June 2025 | Oceanography
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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