Lava dome cycles reveal rise and fall of magma column at Popocatépetl volcano

Lava domes form by extrusion of viscous magma near volcanic vents and display a wide range of morphologies governed by magma rheology, extrusion rate, and topography. Their growth mechanisms (exogenous vs. endogenous) and changing permeability influence hazards including rockfalls, collapses, and pyroclastic density currents, as well as transitions between passive degassing and explosive behavior. Remote sensing methods, especially optical/IR photogrammetry, have been used to study domes but are limited by visibility. Synthetic Aperture Radar (SAR) overcomes cloud and plume obscuration, enabling tracking of dome and crater morphology, though interpretation is complicated by radar geometry and speckle. At Popocatépetl, since reactivation in 1994, cyclic emplacement and destruction of low, pancake-shaped domes have occurred. The volcano shows strong excess degassing, suggesting that unerupted magma degasses via a permeable conduit or convecting magma column. This study aims to resolve the shallow magma dynamics governing repetitive dome cycles and excess degassing by combining high-resolution TerraSAR-X and Sentinel-1 SAR imagery (2012–2020) with SO2 emissions (OMI) and thermal radiation (MODIS, MIROVA) over 2005–2020. The objectives are to quantify inner-crater depth variations and dome morphologies at high cadence, relate these to heat and gas fluxes, and develop a framework in which gas retention and escape control short- and long-term magma column behavior and associated hazards.

Prior studies have classified lava dome morphologies and linked growth style to stability and hazards, with transitions affecting explosive potential. Modeling (analog and numerical) and remote sensing (ground, aerial, satellite optical/IR) have advanced understanding, though clouds and plumes limit optical methods. SAR amplitude has been used to track syn-eruptive morphological changes at several domes, and InSAR has resolved broader volcanic deformation. At Popocatépetl, a long record shows cyclic dome emplacement/destruction since 1996, infilling and later excavation of the crater, and strong excess degassing. Hypotheses to explain excess degassing include degassing of deeper magmas through permeable conduits and convection-driven degassing in the upper conduit. Thermal monitoring (e.g., MIROVA) and SO2 measurements (OMI, traverse) have revealed decadal-scale variability and episodes attributed to magma recharge. Theoretical and petrological work suggests buoyancy variations from dissolved/exsolved volatiles can drive dome rise/fall and that passive degassing can depressurize reservoirs, influencing magma column height over years.

Remote sensing and image processing: 1) SAR datasets: 158 TerraSAR-X (TSX) descending acquisitions (2012–2019) in spotlight and stripmap modes (incidence ~44°), and Sentinel-1 (S1) acquisitions. Images analyzed in radar geometry to exploit line-of-sight shadowing. A custom convolutional neural network (Noise2Noise-based, trained on Colima TSX, tested on Popocatépetl) was used to despeckle intensity images, improving small-scale morphology detection. 2) Crater metrics from SAR: Inner-crater depth estimated from radar shadow length using trigonometry with incidence angle; depth per profile determined by counting maximum consecutive shadow pixels and multiplying by cos(incidence). Inner-crater diameter extracted via edge detection (Sobel, azimuth-direction profiling) with manual checks. Volumes computed assuming an inverted truncated cone with slope 60° (range 40–80° for uncertainty). Vertical resolutions: ~0.65 m (TSX), ~1.8 m (S1). Main crater infill tracked from outer-wall shadows south of inner crater. Methods validated against a high-resolution DEM (>98% accuracy). 3) Thermal data: MODIS thermal anomalies processed with MIROVA to derive Volcanic Radiative Power (VRP), with ~±30% measurement error; ~4 scenes/day combined from Terra/Aqua. Volcanic Radiant Energy integrated for magma budget. 4) SO2 emissions: OMI SO2 monthly masses (UV 270–500 nm) stacked over 0.05° grids, cloud/row anomaly filtered, then converted to monthly fluxes via calibration against traverse methods (R²=0.759; MF=0.751×MM), with seasonal correction. Uncertainty ~50%. 5) Magma budget calculations (Dense Rock Equivalent, ρDRE=2400 kg m−3; ρextruded=2000 kg m−3): - Vdegas from SO2 (petrological method): Vdegas = MSO2/(2·ρDRE·ΔXs), with ΔXs=1500 ppm (tested 500–2500 ppm). - Vthermal from VRP (thermal approach): Vthermal = VRE/Crad·(ρextruded/ρDRE), Crad=1.2×10^7 J m−3 (tested 0.6–1.8×10^7). - Vextruded from SAR crater volume gains ΔV>0 adjusted by density ratio. - Vtephra assumed = (1/3)·Vextruded (semi-empirical from historical Popocatépetl dome destructions). Monthly fluxes Q derived via 365-day smoothing and monthly averaging. 6) Short-term subsidence model: Foam compaction model for the upper magma column segment H1 with initial gas fraction Xgas0. With constant magma mass and negligible gas mass, magma level change ΔH = H1(1−Xgas0)/(1−Xgas) − H1. Parameters: Xgas0=0.5, H1=100 m ±75 m, linear decrease to Xgas≈0.2 in ~30 days; ρDRE=2400 kg m−3. 7) Long-term depressurization model: Analytical model for reservoir/conduit pressure decrease ΔP(t) due to constant passive degassing (scenario of low-pressure exsolution via conduit convection). ΔH(t)=ΔP(t)/(g·ρmc). Inputs include conduit length L=10 km, initial reservoir volume V0=2 km³, host viscoelastic medium (bulk modulus k=10^10 Pa, viscosity μ=10^18 Pa), magma properties (mixture; ρnd=2400 kg m−3; μnd=10^9 Pa·s; α=3 wt% H2O; nc=2 wt% exsolved; Δρ1,2=59 kg m−3; μ1=10^5 Pa·s). Degassing flux set by magma upflow Q via convection parametrization; end-members Q=10 and 1 m³/s imply conduit radii R≈14.8 m and 8.3 m, respectively; intermediate Q=7 m³/s also considered. Model outputs compared to observed crater deepening (2012–2020).

• Repeating dome construction–destruction cycles were imaged at high spatial/temporal resolution from 2012–2020; cycles are consistent with gas-driven rise (extrusion) and fall (subsidence/drain-back) of the upper magma column. - Dome construction is rapid (hours to days), often captured in ≤1–2 TSX scenes, producing pancake-shaped domes (diameter ~45–270 m; edge thickness ~3–6 m). Occasional central lobes and annular fractures observed; construction commonly accompanied by strombolian-like activity. - Destruction proceeds via: (i) progressive subsidence beginning within ~2 days, lasting days to months, with features including central pits (~23–47 m diameter), peripheral bulges, ring fractures, and piston-like collapses; (ii) vulcanian explosions days–weeks after emplacement that fragment domes and excavate depressions of irregular morphology. - Long-term inner-crater change: from early 2012 (~220 m wide, few meters deep) to late 2019 (~350 m wide, ~150 m deep) with inner-crater volume loss ~8 Mm³ ±2 Mm³. Meanwhile, the main crater floor rose by ~15 m (±5 m), implying ~2.8 Mm³ (±1 Mm³) pyroclastic infill—far less than the inner-crater excavation. - Heat and gas trends: MODIS VRP decreased by ~20 MW; OMI SO2 flux declined from 12.8×10^3 tons/day (May 2012) to ~500 tons/day (2020). Over 2005–2020, VRP and SO2 flux are strongly correlated after smoothing (VRP [MW] = 10.5·SO2 [kilotons/day] + 1.8; R²=0.85), excluding Aug-2011–Nov-2015 (high SO2/VRP) linked to magma recharge. - Magma budget imbalance evidences excess degassing and excess thermal radiation: Vdegas − Vextruded − Vtephra ≈ factor −45; Vthermal − Vextruded − Vtephra ≈ factor −15. The average magma input rate Qin from degassing is ~2.55 m³/s, greatly exceeding the average output rate Qout ≈ 0.06 m³/s (Vextruded + Vtephra). - Short-term subsidence magnitudes (−30 to −60 m within ~30 days of emplacement) are matched by a foam compaction model for an upper column segment H1≈100 m with gas fraction decreasing from ~0.5 to ~0.2. - Long-term crater deepening rates over 8 years are consistent with a passive-degassing-driven depressurization model for magma upflow rates Q between ~1–10 m³/s (implying conduit radii ~8.3–14.8 m), with observations closely matching Q≈7 m³/s parameterization. - From mid-2016 onward, excavation rates exceeded extrusion rates, accelerating inner-crater growth (deepening/widening).

The integrated SAR, thermal, and gas datasets demonstrate that short-term dome cycles reflect buoyancy changes in the upper magma column controlled by exsolved gas retention and escape: gas-rich, low-viscosity magma rises and extrudes rapidly, followed by degassing, cooling, and crystallization that increase density, reduce buoyancy, and drive subsidence and drain-back. The subsidence reduces permeability, fostering transient pressurization and subsequent vulcanian explosions that further excavate the crater. Quantitative matches between observed subsidence and a foam compaction model support this mechanism. Over multi-year timescales, the inner-crater deepening and widening occur alongside order-of-magnitude declines in SO2 and thermal fluxes, suggesting progressive gas depletion and depressurization of the conduit-reservoir system. The long-term model reproduces observed depth changes, indicating that passive degassing and magma convection within the conduit can drop the magma column height without significant erupted volume. Excess degassing and excess thermal radiation show that large volumes of magma degas and cool at shallow levels without erupting, coherently explaining the decoupling between input and output magma budgets. These findings reconcile the observed morphology, gas/heat emissions, and dome behavior at Popocatépetl, and align with processes documented at other low-viscosity dome systems and open-vent basaltic volcanoes. The results emphasize the key role of gas pathways and permeability evolution in modulating both short-term eruptive dynamics and long-term edifice morphology and hazard.

High-resolution SAR imagery enhanced by deep learning, combined with satellite SO2 and thermal observations, reveals that Popocatépetl’s dome cycles are driven by gas-mediated buoyancy variations of the upper magma column: rapid extrusion of gas-rich magma is followed by subsidence due to degassing, cooling, crystallization, and drain-back. Superimposed on these cycles is a decadal trend of inner-crater deepening and widening, likely driven by progressive depressurization from persistent passive degassing as input magma flux wanes, with explosions and gravitational wall failures contributing to morphology. Strong VRP–SO2 coupling, along with large excess degassing and excess thermal radiation, indicate substantial unerupted magma degassing and cooling at shallow levels. These insights improve understanding of open-vent volcanic behavior and highlight the value of multiparametric satellite monitoring for hazard assessment. Future work should refine estimates of tephra output, better constrain conduit geometry and permeability evolution, integrate continuous in situ gas/thermal/seismic data with SAR, and apply the framework to other open-vent systems to generalize scaling relationships between degassing, column buoyancy, and surface morphology.

• SAR amplitude-based morphology is influenced by radar geometry; while depth is derived from shadows, complex crater shapes can produce discontinuous shadows and require assumptions, including a truncated cone geometry and fixed slope angles (60° ±20°). - Effects of explosions on dome/crater morphology between SAR acquisitions are difficult to isolate, introducing uncertainty in separating subsidence from explosive excavation. - Thermal (VRP) and SO2 flux estimates have substantial uncertainties (VRP ~±30%; SO2 ~±50%), and seasonal corrections and calibration may introduce additional error. - Magma budget relies on assumed parameters: sulfur loss ΔXs, radiant density coefficient Crad, dome bulk density, and a semi-empirical tephra factor Vtephra = (1/3)·Vextruded that likely varies through time. - Short-term foam compaction model simplifies gas-magma dynamics (e.g., ignores gravitational loading of crystallized dome and concurrent drain-back), treats gas fraction evolution linearly, and assumes constant conduit radius and negligible gas mass. - Long-term depressurization model assumes constant degassing rates, constant conduit radius, conduit convection, and no deep replenishment; real systems may experience variable fluxes, permeability changes, and complex reservoir geometry. - Limited temporal sampling of TSX (11–33+ day intervals when gaps occur) constrains resolution of rapid transients.