The rift zones of Hawaiian volcanoes are long strips 1-2 miles wide in which eruptions can occur. They radiate from the volcanic summits, where eruptions occur most frequently. Most Hawaiian volcanoes have at least two rift zones. Volcanologist’s interpretations of the workings of rift zones are based primarily on close observations of eruptions at Kilauea and Mauna Loa. Magma is believed to come up from depths of at least 60-80 km into a holding area, a magma reservoir, 3-5 km beneath the summit. This part of the process can be monitored with seismometers, tiltmeters, GPS receivers, and gas sensors.
As the magma rises, it starts to bubble off carbon dioxide and stress the surrounding rock, causing earthquakes. As more and more magma is stored in the magma reservoir, the volcano swells; the swelling can be detected by tiltmeters and GPS receivers.
To feed eruptions in the rift zones, magma moves from the reservoir beneath the summit through some sort of conduit. It is uncertain whether this conduit is about 1 km deep or 3-5 km deep below the volcano’s surface. Whatever the depth, the magma moves toward the surface to erupt and become lava. In the last few minutes to hours of its upward ascent, the magma starts to bubble off sulfur dioxide.
The east rift zone of Kilauea Volcano starts at the summit, trends southeastward, and is marked in its upper part by a chain of pit craters – from Lua Manu (the first pit crater on the Chain of Craters Road) to Pauahi Crater. The next main feature downrift from Pauahi Crater is Mauna Ulu shield, the site of a prolonged eruption between 1969 and 1974 that buried two pit craters – `Alo`i and `Alae. Downrift from Mauna Ulu are Makaopuhi crater and Kane Nui o Hamo shield. The rift turns more eastward here to Napau Crater. Four kilometers (2.5 miles) downrift from Napau is Pu`u `O`o cinder and spatter cone – until early this week, the vent for lavas that have been flowing into the ocean.
Early on Father’s Day morning at 2:15 a.m., June 17, HVO instruments detected and began to track a swarm of earthquakes. The “hive” appeared to be under Pauahi Crater in the Mauna Ulu area. The summit began to deflate immediately, and volcanic tremor started to increase, indicating that the magma stored there was on the move. Rapid tilting of the ground near Mauna Ulu suggested its destination. For a few tense hours, the earthquakes lessened slightly, but also started to occur in the rift zone closer to the summit.
At about 8:00 a.m., the earthquake bees started to swarm again, this time east of the “hive” toward, and under, the Makaopuhi Crater/Kane Nui o Hamo shield region. At about the same time, this part of the rift zone started to widen – slowly, at first. Within a few hours, the rate of widening was a steady 2.5 cm/hr (1 inch/hr).
At this point, HVO scientists knew that they were dealing with an upper east rift zone intrusion – an eruption “wannabee” (all eruptions start with magma rising from the magma reservoir or the rift zone magma conduit, but not all reach the surface). The rift zone, opening in the vicinity of Mauna Ulu and Makaopuhi, was providing a path through which magma could rise. And magma rose to the occasion. The summit continued to deflate, and seismic tremor continued to increase in intensity.
Most of the signs accompanying this event suggested an intrusion that wouldn’t erupt, like the September 12, 1999, event, or a small eruption, like the January 30, 1997, episode 54 eruption. We continued to monitor the activity.
The situation seemed to have stabilized until about 3:00 p.m. on June 18, when another scattering of earthquakes occurred even farther east – just east of Makaopuhi Crater. By 9:00 pm, tremor beneath the summit started to taper off.
The next day (June 19), not long after sunrise, an HVO crew on an observation helicopter flight discovered two new intensely fuming and smoking areas, one with a new lava flow in a beautiful `ohi`a forest just east of Kane Nui o Hamo. This was the area above which the most recent earthquakes were occurring. The lava flow was small – no more than 200 m (600 feet) by 50 m (150 feet) – an area of about 2 acres. The erupted volume was a minute fraction of what had been intruded underground.
When seen, the lava was no longer active and soon completely solidified. The fuming and smoking diminished quickly. But the widening of the rift didn’t slow down until late that night, when the seismic tremor beneath the summit also started to diminish in intensity.
Throughout this series of events, the activity at Pu`u `O`o had been declining. The last incandescence seen inside the crater was the night of June 18. By the next morning, the crater was shrouded in dense steam. Occasional glimpses revealed a major collapse of the floor, signaled by instruments tilting inward toward the cone. No signs of activity were apparent on the surface of the entire flow field. The last view of any lava entering the ocean at Poupou was on June 21. The Pu`u `O`o eruption has shut off.
This sequence of events has happened before. In 1997, very similar events led to a complete collapse of the crater inside Pu`u `O`o cone, the cessation of episode 53 lava erupted from the cone, and the eruption of a small pad of lava known as episode 54. Episode 55 started 23 days later, again at Pu`u `O`o.
In 1999, similar events also led to a partial collapse of Pu`u `O`o, and another cessation of lava erupted from the cone. Episode 55 resumed 11 days later.
What will happen next at Kilauea? Will the eruption continue farther down the rift zone? It happened in 1968, and HVO is watching for that possibility. Will Pu`u `O`o pick up where it left off erupting lava? The eruption has resumed after similar interruptions in the recent past; in 1997, the summit caldera had to reinflate fully before Pu`u `O`o resumed eruption. The reinflation has only just started.
Is the June 17/18 event the latest episode of the Pu`u `O`o eruption or is it a separate eruption? This question is a current topic of scientific debate. The issue is an interesting one, and how it is called will be based, in part, on the role of Pu`u `O`o. If the June 2007 eruption was fed directly by magma from the summit, which robbed the supply to the Pu`u `O`o vent, then it, like the 60-plus eruptions of Kilauea over the last 180 years, deserves to be labeled as a separate eruption. However, if the magma is found to have come from Pu`u `O`o, then it could be called episode 56 of the current eruption. Stay tuned!
This past week, activity levels at the summit of Kilauea Volcano remained elevated until 2:15 a.m. on June 17, when they increased dramatically to signal an east rift intrusion. A small pad of lava was erupted on the northeast flank of Kane Nui o Hamo shield by the morning of June 19. Earthquakes continue to be concentrated in the upper east rift zone. The summit caldera significantly deflated between June 17 and 19, then started slow inflation.
The eruptive activity at Pu`u `O`o started to decline in the early morning hours of June 17 and, as of this writing on June 21, has completely shut down. There are no active surface flows visible anywhere on the flow field or at the Poupou ocean entry. This is the first pause since December 15, 2000.
Eight earthquakes beneath Hawai`i Island were reported felt within the past week. Five were associated with the earthquake swarm accompanying the June 17-19 intrusion and eruption. The largest was a magnitude-4.0 that occurred at 6:15 p.m. H.s.t. on Sunday, June 17, and was located 5 km (3 miles) southeast of Kilauea summit at a depth of 1.3 kms (0.8 miles). Three earthquakes occurred in other areas of the island. A magnitude-2.1 earthquake occurred at 5:22 a.m. H.s.t. on Sunday, June 17, and was located 27 km (17 miles) southwest of Hawi at a depth of 13 km (8 miles). A magnitude-2.8 earthquake occurred at 10:56 p.m. on Sunday, June 17, and was located 2 km (1 mile) northeast of Kilauea summit at a depth of 2 km (1 mile). A magnitude-2.6 earthquake occurred at 7:50 p.m. on Monday, June 18, and was located 4 km (9 miles) east of Pahala at a depth of 29 km (18 miles).
Mauna Loa is not erupting. No earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates which have slowed further since May 2007.
This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by HawaiiNews.com with permission.