June 2025

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June 2025 | Oceanography

15

by heat from a continuous axial magma chamber (Jamieson and

Gartman, 2020). Over at least the last 2,000 years, there have

been no large-scale eruptions with significant lava flows that

could bury these vent fields (Jamieson et al., 2013; Clague et al.,

2014, 2020). Krauss et al. (2023) attribute this to the degassing

of the axial magma chamber, which limits extrusive magmatism

and results in the mature chimneys, edifices, and mounds, both

active and inactive, that define the Segment. Concurrently, the

underlying shallow magma chamber ensures sufficient heat and

chemical flux to sustain regular hydrothermal circulation and

consistent sulfide structure growth.

Long, continuous time series of physical parameters provide

opportunities to reveal complex dynamics and ongoing evolu­

tion of the vent system. For example, Barreyre and Sohn (2016)

correlated vent fluid temperatures with bottom pressure fluc­

tuations and estimated the permeability of the shallow upflow

zones near hydrothermal venting using poroelasticity theory.

The study revealed that the Main Endeavour Field (MEF) pos­

sesses geospatially distinct shallow upflow zones characterized

by different effective permeabilities, which sets it apart from

Lucky Strike and the East Pacific Rise, sites with different geo­

logical characteristics and spreading rates.

At Smoke and Mirrors, located near the southern Benthic and

Resistivity Sensors (BARS; shown in the MEF inset in Figure 3),

Barreyre and Sohn (2016) modeled higher effective permea­

bilities characteristic of a slow spreading center, such as Lucky

Strike, with lower heat flux and a thicker extrusive layer that has

ample permeable pathways. Just 150 m apart at Grotto (located

near the northern BARS shown in the MEF inset in Figure 3),

they modeled higher effective permeabilities that are character­

istic of a fast spreading center, such as the East Pacific Rise, with

higher heat flux and a thinner extrusive layer more frequently

paved by volcanic activity.

Rather than attributing these differences to spreading rate, the

effective permeability likely varies due to output from the irreg­

ular distribution of the underlying magma body. This is corrob­

orated by anecdotal visual evidence from repeated visits show­

ing the southern part of the MEF waning in black smoker output

(e.g.,  Smoke and Mirrors edifice), while the northern part of

the field (Grotto edifice) is growing and gaining in vigor. These

results and visual observations also imply that the magma sup­

ply within intermediate spreading centers can vary in space and

time (possibly rapidly) and therefore regionally modify the ben­

thic environment to host biological communities that are more

suited to fast or slow spreading centers.

Continuous geophysical monitoring, primarily using cabled

seismometers and bottom pressure recorders (BPRs, Table 1),

tracks the tectonic activity that can drive these changes in the

FIGURE 3. ONC infrastructure within the area of the Endeavour vent fields and adjacent ridge flanks. The node (orange square) powers the instru­

ment platforms (white circles) that host scientific sensors connected to the internet via fiber-optic cables (white lines). Moorings—both autonomous and

cabled—are shown as yellow circles. Image credit: ONC

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