The vents and lava flows of the Pu`u `O`o-Kupaianaha eruption dominate the middle east rift zone of Kilauea. The zone extends 17 km from Napau Crater to the Heiheiahulu lava shield. More than two thirds of the middle rift zone is covered by lava extruded from the Pu`u `O`o and Kupaianaha vents. Most of the remaining area was paved by lava flows during the past two centuries; six eruptions took place in this area between 1961 and 1977. Older rocks are found only in a handful of kipuka, areas of vegetation surrounded by younger lava flows. Probably no rock at the surface of the middle east rift zone is older than about 2,000 years.
The middle east rift zone begins at an elevation of 790 m above sea level and drops to about 520 m at Heiheiahulu. It is the most isolated part of Kilauea, because roads do not exist in this part of Hawai`i Volcanoes National Park and adjacent state land, and the dense rainforest prevents easy access. A long and strenuous hike provides the only access to those parts of the middle east rift zone not closed to the public because of the ongoing eruption.
Photograph by J. Kauahikaua on 2 February 2001
|The Pu`u `O`o cinder and spatter cone is the largest constructional feature along the entire east rift zone. Since its birth in 1983, the cone has changed constantly. Pu`u `O`o grew rapidly to a height of 255 m during 47 episodes of lava fountaining between 1983 and 1986. When activity resumed at the cone in 1991, continuous extrusion of lava flows built shields on its south and west flanks. Beginning in 1997, the west flank of the cone collapsed to form a deep gap in the crater rim; subsequently, many additional collapse features have formed on the southwest flank (see more about the changing cone).|
Graben structures. Several prominent graben structures extend downrift
from Napau Crater. Graben are elongate blocks that have dropped down along
relative to rocks on either side. Graben are found along the entire east
rift zone in areas of active volcanism; they typically form above an intruding dike as it nears the
surface. Graben also form by extension, as one side of a fault moves
horizontally away from the other. During the October 1968 eruption, a
narrow graben located about 400 m south of one of the vents became reactivated.
Here's a description of the graben:
"Before the eruption, the graben was 30-40 m deep, 10-15 m wide, and 300 m long. Lava erupted from near vent E on October 7 cascaded into the graben, which eventually filled and overflowed to the south. After the lava flow had stopped, probably on October 8, the graben subsided along its preexisting bounding faults. Subsidence had started before the new lava was completely solidified, and cool sticky lava dribbled down the newly formed walls of the graben. Continued subsidence lowered the October 7 lava fill by 60-70 m." Quote from Jackson and others (1975).
Jackson, D.B., Swanson, D.A., Koyanagi, R.Y., and Wright, T.L., 1975, The August and October 1968 East Rift Eruptions of Kilauea Volcano, Hawaii: U.S. Geological Survey Professional Paper 890, 33p.