Chasing a "Magma Gopher": Insights on Intricate Subsurface Magma Pathways at Sierra Negra Volcano, Galápagos

Starting on 26 June 2018, following thirteen years and more than 5 m of intra-caldera inflation since its last eruption, Sierra Negra Volcano started erupting vigorous lava flows from several fissures scattered on the northern flank of the volcano. We present space-geodetic measurements spanning this ongoing event, showing more than 7 m of caldera subsidence and unprecedented evidence for multiple shallow, sub-horizontal intrusions that propagated for several kilometers before feeding fissure eruptions. We use interferometric synthetic aperture radar (InSAR) measurements from a constellation of satellites (the European Space Agency's Sentinel-1A/B, C-band, and the Japan Aerospace Exploration Agency's ALOS-2, L-band, satellites) and build a stack of interferograms, each featuring meter-scale surface displacements. The Sentinel-1 InSAR data, acquired with a 6-day revisit time, allow us to closely follow the intriguing evolution of a narrow sill intrusion that propagated radially away from the summit for 20 km, taking a 90-degree turn along its path to wrap around a second intrusion and possibly align with the local stress field before reaching the surface. Then, the distal portion of this sill inflated and deflated multiple times in phase with the discontinuous eruptive activity, as recorded by satellite thermal emission data (e.g., MODIS). Despite the longer revisit time, ALOS-2 InSAR data (see Figure) maintain coherence in areas where displacement gradients are too large for C-band data, and the dramatic surface changes are used to characterize the geometrical properties of the magmatic intrusions. Sub-horizontal sills have previously been invoked as the preferred means of magma transport at Galápagos volcanoes but the new InSAR datasets presented here offer unique insights on the complexity of such intrusions. Finally, we use optical (e.g., Sentinel-2) and radar data to map lava flow emplacement and obtain estimates of erupted volumes, and compare them to volumes intruded in the subsurface and withdrawn from areas of storage, with the aim of constraining the physical properties of the magma.

Figure: ALOS-2 co-eruptive interferogram over Sierra Negra Volcano. Each color cycle represents 0.20 m of displacement in the line-of-sight (LOS) direction, between the ground and the satellite.