Pu`u `O`o cone and surroundings as of July 2000, showing the area
covered (mauve color) by lava since February 1997 during episode 55. Inside
the crater of Pu`u `O`o, the "trough" is the drained lava pond of
September-October 1999. The central portion of the trough was briefly
filled with lava in February 2000.
Pu`u is the Hawaiian word for hill. `O`o, a bird, now extinct, that was once thought to have lived in this part of Kilauea?s east rift zone. Pu`u `O`o is the cinder and spatter cone built primarily between 1983 and 1986. The cone once towered as much as 200 m above the surrounding terrain. In the years since about 1992, however, lava from flank vents have buried the base of the cone, and in January 1997, a large part of the upper part of the cone collapsed. Now Pu`u `O`o has only about 90 m of relief above the surrounding flow field.
Flank vents active in 1997 have built the south shield, minishield, and episode 55 cone. The south shield has about 20 m of relief; the minishield, less than 10 m. The episode 55 cone was about 10 m high, but it has subsided into a slowly expanding collapse crater; the crack symbols show the continued expansion of pit in the episode 55 cone. These cracks are as wide as 1-2 m, and some have slight vertical offset across them. Major subsidence of these features occurred in abrupt stages. Entire collapse craters as large as 50 m in diameter and 30 m deep formed in a few hours or less. The cracked ground then remained stable for weeks or months until the next episode of subsidence occurred.
Subsidence isn?t unique to the flank vents. Pu`u `O`o itself has been undermined by magmatic stoping--sudden withdrawal of support--when magma drained away and intruded into the rift zone. The west gap, which formed in January 1997, is the result of subsidence along the trend of the east rift zone. Puka Nui is a slowly expanding collapse crater that has consumed part of the tephra cone and surrounding shield on Pu`u `O`o?s southwest flank.
An east-rift intrusion in September 1999 led to a temporary shutdown of volcanic activity at Pu`u `O`o. When activity resumed, small new spatter cones were active briefly, shedding the lava flows shown as 1999 flows on the sketch map.