maandag 27 mei 2013

Did Scrat just trip?

This weekend we watched the fourth part of the Ice Age movies, called Continental Drift. Scrat the squirrel, always finding a spot to bury his beloved acorn, breaks a mountain and falls into the core of Earth. Arrived at a very small and solid core, he accidentally rotates the core, causing old continents to break up and drift appar, forming the current continents (Australia, Antarctica, Africa, Europe, South America and Asia. North America is formed at the end of the movie, where Scrat pulls the plug of Scratlantis (Yes, again Atlantis, it just must exist), forming North America.). A really funny scene and it gave me inspiration for this post. This weekend I was struggling to think about a blog subject. Thanks to Scrat, I found my acorn: Continental Drift.

In my research I work with topography, gravity and seismic observations. I am trying to combine all these observations in a new way, such that I get better insight of the old continental structure under Scandinavia, Finland and northwest Russia. The continental drift in that area is not the most interesting effect observed, so we travel to Hawaii (which is always a good thing to do.).

Hawaii is a very interesting place on Earth (yes, the Hoolahoop girls and cocktails, but also geophysically). The islands are big volcanos erupted from the deep ocean floor. This Google maps figure (since Google maps, geology can be done from your lazy chair at home) clearly show that the islands of Hawaii are situated in the middle of the Pacific Plate. This oceanic plate is one of the largest plates on Earth. The boundaries are visible as subduction zones in the northwest (Japan and Indonesia), where a lot of earthquakes and volcanos are present. In the east (USA) a famous transform fault is visible (just use Google and zoom in at the area between Los Angeles and San Francisco, or look up the San Andreas fault on the internet). However in the middle of the Pacific plate, no boundaries are present. So why is there a large volcano?

What I like about this story is when you inspect the bathymetry (the ocean floor topography, keep up, I told you this already) you can see an underwater mountain range situated to the northwest and then suddenly bending straight north. As a little kid, scrolling through many atlas maps, I was amazed by this feature, wondering about the cause. Many years later I found it out.

Cause: Continental drift and a very special volcano. Hawaii is a special kind of volcano: a hot spot. From the deep interior (most scientist say at the core-mantle boundary) hot mantle material is brought up (because hot things always go to the top) and penetrates (...) the Earth's crust creating large volcanos, like those at Hawaii. This would create a single volcano in the middle of the Pacific plate, if the plate wouldn't move. However thanks to Scrat (or mantle convection, we're still debating on this), we have continental drift. The solid top layer of the Earth moves, very, very slowly. This can be observed with for example GPS stations. 

I got this figure from here. It is a good website to start with, if your interested in the topic. The black arrows represent the direction and magnitude of the absolute motion of the individual plates. You can see that the Pacific plate is quite large, covering almost the whole Pacific Ocean. When you look at Hawaii you can see a black arrow pointed in the same direction as the underwater mountain chain (This is no coincidence). With respect to the fixed (we assume it is fixed, we're not really sure, but we're not really sure of a lot of things) hot mantle plume, the plate moves over this hot area, generating several volcanos on its trajectory. So in some sense the underwater mountain chain is a historic scar of the Earth's surface recording its behavior. 

Looking at the magnitude of the velocity of the Pacific plate (look lower left of the figure), we can say that the plate moves with around 25 cm/yr. Knowing this and the length of the straight mountain chain, we can say that the Pacific plate is moving in the same direction for about 50 million years (if the hotspot is fixed, which it is, we think...). Before that an event happened (maybe a large collision with an other plate) causing the plate to rotate in the current direction. The scar before this event was situated to the north (maybe Scrat tripped).

Just looking at topography (and bathymetry, otherwise you only see ocean waves) and GPS observations can solve my childhood mystery. I think Earth sciences are great, and even more today, with access to all this beautiful satellite data (You must say satellite every day, mustn't you. YES!!!). You can play with volcanos and moving plates. And you can write something after seeing Ice Age 4: Continental Drift, hehe...

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