Monday, January 6, 2014

Geo 730: Jan. 6, Day 371: Sheeted Dikes

Unfortunately, we only made one stop with really good exposures of sheeted dikes, but it was a good exposure. Originally, these planar intrusions of basalt would have been more or less vertical in orientation, but they were rolled over onto their side during the accretion of this terrane. Indeed, it's that roll-over that makes for the nice, continuous sequences in the Josephine: if they were still close to vertical, one would observe little variation in the discrete blocks between faults. As it stands, geologists can walk steadily up- or down-section, not continuously, but in segments that allow reconstruction of an entire sequence from the upper mantle to a thick pile of sea floor sediments.

I can't say for certain at this point, but it looks as if the coarser, salt-and-pepper looking material under the hammer's head is a remnant of the gabboic magma chamber, but the majority of the rock in this photo is comprised of dikes intruded into dikes intruded into dikes. This was once described to me as "all dikes, no host rock," which was terribly confusing. I think a better way to describe it is, "such a profusion of dikes, they are their own host rock." The diagram below gives a good idea of the general setting of where and how such rocks are formed, atop a spreading ridge. As the two plates spread apart, dikes repeatedly intrude as sheets between the melt lens and and the axial summit graben.
Image credit: Ocean Drilling Program Scientific Prospectus No. 106 (July 2002). Image source.

Now why belabor this? The fact is, some three-quarters of the earth's crust was and is being formed in settings just like this. But being able to walk on and look at these rocks directly is an extraordinarily rare opportunity, despite their overwhelming abundance. First, the majority of them are deep under water. Second, the majority of them are covered with at least thin, and in most cases thick, beds of sediment. Finally, these are, as a rule, under yet another thick pile of volcanic basalt- according to the above diagram, 200-800 meters worth. Furthermore, due to their relatively high density, they tend to be recycled into the mantle during subduction. It's an unusual situation in which a chunk like this gets smacked up onto continental crust and uplifted to be eroded and put within human reach.

So, as bland and unremarkable as this outcrop might look to non-geologists, understanding what we're really seeing is a breath-taking experience. And the scenery's not shoddy, either.

Photo unmodified. May 8, 2013. FlashEarth Location is, shall we say, "mostly speculative."

Addendum: By request, I've annotated a few of the dikes and other features that stand out to me. The problem with this particular view is that much of the surface on the closer side of the river is more or less parallel to the strike and dip of the dikes themselves. You can get a sense of their orientation by looking at the rough lamination of the dikes on the other side of the river. The two bits I've labeled as "klippes" are likely  high-standing remnants of the same dike, which may or may not be the same as the one I've highlighted under the handle of the hammer. Still confused? Don't worry, I have a few more photos of this spot coming up, some of which are much clearer.


Callan Bentley said...

So, I think I see moderate-to-shallowly dipping dikes in cross-section on the far side of the river. Is there anything to see in the foreground? If so, could you please call it out for me in an annotation or something? I just see lots of pale green.

Lockwood said...

Coming up shortly... for someone who nominally doesn't work, I find myself awfully busy much of the time.