Off the coast of Oregon, scientists have been monitoring Axial Seamount, a restless underwater volcano, for decades. Using seafloor instruments, they track its magma fill-ups, tremors, and approximate eruption timing. However, a direct view of its internal structure remained elusive until now.
Upon finally generating a 3D map of the volcano’s interior, the team discovered it defied textbook predictions. Something fundamental was missing—something geologists had assumed was a universal feature.
For years, geologists operated under a standard model. This model posited two distinct layers above a deep magma chamber: lava flows on top, and beneath them, a thicker zone of vertical cracks filled with solidified magma.
Evidence from rock samples and ancient seafloor formations exposed on land supported this theory. These cracks were believed to feed every eruption, with lava cooling and new oceanic crust accumulating layer by layer.
This model holds true for many mid-ocean ridges. But what about locations where a hotspot—a persistent heat source from Earth’s mantle—has been supplying magma to the same spot for millennia? This scenario presents a different picture.
Satish S. Singh of the Paris Institute of Earth Physics (IPGP), along with a team of US colleagues, embarked on a quest for evidence. The findings of their new study were published in Nature Communications.
Located approximately 300 miles off the Oregon coast, Axial Seamount sits at the junction of the Juan de Fuca Ridge, a Pacific Ocean seafloor rift, and the Cobb Hotspot. Its horseshoe-shaped caldera rests at a depth of 4,500 feet.
Axial Seamount has erupted three times within living memory: in 1998, 2011, and 2015. In 2019, a research vessel deployed acoustic cables across a 16-by-10-mile area of the seafloor above Axial. This equipment sent sound waves into the rock and recorded the returning echoes.
Singh’s team processed these recordings with enhanced methods compared to previous studies, effectively mapping the volcano’s internal structure down to the molten rock beneath.
The novel 3D visualizations revealed a structure that contradicted textbook expectations. The anticipated thick zone of vertical cracks, known as sheeted dikes, should have been situated between the lava and the magma. However, it was absent.
Instead, layers of lava flows extended over 10,000 feet deep, reaching the very top of the magma reservoir, with nothing in between.
While some vertical fractures must exist to allow lava to reach the surface during eruptions, the classic dike layer appears to be missing across most of the surveyed area.
The lava layers themselves exhibited unusual behavior, tilting inward towards the central crater rather than lying flat. Deeper layers showed a more pronounced tilt—up to 18 degrees near the underlying magma.
Singh’s team proposed two likely explanations for this phenomenon. Cracks on the volcano’s flanks might have gradually widened, creating space beneath. Alternatively, substantial eruptions could have depleted the reservoir, causing the seafloor to sag, with subsequent fresh lava filling the resulting depression.
Here, the surprises continued. The 3D images showed molten rock spreading outward within the lava layers themselves.
These horizontal intrusions of molten rock, termed intrusive sills, were being injected laterally into older lava, rather than ascending through vertical fractures.
This pattern of lateral intrusion directly observed within the upper crust of an active volcano had never been documented before. This lateral movement suggests the volcano’s plumbing system operates differently from the standard model, possessing a unique architecture.
The deeper lava layers are in direct contact with the hot magma reservoir, as evidenced by the 3D imagery. Water-rich oceanic rocks melt at around 800°C, a temperature significantly lower than the over 1093°C of fresh magma.
The Singh team hypothesizes this proximity is sufficient to melt the older lava, reincorporating it into the underlying magma. This suggests that crustal growth isn’t the sole process occurring; significant recycling may also be taking place.
The concept of mixing isn’t entirely new, as geochemists had inferred it from unusual lava compositions. However, the 3D imaging now provides concrete evidence of the physical structure enabling such processes.
These tilted lava layers have only been observed in one other location: Iceland, which also sits atop a hotspot. Similar to Axial Seamount, the layers in Iceland tilt inward, towards the spreading ridge.
Previous explanations for this tilting attributed it to the immense weight of accumulated lava. However, the researchers now suggest that the deflation of the magma reservoir following major eruptions could be the primary driver of this bending.
If this theory holds, the mechanism of crust formation at hotspot-influenced ridges, including the one that formed Iceland, might be considerably different. Thick dike formations might only occur where magma supply is more limited.
Axial Seamount has been building pressure and is long overdue for a significant eruption. Since 2015, the volcano has been re-energizing, with seismic monitoring indicating it is approaching another eruptive event.
Instruments at the connected observatory will record the system’s dynamic responses: the rise of new magma, reservoir deflation, and seafloor subsidence.
This discovery fundamentally alters our understanding of the upper crustal structure in hotspot-affected ridge environments. It’s not a neat, two-layered cake but rather a stack of lava flows interleaved with molten intrusions, partially melted where they meet the underlying reservoir.
Researchers studying Iceland, ancient seafloor exposures on continents, and other hotspot-fed mid-ocean ridges now have a new comparison model. The conventional narrative of how oceans construct their floors clearly requires substantial revision.
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