The November 29, 1975 Eruption
Dawn, November 29, 1975
Spatter from a part of the fissure on Kīlauea Caldera floor, east of Halemaʻumaʻu.
(from Tilling, et al, Earthquakes and Related Catastrophic Events, Island of Hawaiʻi, November 29. 1975: A Preliminary Report; Geological Survey Circular 740).
About half an hour after the main shock, seismographs near the summit of Kīlauea began to record shallow, high-amplitude harmonic tremor, a distinctive type of seismicity associated with movement of magma. At 0532 the sky over Kīlauea glowed yellow-orange as fluid, gas-rich lava began to fountain from a N. 85° E.-trending fissure about 500-m long on the caldera floor, just northeast of Halemaʻumaʻu (A). The new lava issued from a continuous line of fountains as much as 50 m high. Within 15 minutes, the fountains decreased to 5-10 m high, eventually dying out about 0700. The fountains fed a small basaltic pahoehoe flow that spread mainly north from the eruptive fissure, eventually covering an area of about 0.25 km2 on the caldera floor. As fountaining abated, emission of gases became increasingly vigorous. After eruption of lava had ceased, jetlike, noisy degassing continued from the vents.
Map showing the eruption vents B and C, with relationship to Halemaʻumaʻu Crater (A) and to the Hawaiian Volcano Observatory (HVO).
Weak eruptive activity resumed at 0830 with copious fuming from a point on the northeast wall of Halemaʻumaʻu, 21 m above the crater floor (B). The rate of fume emission increased rapidly, and within a few minutes lava began to erupt in 4-5 m high fountains that fed narrow, sluggish rivulets moving slowly down the sides of a low spatter cone built around the vent. A small pool of lava gradually accumulated on the floor of Halemaʻumaʻu at the base of the cone, while jetlike emission of gas continued at the vent.
At 0953 another vent (C) opened on Halemaʻumaʻu's northeast wall 18 m above the crater floor, 107 m southeast of vent B. The activity at vent B diminished somewhat as lava began to spurt to 3-5 m heights at vent C. The proportion of gas to lava emitted by both vents gradually increased, and after 1100 the material erupted was mostly incandescent blocks sporadically thrown nearly 100 m high during loud gas bursts. The ejecta largely fell back into the vents, and by 1330, molten lava was no longer visible and the rate of degassing had greatly decreased.
At about 2005 eruption resumed intermittently in Halemaʻumaʻu from these same two vents, producing lava fountains at times 100 m high and accompanied by vigorous degassing. By 2200 the eruption had ended, but noisy degassing continued until 0200 on the morning of November 30.
The lava from vents B and C in Halemaʻumaʻu, judged by its high viscosity, probably was erupted at a relatively low temperature, perhaps from a body of magma trapped in a pocket high within the volcano. The total volume of lava produced during this short-lived eruption was small, approximately 250,000 m3 from vent A, 3,200 m3 from vent B, and 300 m3 from vent C.
The eruption was not unexpected, as tiltmeter and other geodetic measurements had indicated that Kīlauea had been inflating steadily since its last eruption in December 1974. The amount of inflation caused by gradual accumulation of magma at shallow depths within the volcano had reached a level approaching that of several recent eruptions. The November 29 eruptive activity was apparently triggered by the 7.7-magnitude earthquake. The small volume and brief duration of the eruption suggest that the shallow magma might not have reached the surface under its own buoyant energy without a triggering mechanism apparently provided by the violent ground shaking.
Updated: 14 Dec 1998 (pnf)