Rockfalls and Lava Lakes: What happens when they intermix?

Figure 1. The lava lake at Erta Ale volcano, Ethiopia. Source

Title:  Explosive eruptions triggered by rockfalls at Kilauea volcano, Hawai’i

Authors: Tim Orr, Weston Thelen, Matthew Patrick, Donald Swanson, and David Wilson

GEOLOGY, February 2013

Article url:  http://geology.gsapubs.org/content/41/2/207.full.pdf+html?sid=e7aadd52-ea48-4af4-87b2-1296a496130b

Lava lakes are fascinating enough by themselves, while rockfalls can be entertaining to watch, given a safe distance. But what happens when rock falls into a lava lake? Kilauea volcano in Hawai’i (Figure 2) has erupted continuously since 2008, and webcams have recorded the eruption since its onset. The lack of dangerous, explosive eruptions from Hawaiian volcanoes allows recording devices to exist close by and provide a constant view of the volcanoes and their craters. Cameras coupled with seismic detection instruments produce an excellent record of the eruptions and dynamics of the lava lake in Kilauea’s summit (Figure 3). One of the more interesting features of this volcano is the visible interaction of rockfalls into the lava lake. The overhanging crater walls frequently collapse into the lava lake, resulting in explosive outgassing and sometimes the ejection of tephra.

Figure 2. A: A map of Kilauea volcano, Hawai’i, summit showing the eruptive vent for this study. Acronyms are listed in original-article-text. A – A’ corresponds to the cross-section in B. B: Profile of the crater and the lava lake. The webcam is located on the southeastern rim of the crater and is looking down into the lava lake. Dashed line represents the highest lava lake level. Image is from original article.

Figure 3. The lava lake as viewed from the webcam. Image from original article.    

                  The lava lake was first visible in September 2008 and has since fluctuated in level between 65 and 210 m below the rim of the vent. The best observed rockfalls occurred between January and March 2011, during a period of rising lava level. Video feed from a camera stationed along the rim of the crater (Figure 4) was used to time rockfalls and subsequent explosive events. Most of the rockfalls disaggregated and created a cloud of dust during descent (Figure 3). Relatively small rockfalls produced small ash clouds when they impacted the lava lake surface, while larger rockfalls resulted in larger clouds that deposited tephra outside of the crater. The preexisting state of the lava, such as whether the lava surface was calm or still responding to a previous rockfall impact, appeared to have no effect on the eruptive response to a rockfall.  This indicates that subsequent rockfalls are just as likely to produce an explosive response as any previous rockfall event, despite the previous disruption of the lava lake surface. While the camera footage shows that all rockfalls trigger some explosive response, the reason is largely unknown.

Figure 4. A rockfall in progress. A: The rockfall momentarily after it begins to fall. B: The ash plume formed after the rockfall impacted the lava lake. Image from original article.

                  The scientists proposed an explanation for the explosive response to rockfalls where the impacting rockfall caused a “backsplash” effect. These splashes are called Worthington jet; a common, everyday example is the falling of raindrops into a puddle. These Worthington jets can be 20-30 times faster than the speed of whatever caused the jet and can sometimes rise higher than the height of the dropped object. This appears to match with the observations of a more explosive “jet” with increasing rockfall size, though the largest rockfalls tend to produce too much ash for definitive confirmation. It is thought that these rockfalls are creating a momentary impact crater in the lava, which then collapses and forms a Worthington jet that ejects small bits of lava into the air.

Worthington jet animation. From http://lcni.uoregon.edu/~dow/Projects/Splashes/Worthington_mercury_droplet_splash.html

                  Direct camera observations are important for understanding the active processes at Kilauea. These observations show us that rockfalls impacting a lava lake can produce some explosive events and may form a Worthington jet, which has not been documented before. Similar explosive events previously reported at Kilauea and at other volcanoes may have resulted from such a triggering mechanism.