The 2018 Kīlauea caldera collapse revealed pressure changes in two magma reservoirs

A volcanic collapse as a window into the magmatic system

The 2018 collapse of the Kīlauea caldera made it possible to study how magma pressure beneath the volcano’s summit changed. In a paper published by the U.S. Geological Survey (USGS), researchers analyzed geodetic data collected between May 16 and August 4 to reconstruct the evolution of two reservoirs: the shallow Halema‘uma‘u (HMM) and the deeper South Caldera (SC). The USGS study describes distinct pressure trajectories in the two structures during the collapse sequence.

The moving ground provided indirect evidence about the underground system. To estimate the geometry of the reservoirs, the team used displacements measured with the Global Positioning System (GPS) and interferometric synthetic aperture radar (InSAR). They then inverted high-frequency GPS time series and ground tilt recorded in a borehole to estimate pressure changes over time and shear stress on the caldera’s ring fault.

The analysis represents the reservoirs as pressurized spheroidal cavities. Therefore, the pressures obtained are estimates derived from geodetic observations and a model, not direct measurements of magma. This distinction matters when interpreting both the figures and the relationship between pressure changes and collapse episodes.

Two reservoirs, two estimated evolutions

According to the study, the shallow HMM reservoir experienced episodic pressure increases coinciding with collapse episodes, although the overall balance across the sequence was a decrease. In the deep SC reservoir, by contrast, pressure declined monotonically. The reported figures are medians estimated by the analysis.

In HMM, the episodic increases had a median of 1.9 MPa, while the net decrease was 5.8 MPa (median). In SC, the estimated decline was 6.7 MPa (median). The comparison distinguishes short-term pressure rebounds in the shallow reservoir from its net decline over the sequence. For SC, the study describes gradual depressurization, with no major pressure changes coinciding with each collapse episode.

The team also estimated the shear stress acting on the ring fault, which is associated with the caldera’s subsidence. The available excerpt does not report a figure for that stress, so its magnitude cannot be described here.

What the result allows us to infer about magma

The authors note that the virtually zero pressure changes in SC during collapse episodes imply that this reservoir was not the exclusive source of magma to the eruptive conduit. This statement by itself does not establish whether SC supplied magma or how much it may have supplied.

The study also describes SC as having undergone longer-term deflation and as not having exclusively fed the eruptions located 40 km to the east. The pressure estimate does not by itself establish the entire path of the magma or quantify the relative contributions of different sources. The result concerns the modeled evolution of pressure and the limited conclusion that SC was not the exclusive supplier.

The general geological context helps situate the topic, but does not independently confirm the study’s results. USGS explains that knowledge about Earth’s interior combines, among other evidence, the analysis of seismic waves and laboratory experiments (Earth’s interior). That reference describes general methods for studying the subsurface; it does not validate the pressure estimates for Kīlauea’s reservoirs.

Similarly, the USGS explanation of earthquakes describes how slip on a fault releases energy in the form of seismic waves (earthquake science). This is a general framework for understanding the relationship between faults and seismic activity, not independent corroboration of the specific collapse mechanism or the reconstructed pressures.

A reconstruction conditioned by the model

The interpretation depends on representing the reservoirs as pressurized spheroidal cavities and combining the geodetic observations described by the authors. Consequently, the figures are estimates conditioned by that representation and by the data used. The excerpt does not provide uncertainty intervals, sufficient details to assess all modeling decisions, or a quantitative comparison with alternative geometries.

With those limitations, the main contrast presented by the analysis is clear: HMM had episodic increases and a net decrease, while SC underwent monotonic depressurization. The conclusion about magma supply is also limited: the results indicate that SC was not the exclusive source of magma to the eruptive conduit; they do not establish whether it contributed or the full contribution of each reservoir.