Still kicking..

I realize is has been eons since I last made a post, but with my current schedule I haven't had a chance to put together anything worth reading about. Doesn't look like my schedule will ebb up much this term, but I will try to find something to put together for a blog post. I've started quite a few posts, but in the end deleted them. However, as time zips by I figured I had better post something to show this is till an active blog, if not on a semi hiatus from daily posts.

I will try to make more of an effort to find something worthwhile to write about in the future, but for now just wanted to let you all know I was still kicking. ;0)


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It's the beer..

Came across this and had to laugh... thought my fellow geologists would enjoy.

Destructive collecting

This past weekend I took Little Guy to find a carnelian. One of the iphone apps I have gave directions to the site, which took us about an hour and a half to reach. It was a nice drive and to my surprise Grim and Mandy had decided to tag along. Upon reaching our destination we were confronted with signs that made no bones about how the property owners (a tree company) felt about looking for said rocks. I was pretty surprised, until I started to take a look around. Almost very bend I could see in the creek had huge areas dug out along the banks. The hills beside the gravel road also showed massive amounts of destruction from individuals digging away the sides. Further up the creek piles of rock and mud were evident from screening and dredging for these rocks. Since when is mining activity attributed to a "hobby"?

I came across two locals on horseback and they told me that the area used to be a madhouse on the weekends with groups of people who identified themselves as being from rock clubs. While not all bad behavior can be directly attributed to these types of clubs, I do think they bear a big part of the responsibility for the site being closed. To overtax an area with too many people can take a huge toll on the locals who live in the area and want to enjoy some peace and quiet over the weekends. Not capping arranged trips to a certain amount of members can result in chaos and possibly bad behavior due to competition for getting that "rock". I've seen it happen before at other sites when there is too little room, too many people. Total madness.

In addition, if you are using mining techniques to get said rock, you are no longer a hobbyist- you are a miner! Buy a plot of land and mine it, but don't ruin rock collecting for those who like to casually walk streambeds and pick up rocks already exposed. At the very most, only digging out a rock that is half in/half out with a rock hammer. But to tear away a creekbed, adding to the sediment load.. bad! If you can remove a rock and it makes little to no visual difference, then that is a hobby or getting a rock sample. But if you are removing massive amounts to the point an entire bank is cut away, you are destroying the environment!
So thank you very much greedy people- you just effectively ruined one of the few FREE & educational activities I enjoyed with my children. At least for that area. I'm sure if this continues unchecked, more and more sites will become off-limits.



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Subduction styles/EQ magnitude

Strong Earthquakes in Latin America can be attributed to a greater release of seismic energy associated with flat slab vs.steep slab segments. The geometry of these flat slab regions allow for more stress to build up between the plates (Gutscher, et al.,2000).

Flat Slab segments lack the arc volcanoes which are prominent in western America from subducting young oceanic lithosphere, as depicted in the image above of Normal slab subduction. Normal slab subduction is characterized by maintaining a distinct low viscosity and high temperature melt in the boundary between the slab and overriding plate. Flat subduction occurs when a distinct decrease in the dip angle of the slab results the slab tip flattening underneath the plate overriding it. The wedge corner is subject to high stresses as the slab-tip is pushed into the mantle thereby constricting the corner flow above the slab.

Flat slab segments are attributed to the high magnitude earthquakes in LA, and one possibility behind the differing slab geometry is the buoyancy between the two slabs with the younger buoyant slab resisting subduction. The subduction of young buoyant lithosphere can be modified when encountering denser older lithosphere changing the thermal structure at the margin and displacing the asthenosphere away from the trench. The cooler temperature at the forearc predicts a larger locked zone therefore increasing the risk of great interplate earthquakes.



While Earthquakes occur all along the subduction zone, those in the ‘Seismogenic zone are most destructive. This is because they occur near the surface. The earthquake cycle in the Seismogenic Zone can be said to occur in two stages, the Interseismic Period and the Coseismic Period as described below:

The Interseismic Period Time between earthquakes: (10’s to 100's of years) where plate convergence continues at approximately 8cm a year (between Nazca and S. American plates) with the two plates are locking over a portion of the subduction plate boundary. This results in not only uplift, but also a horizontal shortening of the overlying plate margin.

The Coseismic Period/Earthquake rupture occurs over a few minutes. Once the accumulating stress exceeds the strength of the fault,  failure occurs in the locked zone and great earthquake occurs. During this episode stored elastic strain is released which in turn results in subsidence and horizontal extension where slow uplift and horizontal shortening had previously accumulated. In addition, the underwater displacements can cause tsunamis. Once the stress is relieved, the cycle resets and stress begins to build again.


In Latin America there is a combination of Steep slab/Flat slap segments, with the greater earthquakes occurring along the zones of flat slab subduction.  (Fig. below shows a run down of where earthquakes of greater magnitude occur as opposed to those of lesser magnitude).

Note: I couldn't find a comprehensive enough image to suit my purpose for this blog post so I created my own from a variety of figures (upper two images of steep slab/flat slab), with the principle source being from Gutscher, 2002.





Sources:
http://gsc.nrcan.gc.ca/geodyn/eqcycle_e.php
Gutscher, MA., 2002. Andean subduction styles and their effect on thermal structure and interplate coupling. Journal of South American Earth Sciences 15: 3
http://www.geo.arizona.edu/geo5xx/geos577/projects/flesch/The_Ecuador_Peru_Gap.html
http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_55-56/010021658.pdf





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