Kilauea Crater Rim Drive
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Crater Rim Drive, Stop 6 - Southwest Rift Zone
About 200 years ago, as Kilauea caldera formed, large volumes of ash erupted at the summit and lava erupted on the lower east rift zone. The ash blankets the summit of Kilauea volcano (photograph by J.D. Griggs, U.S. Geological Survey, August 14, 1986).
Hawaiian eruptions are characteristically very gentle. However, under certain conditions, they can be violent. The following quote describes the events at the summit of Kilauea during the 1790 eruption as witnessed by Hawaiian warriors.
Written by Kamakau and reported in Westervelt's Hawaiian Legends of Volcanoes.
The eruption also killed eighty Hawaiian warriors about six miles (10 km) southwest of the summit. The warriors were returning to the Kau district to defend it from an attack by Kamehameha. Some viewed the eruption, which reduced the strength of a rival chiefs army, as a demonstration of Pele's favor of Kamehameha. Kamehameha ultimately united and ruled all of the Hawaiian Islands.
The southwest rift zone is one of two rift zones on Kilauea volcano. The rifts are weak areas in the volcano where it is being pulled apart. Magma, a few miles below the surface, moves from the summit down and through the rift. It pushes sideways against the volcano, causing it to spread. The brittle rocks above this intrusion cannot stretch. Instead, they crack. The cracks on the southwest rift zone are associated with a magnitude 7.9 earthquake that occurred in 1868. Shortly after the earthquake, a batch of magma left Kilauea's summit and traveled down the rift zone. As the magma moved down the rift it caused the cracks we see at the surface today. Crater Rim Drive is near the top of the photo. Photograph by J.D. Griggs, U.S. Geological Survey, March 4, 1985.
Sometimes the magma in the rift rises to the surface to generate an eruption. This happened twice in the early 1970s on the upper southwest rift zone. In September of 1971, an eruption began on the floor of the caldera and migrated into the southwest rift zone. It was the first eruption on this rift since 1921. Although inflation of the summit of the volcano and earthquake activity had increased in previous weeks, harmonic tremor, an indication that magma was moving, began only 50 minutes before the eruption. The geologists living in the park were altered by an alarm that was triggered by earthquakes. They managed to get to the observatory in only eight minutes and witness the onset of the eruption. The eruption lasted five days producing lava fountains, cascades of lava into Halemaumau, flows which crossed the road, and new cracks in the rift zone. History repeated itself in September of 1974. Again, an eruption began on the floor of the caldera and migrated into the southwest rift zone. The eruption lasted four days and covered much of the lava erupted in 1971. This photograph, taken from the rim of the caldera, shows the fissure on the caldera floor. Halemaumau is on the top, right corner of the photo. Photograph by J.P. Lockwood, U.S. Geological Survey, September 19, 1974.
The vents for the September 1974 eruption can still be seen from the rim of the caldera. Compare this photo to the one above.
Crater Rim Drive crosses the main fault that defines the southwest edge of Kilauea caldera. The fault is draped by the 1790 ash deposit, indicating that the caldera was already in the process of forming when the ash began to erupt.