Geomorphology of Two Seamounts
Offshore Ascension Island,
South Atlantic Ocean
By:
Geoffrey Faneros and Frederick Arnold
Purpose of Survey
• Cable route planning and engineering for
Hydroacoustic Data Acquisition System
array site, which will be part of the
monitoring system for the
Comprehensive Nuclear Test Ban
Treaty.
• Nearshore survey, deepwater survey, and
sound velocity casts to verify depth of
sound channel.
• Processing conducted on-board for near
real-time route planning, array site
selection and data acquisition quality
control.
• Four days of continuous survey work.
Ascension
Island
Offshore Survey
• Offshore survey extended from
shore to seamounts through
water as deep as 3500m.
• Nearshore high-resolution
survey included single-beam
bathymetric and side-scan sonar
survey.
• Deep water survey consisted of
hull-mounted swath
bathymetric survey with
imagery data collected from
multibeam echo sounder.
•Data products included bathymetric
charts, slope charts, and geologic features
Location
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~8º South Latitude
~14º West Longitude
80km west of MAR
50km south of AFZ
Permanently settled in
1815
• Governed by UK
• RAF and USAF Bases on
Island
• Giant Sea Turtle nesting
site
Mid Atlantic Ridge 6° to 10° South Latitude
• Between AFZ and BVFZ
~1000km
• Slow Spreading ~ 35mm/yr
• Well defined axial Valley
• Three ridge offset divide ridge
into 4 second-order segments
• Ascension Island and Northern
Seamount lie within first
segment from AFZ on ~5 to 6
Ma seafloor
• Southern Seamount lies on
boundary between first
segment and next segment to
the south
• Two “Proto” Ascension Island
seamount lie to west of AI
•Anomalous shoal on MAR south of AI
A1
A2
BVFZ
Ascension Island Geology
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Sub aerial portion of 98km2 is only
~1% of total volcanic edifice
volume.
Highest point is Green Mountain at
879m above sea level.
Base of Ascension Island is in about
3200m of water with a diameter of
approximately 60km.
Onshore volcanic rocks include
rhyolite lava flows, trachytic domes,
scoria cones and pyroclastic
deposits.
There have been no historic
eruptions since the island has been
permanently inhabited in 1815,
however physical textures suggest
recent eruptions erupted less than
500 years ago.
Northern Seamount
•550km2 conical structure with
a basal diameter of
approximately 16km.
•Base of seamount at water
depth of 3600m with peak
rising to 800m.
•Craggy and elongated with the
same trend as prominent ridge
higher on island flank.
•Some individual flow units can
be seen.
•Superposition suggests
seamount is younger than base
of Ascension Island.
No.Seamount: backscatter over bathymetry
Slopes
• Areas of very
steep irregular
slopes.
• Terraces or
lobate
appearance.
• Potential slope
failure on
seamount flank.
Material types
• Rock
• Scattered rock
• Rock partially covered by
sediment
• Slope and channel debris
• Course to fine grained
volcanic materials
• Pelagic sediments
Geomorphic Features
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Radial channel pattern
emanating from summit.
Channels could have
formed from slope failures
of unstable volcanic
materials or by pyroclastic
flows during eruptions.
Terraced appearance from
individual flows.
No evidence of faulting.
However seamount
summit and associated
ridge correspond to faults
exposed on the island.
Absence of summit
caldera
Northern Seamount Discussion
The lack of a summit caldera could suggest the absence of a shallow
magma reservoir. However we believe that the orientation of the semielongated summit and up-slope ridge, and the younger age of seamount
eruptions compared to the base of the island to be further evidence of a
flank eruption of the main Ascension Island magma chamber as the
origin of the northern seamount.
Southern Seamount
•204km2 relatively flat-topped
structure with a basal diameter of
approximately 19km by 9km.
•Base of seamount at water depth
of 3200m with peak rising to
1500m.
•Oval shaped in plan view with a
somewhat truncated eastern end.
•Superposition suggests seamount
is younger than ridge and graben
structure to the north.
Slopes
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Steeper “leading edge”
with terraced
appearance on
southwest flank.
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Steep sided with a few
prominent crags along
the central axis.
So.Seamount: backscatter over bathymetry
Material Types
• Rock
• Scattered rock
• Rock partially covered by
sediment
• Slope and or channel
debris
• Course to fine grained
volcanic materials
• Pelagic sediments
Geomorphic Features
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Fewer channels, on
slopes, not radiating from
center.
Terraces and individual
flow unit not as obvious.
Absence of summit
caldera.
Curious “wishbone”
feature on south slope.
Fracture could have acted
as conduit of magma and
the eruptive center of the
seamount.
Southern Seamount Discussion
• A relation in submarine lava
flows is believed to exist between
the rate of eruption and the flow
morphology, where higher
eruption rates tend towards
smoother lava flows. Low angle
flows may exhibit steep leading
edges.
• Southern seamount could resulted
from intermittent rapid eruptions
on low angle flank of MAR.
• Lack of summit caldera suggests
the absences of shallow reservoir.
•The craggy central area of the
southern seamount may suggest a
Peléean type of dome origin.
• These domes have lava spines
flanked by cold slope debris failues
from collapsed pinnacles.
Seafloor between so. seamount and AI
•North – northwest ridges and
intervening depressions.
•North –northeast ridge structure
appears to truncate N-NW ridge.
•Spacing between ridges is ~3km.
•Could have resulted from mass
loading of AI on young lithosphere.
•Could have formed on MAR, translated
west by spreading.
Conclusions
• These two seamount were constructed in relatively the same tectonic
setting, water depths, and proximity, yet they display widely differing
morphology.
• The northern seamount is a large conical structure, with structural
trend similar to AI, and overlapping relationship indicating a younger
age than AI while the southern seamount is flat topped, appears to be
composed of lava sheet flows and/or dome growth and overlies a
seafloor fracture that closely parallels the MAR.
• We believe the seamount had different magma sources.
• The northern seamount is a flank eruption of AI, having shared a
similar magma source - hot spot related.
• The southern seamount is related to MAR volcanic processes and may
have formed earlier and has been translated westward.