Volcanic Topography at the Mid-Atantic RidgeIntroduction
The study of seafloor volcanic features at mid-ocean ridges provides information on the geologic processes that control crustal melt supply and delivery to the ridge, and consequently, the construction and evolution of the oceanic crust. Here we show high resolution side-scan sonar images of the axial zone of the slow-spreading Mid-Atlantic Ridge (MAR). The images were collected in 1992 within a pre-existing seabeam multibeam bathymetry survey between the Kane and Atlantis transforms [Purdy et al., Mar Geophys. Res., 1990]. An analysis of these data shows that the construction of discrete volcanic edifices (seamounts) is the dominant style of volcanism on the inner valley floor.Moreover, the side-scan sonar images reveal that these volcanoes have distinct and diverse morphologies not evident in the multibeam bathymetry. Although the resolution of the side-scan sonar is not good enough to discriminate between individual flow morphologies such as those of sheet flows and pillow basalts, two distinct seamount surface textures are evident on the scale of tens of meters: hummocky (bulbous), and smooth. Examples of these hummocky- and smooth-textured seamounts are shown below.
The format of each of the figures is the same and is described here. Bathymetry data are shown above the side-scan sonar images. These two data sets are not co-registered, however, and matching the two is often subjective. Color corresponds to water depth, and 20-m contours have been plotted. Yellows/oranges are shallower than 3400 m; greens between 3400 m and 3700 m; blues between 3700 m and 4000 m; and purples and roses are deeper than 4000 m. The bathymetry has been shaded using the intensities from the side-scan sonar data; thus the side-scan textures can be seen "through" the bathymetry. Each plot shows a patch of seafloor approximately 2.5 km wide and approximately 3.25 km long.
Volcanoes
Hummocky Seamounts
Two examples of
hummocky seamounts. The left panel presents a hummocky seamount surrounded by
smooth-textured terrain; faults cut its flank. Its summit is located in a gap in the
bathymetry data so that the contours running east-west from the summit are artifacts of
the gridding program. Extrapolating from contours of only the northern flank of the seamount,
we estimate it to be ~100 m in relief. It is built from numerous hummocks, with diameters
of a few tens of meters, piling on top of each other. The right panel shows a
hummocky seamount located at the top of the axial volcanic ridge. These types of hummocky
seamounts located on the tops and sides of axial volcanic ridges are very common
at this section of the MAR. It is approximately 270 m high, with basal diameter of ~2225 m.
Large Flat-Topped Seamounts
Two
large flat-topped seamounts. The left panel shows a cratered seamount; it i
s ~220 m in relief with a basal diameter of ~1500 m and a summit diameter of
~600 m, most of which is taken up by the crater. The summit is cut by faults and
fissures, and down-dropped blocks are observed on each side of the crater.
Hummocks cover its flanks. The right panel shows the tallest seamount identified
by Smith and Cann [J. of Geophys. Res., 1992]. It is ~350 m in relief with a
basal diameter of ~ 2200 m and summit diameter of ~ 600 m. Numerous faults cut
its flanks, and hummocks cover parts of the flanks and summit. A shallow graben
is observed at the summit.
Small Flat-Topped Seamounts
Two
short flat-topped seamounts. The seamount shown in the left panel has a summit
height of ~ 70 m, a basal diameter of ~1600 m, and a summit diameter of ~1100 m.
The summit crater appears to be filled with hummocks. Hummocks also occur at the
edges of the flat top. The seamount shown in the right panel has a summit height
of ~120 m, a basal diameter of ~1350 m, and a summit diameter of ~625 m.
Small ridges extend to the north and south of this features, and it is possible
that the seamount was built when an original fissure eruption collapsed to a single feeder.
The top is faulted and a down dropped block has formed on its western side.
A smooth textured patch occurs between two of the ridges to the south.
Hat Seamounts
Two examples of
hat-shaped seamounts.In the left panel the central dome of the feature is at
the left of the image, only half of the edifice is shown. The dome is encircled
by a smooth-textured brim. The summit height of the entire feature is ~80 m,
the basal diameter ~2100 m, and the diameter of the central edifice is ~ 600 m.
A taller hat seamount is shown in the right panel. It has a summit height of ~ 200 m,
a basal diameter of ~1875 m, and a summit diameter of ~800 m. Its smooth surface is dotted with hummocks.
Small Ridges
Two examples of
small volcanic ridges. The left panel shows "Tadpole Ridge". It is ~3300 m long,
~400 m wide, and 30 m high, on average. A circular volcano is located at the
southern end. A smooth textured brim encircles it. The along-axis orientation of
the ridge mimics neighboring faults and fissures, and it is inferred to have erupted
from similar fissures. The right panel shows "Caterpillar Ridge". It is at the center
of the image casting a shadow to its right. It is ~2400 m high, and ~450 m wide.
Its height is not well defined in the bathymetry data. Because of its segmented
appearance, it is thought that an original fissure eruption collapsed to several
point sources along its length.
Discussion
How magma is supplied to the crust and ultimately to the seafloor to build the distinctive topography on the inner valley floor at this region of the slow spreading MAR is currently unknown. Volcanic edifices may be fed from discrete small magma bodies that rise into the crust or from small-scale instabilities that develop in an existing large crustal magma body. In the latter case, there are strong parallels to fields of monogenetic cones in continental areas on Earth. It is, of course, impossible to constrain the crustal plumbing system from the morphology alone, which is only one piece of the puzzle. We believe that linking these small volcanoes with detailed rock sampling and detailed geophysical studies is the next critical step in understanding how the shallow crust is constructed at the slow-spreading MAR.References: