# Volcano Watch — Kupaianaha vent appears to be winding down

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Since at least April 1991, the volume of lava erupted from the Kupaianaha vent has steadily declined. Since late November, the remaining lava flows have been limited to a single area near the 1,850-foot elevation, less than two miles downslope of the vent.

Kupaianaha vent appears to be winding down

(Public domain.)

Since at least April 1991, the volume of lava erupted from the Kupaianaha vent has steadily declined. Since late November, the remaining lava flows have been limited to a single area near the 1,850-foot elevation, less than two miles downslope of the vent.

The last active lava in this area was observed on February 6; since then, no new flows have appeared. The lava pond within the Puu Oo Crater is still active, though the level of the pond dropped slightly between February 7 and 12. The crater floor is 100 feet below the lowest point on the crater rim, and the level of active lava is currently about 26 feet below the crater floor. Several times within the last five months, short-lived lava fountains (lasting only a few minutes) have deposited spatter on the crater rim. However, the crater has never overflowed. In fact, Puu Oo hasn't produced a lava flow since July 1986.

Low-level harmonic tremor is still continuing beneath Kupaianaha as well as at Puu Oo. Harmonic tremor is a continuous vibration of the ground caused by movement of magma at shallow depths. Until the tremor beneath Kupaianaha ceases, we cannot say the vent is dead.

Since December, HVO seismometers have recorded increasing numbers of earthquakes in the upper East Rift Zone, extending from the southeast part of the summit to Pauahi Crater. These earthquakes occur at depths less than three miles, and most are too small to be felt. The elevated earthquake activity in the upper East Rift Zone is doubtless related to the decline of lava output farther down the rift zone at Kupaianaha. The simplest explanation is that as the deep conduit feeding Kupaianaha slowly closes off, magma pressure increases "upstream," i.e., in the upper East Rift Zone.

Since late November, tiltmeters at Kīlauea's summit have recorded gradual inflation of the summit reservoir, the volcano's main "holding tank" for magma ascending from the mantle. This inflation is consistent with the diminishing volume of magma flowing down the East Rift Zone and erupting at Kupaianaha.

As the Puu Oo/Kupaianaha eruption wanes, it is interesting to review how the 1969-1974 Mauna Ulu eruption ended. The final two-and-a-half years of the Mauna Ulu eruption were dominated by lava lake activity, both at the Mauna Ulu vent and at Alae Crater, which was linked to Mauna Ulu by a lava tube. As with Kupaianaha, lava tubes leading from these lava lakes fed long flows that intermittently reached the coast and entered the ocean.

Mauna Ulu continued to produce lava flows until June 1974. However, the average volume of lava erupted per day in 1974 was only half of that erupted per day in the previous two years. This is similar to the steady decline of lava output at Kupaianaha during the last nine months. In the first week of June 1974, the lava lake at Mauna Ulu turned sluggish, and for the next seven weeks the level of the lake slowly subsided.

During this interval, Kīlauea's summit inflated. On July 19, 1974, a fissure opened in the summit caldera and erupted through the 22nd. The lava pond at Mauna Ulu was still visible during the summit eruption, but after the 22nd, it disappeared beneath the rubble on the crater floor, and harmonic tremor at Mauna Ulu ceased.

The Mauna Ulu eruption was closely followed by another summit eruption in September 1974, and an eruption in the southwest rift zone in December. Both of these events lasted less than one day. Similarly, we might expect eruptions on the upper East Rift Zone or at the summit when the Puu O`o/Kupaianaha eruption finally ends.

The Big Island had no earthquakes of magnitude 3 or greater in the last week.