The subduction of seamounts can create large seismic events and with billions of people
living within 60 miles of the world’s coastline it is important to understand and accurately
monitor these features. There are several examples of subducting seamounts and their various
effects on the plate that they are subducting beneath. The Emperor Seamount Chain, Cocos
Seamount Chain, and the Kinana Seamount Chain are some of the most well known and studied
seamount chains. From research done on these seamount chains and other individual seamounts
elsewhere, I investigate the normal and critical stresses involved in seamount subduction and the
geomorphology that results from the interaction between the subducting seamount and the
continental plate. An analog model was created in an attempt to see some of faulting and
geomorphic features and in an effort to understand the critical and normal stresses.
Seamounts – individual volcanoes on the ocean floor, they are distinct from the plate boundary
volcanic system of the mid-ocean ridges because seamounts tend to by circular or conical
(NOAA). Seamounts vary in size and those that breach the ocean’s surface are called oceanic
islands, an example of this would be the Hawaiian Islands.
Accretionary Wedge – is formed from sediments that accumulate and deform where oceanic
and continental plates collide. They are scraped off the top of the downgoing oceanic crustal
plate and are appended to the edge of the continental plate (USGS).
Subduction – takes place at convergent boundaries where one tectonic plate moves under
another tectonic plate as the plates converge.
Thrust Fault – is a low angle reverse fault in which the hanging wall moves up relative to the
footwall.
Subduction Erosion – thins the upper crust by processes of tectonic erosion that detach rock
from the upper plate and transports this material toward the mantle (Scholl et al. 2003). Figure 2
displays a schematic of subduction erosion. As a seamount subducts it increases the normal
stress locally because of its excess mass and buoyancy. Observations show that the subduction of
seamounts results in a seismically locked zone that produces large earthquakes (Scholtz 1997).
Three observations that are important to the model in Figure 3:
1. Erosion on the underside of the upper plate to explain subsidence.
2. Frontal prism that reduces friction and allows subduction of all trench sediment.
3. Middle slope progressively deformed by normal faults until it breaks down into the frontal
prism.
As the oceanic plate subducts downward into the trench stress is relieved by normal faulting
which increases the graben volume.
The Cocos Seamount Chain
From Costa Rica to Nicaragua the Pacific continental margin is formed by a wedge-shaped
body of igneous rock covered by 1-2 km of slope sediment and fronted by a sediment prism. The
ocean plate that is subducting beneath is covered roughly 40 percent by seamounts off of central
Costa Rica. To the northwest of the Nicoya peninsula there is a much smoother morphology.
Important features visible in the Cocos Seamount Chain are frontal and basal erosion features.
Frontal erosion is defined as disrupted topography at the base of a slope and is most evident in
the wake of a subducting seamount. Basal erosion is caused by seamount tunneling and
removing large rock lenses of a distending upper plate. Subduction erosion at present dominates
processes off Costa Rica and most likely extends to Nicaragua (Ranero 2000).
The Emperor Seamount Chain
Research shows that there are subducting seamounts within the Japan Trench. Recent seismic
surveys have shown that the base of the overriding plate is eroded while the decollement is
shifted upward along with the subduction of a seamount. Such effects are expected to induce
local weak interpolate coupling. The damage done by this erosion may prevent the accumulation
of elastic strain energy and elevated pore pressure may reduce the effective normal stress.
The Pacific Plate subducts beneath the North American Plate at roughly 8.3 cm/yr along the
Japan Trench and contains a group of seamounts of various dimensions. Data from the study
shows that subduction of multiple seamounts has had an impact on the base of the overriding
plate, possibly eroding it. The research suggests that a subducted seamount may concentrate
stress at its subduction front where large earthquakes with a broad rupture area can be initiated
(Mochizuki 2008).
Muroto Seamount
The Muroto Seamount is thought to belong to the Kinana Seamount chain in the Shikoku
Basin. Based on the convergence ratio, 4 cm/year, the Muroto Seamount began subducting at
about 0.7 Ma. Based on numerical modeling done by Baba et al. (Figure 7) and in comparison
with observations made in the field (Figure 6), the seamount is crushed downward and sheared.
The normal stress is highest along the landward flank of the seamount because this is where it is
directly driven into the accretionary prism. Faults are expected to develop on the seaward flank
and may generate a more landward-dipping thrust fault. Near the summit the seamount is
compressed in all directions. The deviatoric stress is expected to be low and fractures aren’t
expected to develop.
Materials
To set up the analog model an apparatus was used which included: a backstop that represented
the overriding continental plate and mylar paper that was attached to a dowel representing the
subducting oceanic crust. As the dowel is cranked the mylar paper moves forward and the sand is
collected against the backstop forming a wedge. Other materials that were used included:
Quikrete Premium Play Sand, silly putty, water, sand paper, a measuring cup, a meter stick, a
camera and a tripod.
For these experiments two types of material are used for the subducting seamount, silly putty and
wet sand. The wet sand is used in an attempt to see faulting within the seamount and the silly
putty is used to try and create a more realistic geomorphic profile.
Procedure
1. Place the apparatus on a stable surface.
2. Using dry sand create a layer of sand approximately 80 cm long, 30 cm wide and with a few
centimeters of thickness on the mylar paper.
3. For the subduction of the soft seamount: Using the measuring cup, mix water and sand until
the sand is cohesive enough to hold a shape.
4. For the subduction of the soft seamount: Clear an area in the layer of sand in the location
where the seamount will be and tape a piece of sand paper down.
For the subduction of the hard seamount: do not put down a layer of sand paper.
5. For the subduction of the soft seamount: Create a small mount on top of the sand paper and
level the sand around it so that it is the same height as the rest of the layer.
For the subduction of the hard seamount: Morph the silly putty into a mound shape and stick the
seamount to the mylar paper.
6. Once all the sand is laid out and the seamount has been created, set up the camera, begin
filming and crank the dowel and pull the mylar paper under the backstop. This should begin to
form an accretionary wedge against the backstop.
7. Once the seamount has reached the backstop and begun to subduct underneath the
accretionary wedge, stop cranking the dowel and stop filming.
Video 1. In this video is an analog model of a subducting seamount. The seamount is made of
wet sand and surrounded by dry sand and is approximately 5 cm in height. The video has been
sped up and is 300% faster than the original.
Figure 10. Snapshot from the video used to construct the velocity profile displayed in the
Analysis.
Video 2. In this video is an analog model of a subducting seamount. The seamount in this video
is made of silly putty and is approximately 5 cm in height. The video has been sped up and is
300% faster than the original.
From the analog experiments with both the soft (sand) and the hard (silly putty) subducting
seamounts an accretionary wedge can be seen developing against the backstop and within the
accretionary wedge thrust faults are visible (Figure 11).
The equation σ = ρgh, gives us the normal stress, where: σ = normal stress, ρ = density, g =
gravity and h = height. As the height of the sand increases so does the normal stress and as the
normal stress increases the sand is no longer capable of holding that load, so the accretionary
wedge faults. Once the strength of the sand is too great for the fault another will form in front
where the sand is weaker.
In the analog experiment the geomorphology of the subducting seamount is also similar to
what is seen in the bathymetric images. As the seamount enters the accretionary wedge it
deforms it and small landslides are formed as the seamount continues to move further
into the accretionary wedge. These observations are consistent with observations made of actual
subducting seamounts. If the seamount were capable of continuing further through the
accretionary wedge the area would “heal” on the path that the seamount subducted through.
Some seamounts are capable of retaining their shape at depths of at least 8 km (Kimihro
2008). In the analog experiments faulting was not present within the seamount possibly because,
shear and normal forces weren’t high enough to cause the faulting. Lack of faulting indicates that
the forces were low which would’ve been a result of the seamount being in too shallow of a
position. There is also the possibility that the seamount used in the analog experiment was too
cohesive and again, the shear and normal forces weren’t high enough to fault the trailing end of
the seamount.
Figure 12b. Velocity vector map from Ryan Laracy’s (2005) project.
For my vector maps the grid that I used was too coarse to show the movement of the sand to
the sides and so any calculations and vector maps would not necessarily accurately represent the
movement of the sand. The vector map in Figure 12b shows the direction and magnitude of the
subducting seamount and accretionary wedge. The straight vectors indicate that it is oceanic
crust that has not begun to interact with the accretionary wedge. The outward flowing vectors
represent the base of the accretionary wedge, the sand here is flowing out and down and some is
even being pulled beneath the wedge. Also, as the oceanic plate begins to interact with the
accretionary wedge the magnitude decreases.
Future analog experiments that might be useful for understanding seamount subduction might
include, putting the set up from Experimental Setup into a deep enough tank of water so that the
materials behave similarly to those in a real subducting seamount. Another useful analog
experiment would be to use sand paper instead of wax paper and use wet sand for the entire
oceanic plate and have a frontal prism already formed against the backstop. Also for this setup if
instead on subducting over a flat surface, within the setup allow the oceanic plate to bend slightly
as it subducts in an effort to see subduction erosion in a cross section.
References
Visual Glossary – Accretionary Wedge. (United States Geological Survey).
Scholl, David W. et al. US Geological Survey. Exploring the Hypothesis that Subduction
Erosion formed the Prominent Aleutian Terrace and its Underlying Deep Water Forearc Basin.
2003. http://gsa.confex.com/gsa/2003CD/finalprogram/abstract_51925.htm
Von Huene et al. Generic model of subduction erosion. Geology; October 2004; v. 32; no. 10; p.
913-916.
Scholz and Small, The effect of seamount subduction on seismic coupling. Geology; June 1997;
v. 25; no. 6; p. 487-490.
Ranero and von Huene, Subduction erosion along the Middle America convergent margin.
Nature; vol 404; April 2000.
Mochizuki et al. Weak Interplacte Coupling by Seamounts and Repeating M ~7 Earthquakes.
Science; vol 321; August 2008.
Baba et al. Deformation of a seamount subducting beneath an accretionary prism: Constraints
from numerical simulation. Geophysical Research Letters, vol. 28, no. 9, p. 1827-1830, May
2001.