Introduction

Hello.  I’m glad you have signed up for Tremendous Tectonic Tours, and I will be your tour guide today.  You may refer to me as TG.  We will be taking a tour of at least four different sites on Earth that are tectonic plate boundaries, visiting different types of boundaries including convergent collision, convergent subductions, transform, and divergent.  To start, we will visit the Mariana Trench in Guam, and explore a convergent subduction.  For the second stop, we will head to Iceland to explore a divergent boundary.   Then to the west coast of the US of A to see the San Andreas fault zone, an example of a transform boundary in California.  To polish off our trip, we end in Asia to study the Himalayas, a fantastic specimen of convergent collision. Ready to go? Hold on to your seats!

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Convergent Subduction: Guam

We have arrived at out first stop, Guam!   We are here to explore the Mariana Trench, an example of convergent subduction. 

http://www.theworldsgreatestvacations.com/for-travelers/an-american-in-guam-and-saipan-without-a-passport/

  

At this at this boundary, one oceanic tectonic plate slides under another and sinks into the mantle as they converge.   As they collide, the younger of the two plates, because it is less dense and warmer, will ride over the edge of the older plate. As the older plate very, very slowly moves into the mantle, the heat causes part of it to melt, creating magma.  The magma is less dense, and rises to the surface, creating a volcano. 

https://www.uwgb.edu/dutchs/CostaRica2008/CRAccretion.HTM

Convergent subduction can cause earthquakes, because of the plates colliding, and mountains and volcanoes from the magma pushing from under the top plate. These volcanos then have the potential to form strings of islands, such as the Mariana Islands.  In this situation, the Pacific plate, the largest in the world is subducted under the much smaller Mariana plate. Mariana Trench itself has had no major recorded earthquakes after the 1600s, due to mud.  The mud is a special type of soft rock called serpentine, which allows the stress caused by earth’s movements to be released without causing any earthquakes.  

http://www.ecorazzi.com/2012/03/02/james-cameron-is-diving-the-mariana-trench-soon/

Divergent Fault Line: Iceland

We have reached our second stop, Iceland.  The whole country is sliced in half by a plate boundary, and we are here to see a great example of a divergent boundary.

 

http://www.wired.co.uk/news/archive/2012-10/23/iceland-crowdsourced-constitution

At a divergent boundary, two plates pull apart from each other, creating rift valleys and mid-ocean ridges.  When Earth’s brittle surface layer pulls apart, it usually breaks along parallel faults, tilting slightly outward from each other.  The block of earth’s crust between the fault lines cracks and drops into the interior of the earth.  The absence of that block forms a central valley called a rift.  Magma from the center of the earth seeps up to fill that crack, forming new crust along the boundary.  When a divergent boundary is formed between two continental plates, the rift can be from 30 to 50 kilometers wide.  When one is formed between two oceanic plates, the rift is much narrower, only a kilometer or less across, and runs along the top of a mid-oceanic ridge.

http://www.cotf.edu/ete/modules/msese/earthsysflr/plates3.html

Earthquakes can occur along the fault lines of divergent boundaries, and volcanoes are formed where the magma reaches the surface.  Iceland is splitting along the Mid-Atlantic Ridge, the boundary between the North American and Eurasian plates.  As Iceland is pulled apart, new crust is created by the magma rising through the crack created by the divergent boundary, but it also creates a rift along the boundary.  Iceland will inevitably break apart into two separate landmasses at some point in the future, as the Atlantic waters eventually rush in to fill the widening and deepening space between.  The consequences of plate movement are easy to see around Krafla Volcano, in the northeastern part of Iceland. Here, existing ground cracks have widened and new ones appear every few months. From 1975 to 1984, numerous episodes of rifting, or surface cracking, took place along the Krafla fissure zone. Some of these rifting events were accompanied by volcanic activity. The ground would rise 1 or 2 meters before abruptly dropping, signaling an eruption. Between 1975 and 1984, the displacements caused by rifting totaled about 7 meters.

Krafla Volcano
http://www.willgoto.com/1/146084/liens.aspx

Icelandic Fault Line
http://jscnwy.wordpress.com/iceland/

Transform Boundary: San Andreas fault zone

We are heading to our third stop, California, to see an example of a transform boundary in the San Andreas fault zone.  

 

http://totalfinancialinsurance.com/total-financial-business-blog/2013/02/californias-new-disclosure-to-seniors-rule-is-now-in-effect/

At a transform boundary, two plates are sliding horizontally past one another. Most transform faults are found on the ocean floor. The two plates grind against each other, creating friction, and forming lots of earthquakes and strange land arrangements. They commonly equalize active spreading ridges, producing zigzag plate boundaries, and are generally defined by earthquakes.  Although most occur in the ocean, some occur on land. 

 

http://www.gweaver.net/techhigh/projects/period1_2/Yellowstone/Plate%20Tectonics.html

The San Andreas fault zone in California is a transform boundary that connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda-Juan de Fuca-Explorer Ridge, another divergent boundary to the north. The San Andreas is one of the few transform faults exposed on land. The San Andreas fault zone, which is about 1,300 kilometers long and in places tens of kilometers wide, cuts through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 centimeters per year. The movement at this boundary has caused many earthquakes. The presence of the San Andreas boundary was brought to attention on April 18, 1906, when sudden displacement along the fault produced the great San Francisco earthquake and fire. The earthquake took about 700 lives and caused millions of dollars worth of damage in California from Eureka to Salinas and beyond. The earthquake was felt as far away as Oregon and central Nevada, and was estimated at a magnitude of 8.3 on the Richter scale.  On May 18, 1940, an earthquake of a magnitude of 7.1 occurred along a previously unrecognized fault in the Imperial Valley. Clearly, this fault is part of the San Andreas transform boundary.

http://en.wikipedia.org/wiki/San_Andreas_Fault