawaiian Islands are not part of this global rift system; instead, they grow above a fixed "hot spot" in the Earth's mantle. As the crust of the Pacific Plate slides over this hot spot, new volcanoes form and die.

In a manner similar to the formation of new crust in the Earth's rift systems, part of the growth of Hawaiian volcanoes come from the formation of nearly vertical magma-filled cracks called "dikes" into the volcano's rift zones. The remnants of these rift zones form the spines of many of the islands' mountain ranges, such as the Waianae or the Ko`olau Mountains on O`ahu.

During the 14 years of near-continuous eruption of Kilauea Volcano from the Pu`u `O`o and the Kupaianaha vents, magma moved to the surface through well-established pathways. In the early evening (6:40 p.m.) on January 29, the volcano's plumbing system just uprift of the active Pu`u `O`o vent changed as a new dike began to form below Makaopuhi Crater.

As the new dike opened up, magma poured into it from storage areas in the summit and the east rift zone. Eight hours later (about 2:40 a.m. on January 30), part of this dike reached the surface, and lava erupted sporadically for 20 hours from a 2-km-long line of fissures. The amount of lava that erupted at the surface is only a small fraction of the magma that filled the 5-km-long, 2.5-km-deep dike that formed below the line of fissures. Most of the magma in the newly formed dike will remain underground, slowing cooling. It may someday be exposed as part of the spine of a scenic mountain range.

Our estimate of when, how much, and where magma moved during this intrusive event is based on changes in the shape of the volcano, measured with a network of sensitive GPS (Global Positioning System) stations and tiltmeters. At about the same time that seismometers detected tremor (steady ground vibrations associated with magma movements) and an earthquake swarm below Makaopuhi Crater, our GPS stations detected spreading across the rift zone.

The magma that initially intruded into the dike probably came from an old storage chamber below Makaopuhi Crater. Shortly afterwards, magma began moving into the new dike from storage areas at the summit of Kilauea and in the east rift zone. The lava pond at Pu`u `O`o drained, leaving its unstable sides unsupported, resulting in a partial collapse of the cone.

Our deformation measurements show that this dike continued to open during the 54-day-long pause in the surface flows that followed the dike intrusion. During this pause, the summit magma chamber and pond at Pu`u `O`o refilled. Although the eruption from the Pu`u `O`o vent has resumed, the activity has been more sporadic than before January 29, with surface flows limited to the area around the cone.

We observe continued slow opening of the new dike. The changed character of the eruption is a reflection of the change in the volcano's plumbing system, which has not yet achieved the stability that it had before the events of January 29.

Kilauea Eruption Status--May 30

Kilauea's east rift zone eruptive activity during the past week was interrupted by two pauses that lasted a total of 18 hours. Vents on the south and southwest flanks of Pu`u `O`o fed `a`a flows that travelled up to 1.5 km away from the cone. Occasional fountains up to 15 meters high were observed from the flank vents. Activity within Pu`u `O`o has raised the floor of the crater to within 10 meters of the lowest section of the rim.

There were no earthquakes reported felt during the past week.