Primavera 1998 —On the islands of Iceland and Hawaii, the location and distribution of fissures and vents, lava flows, and other volcanic features provide important information for understanding how magma is fed to the surface crust and then erupts at the surface. Segments of the mid-ocean ridge that are more individual volcanoes, although long and narrow compared to subaerial (terrestrial) volcanoes, are not as well known, mainly because they occur at sea depths of 2,500 meters or more. However, with advanced imaging techniques, we are now getting data over relatively large areas of mid-ocean ridges on the orders of magnitude needed to make the same types of observations and inferences about magmatic and volcanic processes that we make about subaerial volcanoes.
High-resolution side-scan sonar imagery combined with multi-beam bathymetry is providing crucial new views of the seafloor, changing our ideas about how the slowly expanding (roughly 25 millimeters per year) mid-Atlantic oceanic crust forms (nails grow). Comb. This data also enables future detailed geophysical and geochemical studies of the Mid-Atlantic Ridge to be designed at the same scale used to understand subaerial volcanic eruptions.
The Mid-Atlantic Ridge consists of discrete spreading segments tens of kilometers long offset by transform faults and non-transform faults. The Mid-Atlantic Ridge axis is characterized by a large rift valley 1–1.5 km deep, a central floor 4–15 km wide, and ridge-like mountain ranges on either side of the valley spaced 20–40 km apart. The walls of the middle valley are made up of large faults that move the crust up to form the crest mountains. The central valley floor is the main building site for oceanic crust, with most segments containing an axial volcanic ridge running down the middle of the central valley floor. Axial volcanic ridges are composed of smaller ridges, rounded domes, and a variety of topographical features that merge into a single larger ridge. Axial ridges can be 2 to 4 kilometers wide and 100 to 600 meters high and represent a much larger extent of volcanic relief than rapidly expanding ridges, which are mainly characterized by shallow flows.
Ground-level side-scan sonar data collected on segments of the slowly expanding Mid-Atlantic Ridge provides images that show volcanic features on the central valley floor in unprecedented detail. On a small scale, there are two types of volcanic features: some consist of lava mounds 50 to 200 meters in diameter and no more than 10 to 20 meters high, while other features are covered by smooth lava flows, including one that we photographed , which covers most of the central valley floor. The smooth currents are similar to those seen on the fastest-spreading ridges (where rates of spread average 100 millimeters per year).
These two types of small-scale morphology combine to form a wide variety of larger forms. The round, dome-shaped structures are small volcanoes, sometimes made up of hills and sometimes gentle rivers. Some of them have flat peaks with craters in the middle, others rise to sharp peaks. Lava flows flow past the rims of the domes and over the surrounding sea floor, forming rims around the volcanoes. A variety of other features are linear in plan, parallel to the length of the segment, and likely represent eruptions along a fissure. These are commonly seen in Hawaii and Iceland and often consist of a series of mounds resembling a caterpillar.
Some segments have a higher frequency of a trait type than others. A segment near 29°N has a pronounced axial volcanic ridge composed mainly of hills and a few large circular volcanoes whose surfaces are covered with smooth rivers. The axial ridge widens and narrows along its length, but is typically a few kilometers wide and about 150 meters high. In some places, flat topographical elevations covered with smooth textured flows extend east and west from the base of the axial volcanic ridge.
In contrast, a segment at 25°N was recently inundated by gentle lava flows covered by a variety of smaller, recently erupted formations. It is intriguing that these two segments, separating at the same rate, exhibit dramatically different volcanic styles: hill-like flows and the formation of an axial ridge at 29°N versus flows that are flat and diverge from their roots. at 25°N. It is also interesting that the small-scale volcanic morphology in the middle of the 25°N segment is similar to that commonly seen in rapidly expanding ridges. This indicates that the correlation between eruption style and variables such as erupting magma volume and outpouring rates may blur the effects of propagation velocity on eruption styles; In fact, eruptive styles can sometimes be similar between fast and slow propagation. Ridges and different between ridge segments with the same spread rate.
How are the volcanic structures we see on the sea floor formed? An eruption occurs when magma moves up and along the segment through a vertical dike or fissure, crossing the seafloor and flowing to the surface through a fissure. We know the properties of fissure eruptions on land and assume that the same processes are at work on the seafloor. At the beginning of a fissure eruption, currents are violent and tend to spread quickly. If the fissure eruption continues for a long period of time, it will first develop into multiple sources and then into a single source, and over time the flow will lose strength. Magma overflowing from the vent at low to moderate rates tends to concentrate in the channels. If the flow is constant, the channel can be covered to form a lava tube.
Sidescan sonar image of a small volcanic ridge lying at the bottom center of an expanding segment near 25°N. The lighting comes from the right. The ridge is about 3.3 kilometers long, 400 meters wide and 30 meters high. Alignment along the axis of the ridge mimics adjacent faults and fissures and are believed to have arisen from similar fissures. The smooth, flawless flows surrounding the ridge are interpreted as part of the eruption that formed the ridge. These flows cover already existing faults and fissures. Information about small-scale volcanic products and faults derived from such images provides insight into magmatism and tectonics processes along the axis of the Mid-Atlantic Ridge.
Low relief flows are likely to occur early in an eruption when flows are fast and tend to spread quickly. Undulating ridges form when fissure eruption is confined to multiple vents along its length, similar to the spray cones or splash walls that form along fissures in Iceland and Hawaii. A large circular volcano (seamount) forms when lava flows from a single vent, similar to the flow from the Pu'u 'O'o cone that marks the current vent in the East Rift Zone of Kilauea Volcano, Hawaii. Surface currents, ridges, and seamounts are therefore the building products of a fissure eruption. On a larger scale, axial volcanic ridges form from multiple eruptions concentrated in a relatively narrow area of the central valley floor.
Since lava tubes and channels are common on subaerial volcanoes, it is likely that they are also common on mid-ocean ridges. The lava tubes probably feed the streams regularly covering the flanks of the axial ridges and are probably the sources of the lava that formed the semi-circular topographical highs extending east and west from the base of the axial ridge in the segment near 29° N The hill fields observed at the surface of most mid-Atlantic ridge currents may correspond to the "hill" fields common in Iceland and Hawaii. Burial mounds form where the pressure of the fluid in a lava tube or lava flow increases so much that the basalt rock that forms the surface swells and ruptures. Mounds are usually a few meters high but can reach heights of 10 meters and are oblong or almost circular in shape. Lava can erupt from the jagged surface, covering a mound and giving it the appearance of a small opening.
Determining the importance of tubes and channels in transporting lava away from the eruption site, and determining whether or not the surface textures of the mounds observed in the seafloor images are burial mounds, awaits more detailed imagery and sampling, such as that carried out on the Serocki volcano in near 22°N on the Mid-Atlantic Ridge. Geochemical evidence indicates that this flat-topped volcano is fed by lava that originally erupted at the top of the axial ridge of the volcano and flowed through a tube for a distance of about a kilometer before the tube blocked, ruptured, and allowed the lava to come out and spill. form the volcano.
What do we know about the difference in eruptive style between the fast and slow spreading segments of the mid-ocean ridge? Since fast-spreading ridges are primarily characterized by low-relief currents, we believe eruptions there are typically violent and short-lived. Hilly ridges, however, build upon rapidly advancing ridges, so fissure breakouts are sometimes protracted and must be confined to the few vents that form these features. Seamounts are rare in the axis of rapidly expanding ridges. In contrast, dome volcanoes, ordinary hills, and the generally complex volcanic topography of the mid-Atlantic ridge would arise from eruptions that violently begin to produce bas-relief flows and evolve into mountain ranges and seamounts. As the eruption continues at a moderate rate, lava-bearing tubes develop on the flanks of axial volcanic ridges, covering their surfaces with a variety of volcanic features, some producing burial mound fields.
Recent improvements in the capabilities of ground-level side-scan sonar systems now make it possible to obtain co-registered fine-scale bathymetry along with high-resolution side-scan images. As more of this data is collected, we will be able to narrow down the sizes and shapes of small volcanic features and make more rigorous comparisons to subaerial features. In conjunction with this data, it will be possible to design future geophysical and geochemical experiments on the same scale as are currently being conducted on subaerial volcanoes. Studies such as these, combined with existing data, will rapidly advance our understanding of the volcanic processes associated with the formation of oceanic crust at the Mid-Atlantic Ridge.
Debbie Smith travels to the ends of land and sea to study volcanic features. He recently received a grant from the National Science Foundation to take the public and school children aboard an R/V at sea.thompson(University of Washington) via the Internet during a cruise in October 1998 (www.punaridge.org). For a change, he dons his football boots to attend the lunchtime games at WHOI's McKee Baseball Field.
Joe Cann says he's old enough to remember before plate tectonics existed, but still young enough to enjoy exploring mid-ocean ridges. Visit Woods Hole from your UK base at the University of Leeds in the off-season, when rents are at their lowest in the Cape.