What are the Time Scales of Ridge Processes?

John C Mutter and Maya Tolstoy
Lamont-Doherty Earth Observatory

The physical, chemical and biological processes that interact to form mid-ocean ridges, and the oceanic crust they produce, operate at many spatial scales but also at almost the entire range of temporal scales that the Earth exhibits.

Plate motions and large-scale flow in the mantle exhibit the greatest spatial scales of all Earth processes (the Pacific Plate is the largest moving object on Earth) with time scales of 100’s of millions of years.  Flows in hydrothermal systems are very fast and can change in a matter of hours to weeks to years and associated biological systems vary on a similar range of time scales.  Faulting and volcanism in the form of dyking and surface eruption must have a recurrence time on the order of years to decades to be consistent with mass balance calculations, dyke widths and fault spacing.  Crustal magma systems must evolve and change over time scale shorter than eruption intervals while the stability of mantle upwelling zones, though not known, are likely to be on the order of thousands to millions of years.

Mid-ocean ridge physical structure is often described as having an ordering that gives rise to the idea of segmentation scales.   Temporal scales are unlikely to be randomly distributed and processes operating at ridges may give rise to a few dominant time scales. Cyclical processes, such as tidal triggering of earthquakes and tidal modulation of hydrothermal flow are well documented.  Evidence for consistent dike widths and relatively steady state spreading rates suggest a cyclical nature to crustal formation and magmatic processes.  Other processes may be quite aperiodic and possibly have power law frequency/magnitude distributions so common in Earth processes.  Time series sampling to date has not established the full range of ridge processes temporal characteristics.   Technology to monitor deep sea processes with high fidelity over time scales of years has only recently started to become available making the temporal domain a new frontier.

While high-resolution seafloor mapping has provided some sense of the range of spatial scales of ridge processes we presently have little sense of the shape of the spectrum of process time scales or how processes might interact in time. Very little is known, for instance about temporal scales of magma dynamics.  Does the magma body change geometry (enlarge) before an eruption then deflate or instead does the liquid fraction progressively increase and fluid pressure rise until the eruption occurs?  Seismic time-lapse imaging techniques widely used in the oil industry to track depletion of hydrocarbons in producing fields and more recently for monitoring sub-surface storage of CO₂ can be readily applied to study magma dynamics. R/V Langseth’s seismic systems have sufficient resolution and repeatability that repeat surveys can be performed to study changes in shape and properties of the magma body. The 2008 3D cruise to the 9°N area of the EPR where the eruptions took place made the first set of images for this purpose.  Monitoring of the ridge to detect any precursors to eruptions such as increases in microseismic activity would help identify the most critical time for a repeat survey and these measurements as well as others could be integrated to build a picture of ridge dynamics.

Future ridge studies should focus on strategies to assess the wide spectrum of time scales of ridge processes and how spatial and temporal scales interact.