# Volcano Watch — Mother's Day came early this year—January 18, to be exact

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If Puu Oo were human, it would make a great baseball pitcher. It throws blazing fastballs, slow change-ups, and sharp curveballs that leave the volcanologist batter fanning at the breeze. But all pitchers have bad days, and Puu Oo had one on January 18, when it tossed a hanging curve that fooled nobody. Here's the story.

At about 0550 local time on January 18, the tilt at Puu Oo started down exceptionally fast. Such deflation is best explained by movement of magma from a storage reservoir beneath the cone to somewhere else. The precipitous tilt was accompanied by increased strength of volcanic tremor, which denotes movement of magma through underground conduits.

Within minutes, perhaps seconds, a crack opened on the south flank of Puu Oo, and lava began spilling onto the ground surface at the base of the cone. A new vent had opened! Three or four other small vents then opened just south of the cone, and lava welled from them to join that from the first vent.

Lava spread quickly southward in one narrow flow 1.5 km (0.9 miles) long. Lava from part of that flow entered a pre-existing lava tube, last active in 2002, and used it to move more than 500 m (0.3 miles) underground. Other lava formed a flow that headed east some 700 m (0.7 miles) to the national park boundary. Some of the lava collected in a depression just south of the cone, creating a pond that lit the sky with brilliant glow on the nights of January 18-20.

The crater floor of Puu Oo, host to several spatter cones active during the previous month, slowly subsided during the event. The amount of subsidence is only several meters (yards), and it results from sagging rather than piston-like down dropping. The spatter cones ceased their activity once the new vent opened, though they remained incandescent.

The tilt continued its steep deflation until about 0830, when it abruptly flattened. Deflation then resumed in the early afternoon and continued for at least 3 days at a slow but steady pace.

Interestingly enough, the summit of Kīlauea barely responded to this activity, as indicated by the small tilt. Usually, Kīlauea's summit senses events before Puu Oo, because magma that eventually reaches Puu Oo must first pass through the reservoir beneath the summit caldera. Some events at Puu Oo, however, don't follow this pattern. Instead, they start first at Puu Oo and only later, if at all, affect the summit. The January 18 outbreak was one of these.

All of last Sunday's events are reminiscent of those accompanying the Mother?s Day event at Puu Oo on May 12, 2002. They did not surprise HVO scientists, who had earlier realized that the situation at the volcano was primed to repeat the Mother's Day outbreak.

A long period of expansion of Kīlauea's summit and the east rift zone near Puu Oo preceded the Mother's Day 2002 new vent. The idea is that the volcano becomes pressurized and that existing vents at Puu Oo are insufficient to reduce the pressure. The pressure builds slowly, expanding the volcano, and accelerates up to the time that Puu Oo breaks open with a new vent.

About 9 months of accelerating expansion at Kīlauea's summit preceded the Mother's Day 2002 event. Before January 18, the rate of expansion had increased for about 8-9 months. For the past couple of months, HVO scientists were speculating that the increasing rate would culminate in a Mother's Day-like event, some time in January or as late as Valentine's Day.

The January 18 outbreak likely represents the anticipated event. We will have to wait several weeks to be sure, since it takes that long to tell if the summit has stopped expanding, as it did following Mother's Day 2002.

Flows active before the Mother's Day outbreak continued to move for several weeks. The currently active field of rootless shields took a downturn on January 18 but did not shut down. We can only speculate that the rootless shields will gradually die out over the next few weeks.

### Volcano Activity Update

The condition of Kīlauea's eruption is described above.

No earthquakes were felt on the island during the week ending January 21. Mauna Loa is not erupting. The summit region continues to inflate. Seismic activity remains very low, with no earthquakes located in the summit area during the last 7 days.

A small swarm of earthquakes took place at Loihi, the submarine volcano off the south coast of the island, on January 21-22. The largest earthquake to date has been of magnitude 3.6, at 4:49 p.m. on January 21. This is the first significant swarm at Lo`ihi since January 12-16, 2003.