Fig. 1: Observations of hydrothermal megaplumes and deep-marine tephra deposits.

Locations of megaplumes detected by water-column measurements (yellow boxes) and observed deep-marine pyroclastic tephras (red circles). Observations are shown (A) globally and (B) in the NE Pacific. Boxes with solid lines show plumes that have been mapped in three dimensions and therefore have known volumes (~10–150 km³; 7 observations), while dashed lines indicate those with chemical and physical characteristics consistent with a megaplume but without a confident volume estimate (5 observations). Deep-marine tephras have been discovered in multiple locations at both mid-ocean ridges (MORs) and seamounts. These encompass the global range of MOR spreading rates and water depths of up to 4 km. The preponderance of observations in the NE Pacific (shown in (B)) is related to the concentration of marine research in this region. The location of the eruption and tephra deposit used for our inversion (Fig. 2) is shown by the star symbol in (B). Tephra observations, particularly in (B), are from¹⁵ with additional data from^{18,19,20,21,22,23,24}. Megaplume observations are from compilations by^5,6. Black lines in (A) show tectonic plate boundaries. The base map in (B) from geomapp.app.org⁷¹.