Wednesday, September 18, 2013

2011 Lava Flows

Today we arrive in Victoria BC. Dave Butterfield and Jim Holden’s groups will move from the R/V Thompson to the R/V Falkor. The rest of the crew will disembark in Seattle tomorrow.

2011 Lava Flows

 In July 2011 several scientists who are now working on the Axial Seamount 2013 cruise were here doing similar work to deploy and recover Bottom Pressure Recorder (BPR) instruments that record pressure data used to interpret the inflation and deflation of the volcano (see blog entry: Measuring Pressure for more information). But in 2011, several instruments could not be found in the locations where they had been installed. Navigation systems were double- and triple-checked, but all systems were functioning properly. Volcanologists, Dr. Bill Chadwick and Dr. Scott Nooner finally observed fresh pillow basalts in a location where they had not been seen previously, and immediately recognized that there had been an eruption of lava flows since the last they had been to Axial Seamount. Their interpretation was confirmed when they found one of their colleague Dr. Bob Dziak’s ocean bottom hydrophones (OBH, used to record acoustic waves associated with earthquakes; see example below left) buried in the new lava (below right), with only the chain and flotation left coming out of the seafloor. Suddenly, the frustrations of missing instruments turned into the excitement of discovery.
Unburied OBH (left) and OBH found buried in 2011 lava flows (only chain to buoy is visible).

2011 flows with microbial bloom.
The 2011 flows weren’t initially recognized as new because where they were thick, they were covered with a thick microbial mat. Microbiologists came to realize that following the eruption, a microbial bloom occurred and had covered the warm flows (see photo at right), making them appear much older than they really were. The microbes have since died, so flows appear much fresher now than they did just after the eruption!

Northern 2011 lava flows from MBARI mapping.

When BPR data from the previous year were recovered in 2011, Bill and Scott were able to determine the date of the eruption. They saw that on April 6, 2011 the central part of the volcano deflated by 2.4 m (7.8 ft), which occurred when magma left the volcano’s edifice as it erupted. Bill and Scott’s BPR data also show a steady uplift of the seafloor from 2010-2011, at a rate which gradually increased in the months leading up to the eruption. Then, an hour before the start of the eruption, one BPR station recorded a sudden uplift of 15 cm (38 in) in 55 minutes, probably caused by magma making its way to the surface. As described in the blog entry, Red Mat Pillars, colleagues from the Monterey Bay Aquarium Research Institute (MBARI) used a multi-beam sonar on the autonomous underwater vehicle (AUV) “D. Allen B.” to map the seafloor to produce a very high resolution (1 m pixels) bathymetric map of the 2011 lavas (see map of the northern part of the 2011 flows at right).

During the final ROV Jason dive of the 2013 Axial Seamount cruise, Bill and Scott spent some time exploring the 2011 lava flows.

The MBARI multi-beam map made it easy to find the eruptive fissures, collapse pits and flow channels of the 2011 lavas. Large flow channels formed where lava was transported from the vent area to lower elevations. In this case, broad sheet flows formed in the floor of the channel with a striated surface crust that insulates molten lava flows beneath. Flow channels are easy to identify on the multi-beam map (see arrow on MBARI map) and on the seafloor (see photographs below).

Roof structure of lava flow channel on left and a closer view of the channel roof where it has collapsed (right).
After following the main flow channel, ROV Jason made its way to the northern boundary of the 2011 flow, which is marked with pillow basalts that form when molten lava erupts into water and rapidly forms a hard crust. The crust covers all sides of the lava which forms a cylindrical or tube-like shape. Often, the tubes of lava separate and form nearly spherical blobs that look like pillows (hence the name) and are common on the flow margins (see photo at right).
From the far northern margin of the 2011 flows, Jason turned to the east, up and out of the caldera along the southeast caldera wall (photo at right). Here, Bill and Scott were able to see a cross-sectional (side) view of the layers of lava flows- many of them pillows- that make up the wall of the caldera due to hundreds of years of eruptions that built the volcano.

Axial Seamount has a caldera because at some time in the last few tens of thousands of years, the top of the edifice collapsed, likely when a large volume of magma erupted, allowing large fault blocks to collapse as down-dropped blocks. The caldera now has a horseshoe shape (see map at top of blog), which is the result of lava flows filling and then overflowing the southern end of the caldera.

Caldera wall with lava flows and pillow lavas.

For more information on the pressure data that Bill and Scott used to describe the details of the eruption, see their paper (with colleagues) in the July 2012 edition of Nature Geoscience.
To learn more about the emplacement of pillow lavas, check out this movie that shows pillow lava flowing underwater off the coast of Hawaii, after it was erupted on land and flowed into the ocean. Note that the video of the flows were not recorded at Axial Seamount, but probably look similar to what would be seen there. Video footage is from the movie "Pele Meets the Sea" courtesy of Richard Pyle at Lava Video Productions.