April 9, 2015

Thanks Florian! And hello again everybody.

We’re finishing shooting our second and third profiles, and it is probably a good time to tell you a bit more about the science we are doing here. Throughout this blog I’ve been trying to write for a general audience of scientists and non-scientists alike, but assuming the targeted reader: (a) does not need to be convinced of the intrinsic value of basic research, and (b) is willing to glide past technical terms to get the big picture. But that doesn’t mean that a bit of effort isn’t needed on my part to convey the richness of this story.

If one looks at an image of the Earth – using Google Earth for example – you will find that more than two-thirds of its surface is oceanic. The reason the oceans are deeper than the continents is that they are more magnesium and iron rich (“mafic”), and therefore denser as the lithosphere “sinks” into the more viscous asthenosphere. Then, as subduction zones (e.g. under Seattle) pull the oceanic plates, mid-ocean ridges make new crust where the asthenosphere rises up (“upwells”), melts with the dropping pressure, and forms the crust via intrusions and eruptions of that melt. Think about that episode of “I Love Lucy” where they have to keep putting chocolates onto the conveyer belt….

The fastest spreading ridge in the world is the East Pacific Rise, and it forms about a 6-kilometer thick crust of gabbro (the oceanic equivalent to granite) and basalt that spreads off the axis at well over 10 centimeters per year, about the rate that your fingernails grow. In contrast, the Mid-Atlantic Ridge is quite slow, spreading at less than 3-4 centimeters per year. The two systems are remarkably different, with the Atlantic hosting high massifs of exhumed (“dug up”) mantle and lower crustal rocks in areas where, for reasons we don’t really understand, the crust was either not completely formed, or is completely absent! Some of these are well over the size of Mt. Rainer, so they are not trivial objects, and in at least one place seafloor vents on top of them host a spectacular ecosystem.

In 2005, I was on cruise where we drilled into one of these “core complexes,” the Atlantis Massif, and found that most of its center was made of lower crust, rather than exhumed mantle. Though the second such result, it was a bit of a surprise, and we have learned a lot about how melts get introduced into the crust from the mantle from that project, among other things. But if one looks at the gravity anomalies and seismic images of the Atlantis Massif and structures like it, many of them appear to be made of intrusive crust with bits and pieces of mantle scattered within them and adjacent to them. This is certainly not true for all of them, however, and some parts of the Mid-Atlantic Ridge have exhumed mantle and other parts look like normal oceanic crust. A lot of research is going into understanding what determines the kind of oceanic lithosphere we observe, but it seems to have something to do with the duel between mantle-melting and breaking, or faulting of the crust.

A really peculiar aspect of all of this, however, is that when geophysicists sort of gloss over some of the complications I remarked on before, and look at the seismic velocities of different ocean basins, they find that the thickness of areas with crustal velocities stay more or less the same, overall. The big exception is where spreading rates dive below about two centimeters per year, at which point the crustal thickness drops precipitously. The easy explanation for this is that the mantle upwelling is less vigorous in these places, so it melts less easily, and less crust is created. Unfortunately, it is difficult to say much more about these areas because most of the places in the world where this happens are relatively remote, such as along the Arctic ridge system (the Gakkel), and the Southwest Indian Ocean. The Cayman spreading center is thus pretty important because it is relatively accessible, relatively short, yet has all of the features I described: core complexes, basalt fields, hydrothermal vents, etc…Our work here is thus to use the seismic velocities we get via the OBS-recorded refractions to determine the nature of the crust and mantle here, which should allow us to test some hypotheses for why core complexes form, why ultraslow spread crust is so thin (to absent), and how these ultraslow spreading centers evolve over time.

There’s more to it than all of that, and I can hopefully write an entry about how this all affects the price of milk, but I’ll leave it there for today. For an operations update, I can tell you that we are ahead of schedule! By preparing our deployments while recovering the previous profile, and efficiently getting the guns deployed, we shaved off half a day. Doesn’t sound like much, but at sea it can turn into two extra days by the end of it all (knock on wood), and every minute counts!

-Nick

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