Magazine article Oceanus

The Magnetic Thickness of a Recent Submarine Lava Flow

Magazine article Oceanus

The Magnetic Thickness of a Recent Submarine Lava Flow

Article excerpt

Submarine lava flows and their associated narrow feeder conduits known as dikes constitute the basic building blocks of the upper part of the ocean crust. We are only beginning to understand how lava erupts and forms on the seafloor by flooding topographic lows, flowing through channels or tubes, centralizing into volcanoes, or some combination of all of these. (See article on page 11 for a discussion of these volcanic processes.) The style of emplacement along with the extent and volume of individual lava flows and eruption rates are important parameters that help determine the initial properties of oceanic crust, its vertical and horizontal structure, and what processes control the magma supply to the crust.

Less than a handful of seafloor eruptions have ever been monitored in real time or near real time, so when a swarm of seismic events showed characteristics of an eruption off the west coast of the US in July 1993, the American science community mobilized to take advantage of this unique opportunity. The seismic activity was initially detected by seafloor sensors on the CoAxial ridge segment of the Juan de Fuca mid-ocean ridge system at 46 [degrees] 15 [minutes] N, 129 [degrees] 53 [inches] W, but then over a period of just two days the activity marched 40 kilometers north along a narrow band of the sea floor to center on 46 [degrees] 31.5 [minutes] N, 129 [degrees]35 [minutes] W, where activity finally dissipated after a few more days.

Research cruises sent to this latter site discovered a seafloor eruption had indeed occurred, forming a new lava flow up to 30 meters thick, 2,500 meters long, and 400 meters wide. To the north and south of the lava flow, a linear narrow fault-bounded valley, called a graben, was also discovered, oriented along the same trend as the lava flow. This narrow graben is the surface expression of the subsurface feeder conduit or dike zone that fed the lava to this eruption site from the magma chamber located some 40 kilometers to the south.

We were fortunate that the Juan de Fuca region had been mapped relatively recently with modern bathymetric systems and thus repeat bathymetric mapping after the CoAxial eruption allowed the pre-eruption topography to be subtracted from the post-eruption topography to obtain an estimate of the new lava flow's thickness. Pre-emption bathymetric surveys are not always available, however, which precludes using differential bathymetric mapping to determine lava flow thickness for many regions. One possible solution is to measure a property that is proportional to the volume of the new lava, such as its magnetic anomaly. Newly erupted lava is thought to be initially highly magnetized before it degrades to less magnetic minerals through seawater alteration. Highly magnetized lava thus should produce a distinctive magnetic signature relative to the older lava. Individual lava flows are typically thought to be on the order of a few tens of meters thick and a few kilometers long, requiring close-up, near-bottom magnetic surveys rather than distant surface surveys to obtain the requisite resolution for detecting such features. Magnetic surveys also offer some potential advantages over differential bathymetric mapping. Surface ship bathymetry has a relatively large footprint of 100 meters square and a limiting depth resolution of 5 to 15 meters, although near-bottom bathymetric mapping could improve on this resolution by an order of magnitude. Depending on the geometry and density of the magnetic survey tracks, near-bottom magnetic mapping could have both a small effective footprint and the ability to map flows thinner than 5 meters. The magnetic mapping method also has the advantage that it can be done after the lava flow has erupted and does not require pre-eruption surveys.

Six months after the eruption, in late 1993, we used the submersible Alvin to carry out an initial survey of the new floor and found that the new lava flow did indeed have a strong magnetic anomaly associated with it and that the anomaly could be directly attributed to the new lava. …

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