On October 30, 2014, the front of Kīlauea Volcano’s active lava flow stalled only about 155 m (170 yards) from Pāhoa Village Road (lower right corner). As this photo was taken on November 5, 2014, active lava, indicated by smoke from burning vegetation, continued to break out of the lava tube just upslope of the stalled flow front, threatening the Pāhoa Solid Waste Transfer Station (upper center) and other infrastructure. Today, active breakouts are about 15 km (9.5 mi) upslope of Pāhoa Village Road. USGS photo.

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

A lot has changed over the past year on Kīlauea Volcano. One year ago, the June 27th flow was threatening to cross Pāhoa Village Road and, potentially, Highway 130. Lava destroyed one house on November 10, 2014, and was moving downslope towards many others.

Now, there is no imminent threat to Pāhoa, and life in the lower Puna District of Hawai‘i Island has largely returned to normal.

So, what exactly happened to the Pu‘u ‘Ō‘ō eruption over the past year?

In a word: disruption. Pu‘u ‘Ō‘ō has been erupting nearly continuously for over 32 years, but continuous does not imply steady. Instead, fluctuations in lava supply rate have been common, and these disruptions have changed the vigor and reach of the flow field activity over the past year.

Let’s use a car as an analogy. A drop in the lava supply rate is like easing your foot off the gas pedal. The lava flow, like the car, will slow down. If the lava supply rate drops enough, the flow can stall, similar to a car without enough gas.

An increase in the lava supply rate is analogous to pressing down on the car’s gas pedal. The lava flow, like the car, will speed up. But, if the lava supply rate (or a car engine’s RPM—revolutions per minute) gets too high, their respective systems can rupture or fail. Very high lava supply rates can trigger breakouts of lava from the tube system, which rob lava from the flow front, causing it to stall—just as running an engine too high can trigger mechanical failure and cause the car to stop.

As with a car, a lava flow needs to operate within a certain “envelope” of efficiency to continue advancing. Lava activity outside this envelope—too little or too much—causes the flow front to stall. Over the past year, major fluctuations in the lava supply rate have pushed the flow out of this envelope several times, disrupting the flow front advance.

A simple way to track lava supply rate is to look at Kīlauea’s summit tilt. Deflation of the summit magma reservoir is normally associated with a drop in lava supply rate, while inflation means an increase.

The day before Halloween in 2014, a rapid drop in the lava supply rate occurred, causing the tip of the June 27th flow to stall about 155 m (170 yards) from Pāhoa Village Road. About two weeks later, a major increase in lava supply rate triggered upslope breakouts from the tube, completely shutting off the flow front near Pāhoa Village Road.

The lava supply rate held steady enough for the next two months that a new lobe of lava advanced downslope once again, this time towards the Pāhoa Marketplace, in late December. Fortunately, the lava supply rate was low and the flow front essentially stalled in January 2015.

Another increase in lava supply rate triggered breakouts upslope of Pāhoa in mid-March. As in November 2014, these upslope breakouts robbed supply from the flow front near Pāhoa Marketplace, causing it to shut down.

Since March 2015, surface flows have remained active, but haven’t strayed too far from Pu‘u ‘Ō‘ō, staying within about 8 km (5 miles) of the vent. In fact, the farthest reach of Kīlauea’s active breakouts has retreated slightly the past couple months, and, as of this week, was 6.4 km (4 mi) northeast of Pu‘u ‘Ō‘ō.

This is good news, of course. But why haven’t the flows advanced very far over the past six months? It is most likely due to a relatively low lava supply rate—or, in car terms, the gas pedal simply isn’t being pushed very hard at the moment.

With flows now closer to Pu‘u ‘Ō‘ō and farther upslope from residential areas, there is no immediate threat to homes or roadways, as there was a year ago. But, as we’ve seen throughout this eruption, lava supply rates fluctuate, and future increases in lava supply could restart slow advancement of the flow front toward communities.

Changes in lava flow activity can occur abruptly and with little warning, so the USGS Hawaiian Volcano Observatory continues to closely monitor the June 27th flow with on-site field visits, helicopter overflights, webcam imagery, and satellite data.

Time-lapse movie of Pu’u ‘O’o Crater North Flank from the North Rim. October 30-November 5, 2015. Images courtesy of USGS/HVO