Tuesday, November 29, 2011

[Geology2] Why Can’t the Louisville Hotspot Walk in a Straight Line?

Why Can't the Louisville Hotspot Walk in a Straight Line?

Hotspots have been thought for a long time to be fixed underneath the lithosphere. This might lead one to believe they would always form a straight line of seamounts, and yet if the Louisville Hotspot were pulled over for suspicion of driving while intoxicated, and it followed the path of the Louisville Seamount Trail to show that it could walk in a straight line, it would be calling its lawyer right now. Starting at the east end, most of the seamounts are in what is close to a straight line, that is until you come closer to the Kermadec Trench on the west end, when the seamount trail suddenly takes a turn to the north. The reasons for this bend in the trail, of course, are complicated.

Pretty much all of the bumps on the image I pasted below of Pacific seafloor features are seamounts and many of them form trails. Look closely at some of the longer trails and see if you notice a trend. Go ahead, I have time.

You probably noticed that Louisville is not the only seamount trail that changes direction. The long seamount trails are like teenagers succumbing to peer pressure in that, looking east to west, they all conform to stretch out in a west-northwest direction until they somewhat abruptly shift to a north-northwest direction (I put some arrows on the image below to show that more clearly).

As I wrote about in an earlier blog, the Pacific Plate is currently being pushed by a convection current in a west-northwest direction away from the East Pacific Rise, which can be seen in the path of the youngest seamount and islands in each of the trails (if red arrows were superimposed over the eastern seamounts of these trails by some gigantic educational blogger in space, it would show the general direction the Pacific Plate is currently moving). That is not the direction the Pacific Plate has always moved, though. Before 50 million years ago, it was moving in a north-northwest direction seen in the western ends of the seamount trail (the yellow arrows), and then between 50 and 42 million years ago, the plate switched directions slightly. Scientists are not exactly sure why the switch occurred, but one strong possibility is the Pacific Plate started subducting under the Mariana Plate about this time and formed the Mariana, Bonin and Izu trenches in the process (to remind yourself what subduction and ocean trenches have to do with each other, read this blog). When an ocean plate is sinking down into a subduction zone it drags the rest of the plate with it, adding another force to the plate besides the push of the rifts and the pull of all of the other subduction zones along the rims, and so the thought is this new subduction zone started pulling the Pacific Plate in a more westerly direction.

A way to understand all the forces working on the Pacific Plate is to think about Elvis Presley circa 1957 walking into a room full of teenage girls who grab ahold of him and start pulling him in multiple directions. Then just to somehow continue this analogy, lets say that Batman and Robin showed up and started pushing Elvis. Batman is the strongest, so the direction he is pushing is pretty much the direction Elvis is going, but the pull of the teenage girls and the push of Robin are also affecting the direction Elvis is moving. The pull of the teenage girls on Elvis is like the subduction zones pulling the Pacific Plate. Batman is the convection current forming the East Pacific Rise, the strongest force on the Pacific Plate. Robin is three smaller convection rifts that are also pushing on the Pacific Plate, forming the rifts the Juan de Fuca, Explorer and Gorda Ridges near the Pacific Northwest of North America.

Another factor making it difficult for seamount trails to form a straight line is also pretty much the main reason we are currently out on this expedition. A previous JOIDES Resolution expedition (ODP Leg 197) to the Emperor Seamount trail, uncovered evidence that the hotspot that formed those seamounts (and the Hawaiian Islands), was not fixed in the mantle as was previously presumed, but had moved at least 15 degrees of latitude south over the course of its lifetime. This was a big deal, and so the scientists on our expedition are conducting similar tests to the cores we are drilling to see if the hotspot that formed the Louisville Seamount Trail has also been moving around under the Pacific Plate.

One more thing that makes it difficult for a hotspot to produce a straight line of seamounts is it is producing them on the surface of a 3-dimensional globe, rather than a 2-dimensional flat surface, but I've already broken the rules of blog post length, so I won't even try to explain that.

Like I said, complicated.


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