Time-lapse thermal image movie of Halemaumau Overlook Vent. The electronic tilt graph shows the radial tilt at Uwekahuna (UWE), on the northwest rim of KÄ«lauea’s caldera, and PuÊ»u ʻŌʻō (POC), on the north flank of PuÊ»u ʻŌʻō cone, as recorded by continuously operating electronic tiltmeters. Positive changes often indicate inflation of the magma storage areas beneath the caldera, but may also result from heavy rainfall or, occasionally, instrumental malfunctions. The Y-axis is in microradians, an angular measure in parts per million; for example, one microradian represents the tilt of a 1-km-long bar, one end of which is lifted up or down 1 mm. May 21, 2015. Images courtesy of USGS/HVO

Time-lapse movie of Halemaumau Overlook Vent. May 14-21, 2015. Images courtesy of USGS/HVO

Kīlauea Volcano’s summit lava lake dropped from nearly overflowing on April 26, 2015 (left), to about 62 m (203 ft) below the newly created (by multiple overflows) vent rim on May 15, 2015 (right). Photo courtesy of USGS/HVO

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

Kīlauea made the news this past month with many changes, including an elevated lava lake level at the volcano’s summit, which provided spectacular views from the Jaggar Museum overlook in Hawai‘i Volcanoes National Park, and increased earthquake activity. As of this writing, the lava lake level has dropped and is no longer visible from Jaggar Museum, and the earthquake frequency has decreased to background levels.

So, what exactly happened over the past month, and what does it mean?

First, let’s review what happened. Unusual activity began on April 21, 2015, with an abrupt onset of rapid summit inflation and a rise of Kīlauea’s summit lava lake. By April 24, the rising lava lake, which normally is too deep within its crater to be seen by Park visitors, came into view from the Jaggar Museum vantage point. Inflation continued and the lake kept rising, eventually spilling lava onto the floor of Halema`uma`u Crater for the first time on April 28. Along with the inflation and high lava lake levels, earthquakes in Kīlauea’s upper East Rift Zone were also increasing in rate and size.

The inflation, rising lava lake level, and increased earthquake activity all pointed to increasing pressure in Kīlauea’s summit magma reservoir and upper East Rift Zone. Increased pressure in a magma storage system can be explained by either more magma coming into it (due to an increase in magma supply) or less going out of it (for instance, due to a blockage of magma flowing out to the Pu‘u ‘Ō‘ō vent on the East Rift Zone). Preliminary interpretations suggest that the recent increased pressure was driven by a brief increase in magma supply.

Regardless of the cause, increased pressure at the summit means a higher likelihood of new activity on Kīlauea, such as an intrusion (injection) of magma into the rock that makes up the volcano—an intrusion that might, or might not, make its way to the surface and form a new eruptive vent. The most recent example of this occurred in March 2011, when building summit pressure produced an East Rift Zone intrusion that resulted in new vents opening west of Pu‘u ‘Ō‘ō and erupting fountains of lava for about five days. Intrusions (and associated eruptions) act as a pressure release “valve” for the summit, and are generally accompanied by deflation of Kīlauea’s summit as magma leaves the reservoir.

High lava lake levels and inflation were sustained through the first week of May, but around May 10, the summit began to deflate and the lava lake level began to drop. On May 14, HVO recorded the onset of a sharp increase in small earthquakes in the southern part of Kīlauea’s summit caldera. Around the same time, a nearby electronic tiltmeter (an instrument that measures tiny changes in the slope of the ground) began to tilt rapidly away from the south caldera.

Earthquake activity in Kīlauea’s south caldera peaked on May 15, with more than one earthquake occurring each minute. Rapid ground tilting also continued, but diminished to low levels by May 17.

Now, let’s address what all this means. Our preliminary interpretation is that, beginning in late April, an increase in magma supply to the shallow magma storage system beneath Kīlauea’s summit resulted in inflation and a rising lava lake level. By mid-May, the summit reservoir could no longer accommodate the influx of magma. This triggered an intrusion of magma from the reservoir into the southern part of Kīlauea’s caldera, and, as the intrusion reduced pressure in the summit, the lava lake level dropped.

This pattern of inflation, seismicity, and subsequent intrusion in response to excess magma pressure is a common phenomenon at KÄ«lauea. Although the heightened activity diminished to background levels earlier this week, more changes could still occur at KÄ«lauea in the coming days to weeks. Because of this, HVO scientists continue to keep a close eye on KÄ«lauea. Daily eruption updates are posted on the HVO website at http://hvo.wr.usgs.gov/activity/kilaueastatus.php. You can also sign up to receive email notifications of volcanic activity at http://volcanoes.usgs.gov/vns/

Video courtesy of Tropical Visions Video with air transportation by Paradise Helicopters.