Volcano Watch — What happens when all that lava flows to the surface?

"With all the lava being erupted, is there a large, empty space within the Earth where the lava came from?" This is a frequent question answered by the staff of the U.S. Geological Survey's Hawaiian Volcano Observatory, and the answer is "no."

Kīlauea's lava originates as magma, generated by the partial melting of the Earth's mantle at a depth of at least 20 to 40 miles. As magma rises up to a shallow storage area 2 to 4 miles beneath Kīlauea caldera, slow convection currents within the mantle bring in more molten material to replace the ascending magma. When the volcano erupts, magma is drawn from the shallow storage area, and the ground above it subsides to fill the space. If a large amount of magma is withdrawn rapidly from the storage area, the ground will collapse, and this is the process in which calderas or pit craters are formed. Monitoring the movement and storage of magma within the volcano is one of the primary activities of the Hawaiian Volcano Observatory, and the measurement of ground surface deformation is one of the techniques used in this monitoring.

As magma slowly accumulates within the volcano, the ground above the storage area rises and stretches, and when an eruption occurs, magma leaves the storage area, and the ground above it subsides and contracts. A large network of permanent survey markers, stamped metallic disks called benchmarks cemented into bedrock, encompasses the volcano. These markers are used to monitor changes in the ground surface. Vertical changes are noticed by the remeasurement of the elevation of each benchmark, and horizontal changes are observed by the remeasurement of the distance between benchmarks. The slope of tilt of the volcano is also monitored to detect ground surface deformation.

The elevation of a benchmark is usually referenced to a datum, such as mean sea-level, determined from the record of the tide gauge at the Hilo pier. Starting at the tide gauge and using a procedure called leveling, the elevations of benchmarks along the highway up the summit of Kīlauea and down across the rift zone in Puna are determined. Leveling involves a level transit measuring the difference in height between two calibrated rods set on the ground about a hundred meters apart. If one of the rods starts on a tidal benchmark of known elevation, the elevation of the ground under the second rod can be calculated using the height difference between the rods. Holding the second rod and moving the first one another hundred meters in the direction of interest, another height difference can be measured and another elevation determined. Using this leapfrogging technique, the elevations of all the benchmarks along the level route can be determined. After a period of time, the entire process is repeated, and new elevations are determined for the benchmarks. Changes in elevation of the benchmarks are caused by the addition or withdrawal of magma from shallow storage areas under the volcano. The Halema`uma`u overlook parking lot subsided nearly 2 feet at the start of the current eruption in 1983. The same area typically moved up and down about 5 inches during the early cyclic episodes of the eruption.

Horizontal distances between benchmarks are measured with an electronic distance measurement instrument. The EDM is set above a benchmark, and special reflectors are set above a second benchmark within line of sight of the first benchmark. A modulated or pulsed laser beam is transmitted by the EDM and reflected back to the EDM where it is received and compared to calibrated signals. Multiples of 2000-, 100-, and 5-meter wavelength are determined for gross distances, and parts of the 5-meter wavelength are used to determine finer distances down to a millimeter. Changes in line lengths are attributed to the addition or removal of magma, but movement along faults also causes considerable horizontal changes. The distance between the Observatory and the Halema`uma`u parking lot contracted about 2 feet at the start of the current eruption, and the line length varied by about 3 inches between the early episodes of periodic fountaining.

Tilt or slope changes are measured by the precise leveling of a triangular set of benchmarks which determines a plane or by electronic tiltmeters which can sense miniscule changes in slope. Inflation causes the slope of the volcano to steepen.

Assuming that the surface deformation is caused by a magma body, the data can be used to determine the location, size and depth of the magma body. Since the last eruption of Mauna Loa, we have determined from surface deformation measurements that nearly 60 million cubic yards of magma have accumulated in a storage reservoir located about 2.3 miles beneath the southeast section of Moku`aweoweo caldera. This is roughly about 50 percent of the volume that erupted in 1984 during Mauna Loa'a last eruption.