Magazine article Oceanus

Mid-Atlantic Ridge Volcanic Processes: How Erupting Lava Forms Earth's Anatomy

Magazine article Oceanus

Mid-Atlantic Ridge Volcanic Processes: How Erupting Lava Forms Earth's Anatomy

Article excerpt

On the islands of Iceland and Hawaii, the location and distribution of eruptive fissures and vents, lava flows, and other volcanic features provide critical pieces of information for understanding how magma is supplied to the shallow crust and subsequently erupted at the surface. Mid-ocean ridge segments, which are in some ways single volcanoes - although long and narrow compared to subaerial (land) volcanoes - are not as well known, primarily because they lie at water depths of 2,500 meters or more. With advanced imaging techniques, we are, however, now obtaining data over relatively large areas of the mid-ocean ridges at the scales necessary to make the same kinds of observations and inferences about magmatic and volcanic processes there as we do at subaerial volcanoes.

High-resolution side-scan sonar imagery combined with multibeam bathymetry is providing critical new views of the seafloor and changing our ideas about how the oceanic crust forms at the slow-spreading (25 millimeters per year, about the rate fingernails grow) Mid-Atlantic Ridge. These data also enable the design of future detailed geophysical and geochemical Mid-Atlantic Ridge studies at the same scale used to understand subaerial volcanic eruptions.

The Mid-Atlantic Ridge is composed of discrete spreading segments that are tens of kilometers long, and offset by transform faults and nontransform offsets. The axis of the Mid-Atlantic Ridge is marked by a major riff valley 1 to 1.5 kilometers deep, a central floor 4 to 15 kilometers across, and ranges of crestal mountains on each side of the valley separated by 20 to 40 kilometers. The median valley walls are composed of large faults that move the crust upwards to form the crestal mountains. The central valley floor is the primary site of ocean crust construction, and most segments contain an axial volcanic ridge that runs down the center of the median valley floor. The axial volcanic ridges are themselves made up of smaller ridges, round domes, and a variety of topographic features that all amalgamate into a single larger ridge. Axial volcanic ridges may be 2 to 4 kilometers across and 100 to 600 meters high, and represent a very much larger scale of volcanic relief than found on fast-spreading ridges, which are characterized mainly by flat-lying flows.

Near-bottom sidescan sonar data collected at segments of the slow-spreading Mid-Atlantic Ridge are providing images that show volcanic structures on the floor of the median valley in unprecedented detail. At a small scale, there are two types of volcanic features: Some are composed of lava hummocks 50 to 200 meters in diameter and at most 10 to 20 meters high, while other features are coated with smooth lava flows, including one we've imaged that covers most of the median valley floor. The smooth flows are similar to those seen over most of the fast-spreading ridges (where spreading rates average 100 millimeters per year).

These two types of small scale morphology are assembled into a great variety of larger forms. The round, domelike structures are small volcanoes composed sometimes of hummocks and sometimes of smooth flows. Some of them have flat tops with craters in the middle, some rise to sharp peaks. Lava flows run beyond the edges of the domes and across the surrounding seafloor, forming brims around the volcanoes. A range of other features are linear in plan, elongate parallel to the length of the segment, and probably represent eruptions along a fissure. Commonly seen in Hawaii and Iceland, these are often composed of a row of hummocks that resemble a caterpillar.

Some segments have a greater abundance of one type of feature than others. A segment near 29 [degrees] N has a pronounced axial volcanic ridge primarily composed of hummocks and a few large circular volcanoes whose surfaces are covered with smooth flows. The axial volcanic ridge widens and narrows along its length, but is typically a few kilometers wide and about 150 meters high. …

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