Deep crustal hot zones control shallow magma reservoirs in an active transcrustal magmatic system

Understanding volcanic dynamics and assessing eruptive risk require examining the entire transcrustal magmatic system. However, direct imaging of magma distribution and connectivity from the Moho to the surface remains rare. Changbaishan-Tianchi volcano, known for one of the Earth’s largest explosive eruptions, has recently undergone repeated magmatic unrest. To investigate the magmatic system beneath Tianchi volcano, we invert magnetotelluric data to image its three-dimensional resistivity structure, revealing a vertically extensive low-resistivity anomaly system spanning the entire crust. This system comprises three compositionally distinct, vertically connected melt reservoirs: a broad lower-crustal basaltic reservoir (20–35 km), an upper-crustal trachytic reservoir (5–10 km), and an uppermost comenditic reservoir (2–3.5 km). Estimated melt fractions are 2.1–3.7%, 7.0–16.3%, and 18.8–23.8%, respectively. Our results suggest progressive upward magma transfer among these reservoirs via well-resolved conductive pathways, consistent with recent episodes of unrest. Notably, melt volume in the lower crust is about one to two orders of magnitude greater than in the upper crust, a pattern also observed at Yellowstone and likely applicable to active volcanic systems at both divergent and convergent plate boundaries. These findings suggest that deep crustal hot zones may fundamentally control shallow melt volumes and thus the eruptive capacity of polygenetic volcanoes. Deep crustal melt accumulation controls the shallow melt volumes and hence the eruptive capacity of polygenetic volcanoes, according to geophysical and petrological analysis.