Southern Exposure

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Diamonds

As christmas is quickly approching, I was thinking of all the gifts that I'd love to give, but, alas, cannot afford. So instead I thought I'd talk about something that I like. Diamonds.

Ever wonder where a diamond comes from? This is an amazing story of suspense and intrigue. Well maybe...
Ok, the diamond on your hand or on the hand of the person next to you was formed at a very violent place. And no, I don't mean all fo the conflict taking place in Africa over the mining of the diamond. diamonds are formed at a a depth of about 100km (60 miles) down in the mantle of the earth. The diamond itself is a crystallized carbon, which in fact is the hardest known substance on earth. What this means is that a diamond can scratch anything. But then in a moment of great violence, magama and diamonds are thrust up toward the surface through a conduit called a kimberlite pipe. The name kimberlite comes from a town in Africa, Kimberly. Many of these pipes exist in South Africa, but recently, many have been discovered in Brazil and even Canada! The pipes themselves are not very interesting, but as you can see from the diagram, the diamonds are located throughout the pipe- and that is of course what we are all interested in.

The pipe is then mined, and the raw diamonds are found. The diamonds are so valueable that the mining companies often justify the processing of large amounts of rock just to get a few diamonds.


These raw diamonds are then cut to form the many cuts that we enjoy today. I personally like the brilliant cut, below is a guide of how to facet the diamonds:

Tectonic subduction and what it means for you.

I will preface this post with a very brief overview of a fascinating geological process. Lets start with a cross section of the earth.

Beneath our feet, as most of us know, are several large layers of rock, magma, and core materials. (see picture above)...

Our planet, on in its surface, is broken up into large pieces called tectonic plates. Over the course of millions of years, these plates have shifted around to create the continents as we now currently know them.


These processes still continue today, and for the most part are a large cause of many of the earthquakes and volcanoes that we experience today. Along the boundaries of these plates there are different movements that they make. At some locations, plates collide, at others they get pushed apart, and at others they grind past each other. The collisional boundary is called a subduction zone when one plate gets pushed beneath another and eventually is forced back into the mantle and is re-melted. The boundary where the plates get pushed apart is called a divergent plate boundary (and where this occurs in the ocean, it called a mid-ocean ridge).
Here is a diagram explaining these processes in a greater detail.

Hopefully that made sense. Lets take that bit of knowledge and apply it to the really cool part.

Off the coast of Washington, Oregon, and Northern California is a small tectonic plate called the Juan De Fuca Plate. This plate has been colliding with the North American Plate for the past several millions of years (however, it used to be called the Farralon plate). As the plate is getting thrust down into the earth, it goes deeper into the mantle. Obviously this heats it up quite a bit and re-melts some of the crust. That melted crust is then forced upward and becomes the volcanoes that we have in that part of the country.

I like this diagram a lot better...

That is why there are all of the volcanoes in the Pacific Northwest. The magma being erupted is re-melted crust that was initially made out at sea. Isn't it amazing how mother nature recycles herself???

Ok, so essentially you have an area that makes oceanic crust (a mid ocean ridge) and another that consumes it (a subduction zone). I wonder what would happen if the two were to collide? The answer to that is rather simple arithmetic. 1 + (-1) = 0. Such an event DID happen, and that is why we have the San Andreas Fault.

Read on...
"EVOLUTION OF THE SAN ANDREAS FAULT.

This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system."

"Map of the modern San Andreas Fault in relation to the greater plate-tectonic setting of western North America and the northeastern Pacific Ocean basin. The San Andreas Fault represents a great transform-fault boundary between the North American Plate and the Pacific Plate. The San Andreas Fault system connects between spreading centers in the East Pacific Rise (to the south) and the Juan de Fuca Ridge and Mendicino fracture zone system (to the north). The San Andreas Fault system has gradually evolved since middle Tertiary time (beginning about 28 million years ago). The right-lateral offset that has occurred on the fault system since that time is about 282 miles (470 km); however, the fault system consists of many strands that have experienced different amounts of offset."


So that brings us up to today. What will happen in the next few million years? Well, if everything keeps going at the rate its going, the JdF Plate will eventually be consumed and the San Andreas fault will continue unabated north up to Alaska. Of course all of the continent on the pacific plate (ie. Western Cali, Oregon, and Washington) will be making a slow journey north, detaching themselves from the rest of the Continent)

So, to answer Dad and Ethan's question...I'm of the opinion that the long term earthquakes that the last post is in reference to is earthquakes caused by the subduction of the Juan deFuca Plate under Washington and Oregon. These long-term quakes don't make the area any more safe than they would be otherwise. You live on the top of a huge conveyor belt that is always moving, eventually it will snag. Not necessarily all at once, but parts here and there will get stuck and then you'll have your large earthquakes.. Here is a diagram as evidence for my theory:

What this shows is a plot of earthquake location and depth. It is a cross section, with the top being the surface. That nice pattern of dots the arcs gracefully off to the lower left is the actual plate being forced down. this subduction is not a stop-and-go procedure, but a slow, ever-moving event.

RISING TIDES INTENSIFY NON-VOLCANIC TREMOR IN EARTH'S CRUST

Ooh, this is worth a post and discussion... It comes from the NASA Earth Observatory (who, by the way, has an image lab on the same floor as the geology department at ASU. The original page can be found here: http://earthobservatory.nasa.gov/Newsroom/MediaAlerts/2007/2007112225969.html)

For more than a decade geoscientists have detected what amount to ultra-slow-motion earthquakes under Western Washington and British Columbia on a regular basis, about every 14 months. Such episodic tremor-and-slip events typically last two to three weeks and can release as much energy as a large earthquake, though they are not felt and cause no damage.

Now University of Washington researchers have found evidence that these slow-slip events are actually affected by the rise and fall of ocean tides.

"There has been some previous evidence of the tidal effect, but the detail is not as great as what we have found," said Justin Rubinstein, a UW postdoctoral researcher in Earth and space sciences.

And while previous research turned up suggestions of a tidal pulse at 12.4 hours, this is the first time that a second pulse, somewhat more difficult to identify, emerged in the evidence at intervals of 24 to 25 hours, he said.

Rubinstein is lead author of a paper that provides details of the findings, published Nov. 22 in Science Express, the online edition of the journal Science. Co-authors are Mario La Rocca of the Istituto Nazionale di Geofisica e Vulcanologia in Italy, and John Vidale, Kenneth Creager and Aaron Wech of the UW.

The most recent tremor-and-slip events in Washington and British Columbia took place in July 2004, September 2005 and January 2007. Before each, researchers deployed seismic arrays, each containing five to 11 separate seismic monitoring stations, to collect more accurate information about the location and nature of the tremors. Four of the arrays were placed on the Olympic Peninsula in Washington and the fifth was on Vancouver Island in British Columbia.

The arrays recorded clear twice-a-day pulsing in the 2004 and 2007 episodes, and similar pulsing occurred in 2005 but was not as clearly identified. The likely source from tidal stresses, the researchers said, would be roughly once- and twice-a-day pulses from the gravitational influence of the sun and moon. The clearest tidal pulse at 12.4 hours coincided with a peak in lunar forcing, while the pulse at 24 to 25 hours was linked to peaks in both lunar and solar influences.

The rising tide appeared to increase the tremor by a factor of 30 percent, though the Earth distortion still was so small that it was undetectable without instruments, said Vidale, a UW professor of Earth and space sciences and director of the Pacific Northwest Seismograph Network.

"We expected that the added water of a rising tide would clamp down on the tremor, but it seems to have had the opposite effect. It's fair to say that we don't understand it," Vidale said.

"Earthquakes don't behave this way," he added. "Most don't care whether the tide is high or low."

The researchers were careful to rule out noise that might have come from human activity. For instance, one of the arrays was near a logging camp and another was near a mine.

"It's pretty impressive how strong a signal those activities can create," Rubinstein said, adding that the slow-slip pulses were recorded when those human activities were at a minimum.