Physical Geology 2003


This Italian submersible was bought by the US Navy and explored the trench in 1960. For 'deep' thoughts related to this adventure, go to the following website. Photo also courtesy of:


The Marianas Trench did not simply spring into existence. Rather it is the product of intense Geologic Activity that has taken place on this earth for hundreds of millions of years.

We must begin with a brief introduction about the earth and its various layers. Starting from the inner core we travel towards the surface to the outercore with temperatures decreasing as we ascend upwards. We then find outselves in the lower mantle and eventually the upper mantle. This is where most geologic activity occurs that affect humans and their immediate environment. The upper mantle is divided into the lithosphere and asthenosphere. The lithosphere lies closest to the surface with a relatively thin 'crust' on top. There are two classifications of lithosphere; continentel and oceanic, the latter being thinner, older, and composed of denser rock. Since it is heavier it sinks lower into the asthenosphere which is precisely why the sea floor is lower than what we call 'land'.

Headed back towards the earth's center we reach temperatures of 1,280 degrees celsius and find ourselves back in the asthenosphere. Temperatures here are extremely high and the rock is 'melted' enough to flow relative to the cooler and more rigid lithosphere. Because of its rigidity this layer bends and breaks, all the while floating on top of the asthenosphere. These broken pieces of asthenosphere are referred to as plates whose movement determines the earth's surface feature. This theory of plate tectonics and its accounting for continental drift was proposed last century by the brilliant German scientist Alfred Wegener. The theory's premise has long since been adopted by Geoscientists who recognize 12 major plates in the earth.

The points at which these plates meet are called plate boundaries, of which there are three types. For the sake of understanding the Marianas trench we shall only discuss two. A Divergent boundary occurs when two plates move away from one another. The gap created in the sea floor allows magma from the asthenosphere to rise and create new sea floor. In a convergent boundary two plates meet and one subducts underneath the other. Continental lithosphere cannot subduct because it is too bouyant, meaning that it is too light relative to the melted bed on which its rest. Therefore if it meets with more continental lithosphere the only place to go is up. Oceanic lithosphere must be the one to subduct. It should be noted however that slabs of oceanic crust can be obducted onto continental crust during colissions due to the inconsistencies of the plate. Oceanic lithosphere will always be younger. We know this because it is constantly subducting and and being added to the sea floor, whereas continental lithosphere remains on the earth's surface.

So what does all this have to do with the our precious trench? Well, the massive Pacific plate moves towards Asia and meets the much smaller Philippine plate and subducts underneath it. This creates a trench, which marks the beginning of the subduction zone. Once the plate begins to subduct, the rest follows and slowly sinks towards the earth's center, melting along the way. Since a voyage to this part of the earth is absolutely impossible, scientists can only guess what goes on at this stage. It is assumed that convection occurs in the mantle, a cyclic motion of melted and partially melted rock that occurs in loosely defined convection cells. Hence, recently sunken melted rock is reintroduced to the surface at a point further away from where it subducted. The idea that plate motion is a reaction to convection in the mantle is under question. Evidence from laboratory experiments suggests that plate motion is in fact an integral part of the convection process. Anyhow, as the Pacific plate slides past the Philippine plate, melting of lithospheric rock occurs, creating magma which rises to the surface of the overiding plate.

A volcano is formed. This occurs all along the convergent boundary and is a direct product of subduction. So on the overiding plate's side of the convergent boundary a volcanic arc will always be found. Magma constantly finds its way to the surface, cools, and accumulates. After millions of years of this, the accumulation will eventually be large enough to rise above the water's surface and will be called an island arc volcano.

The volcanic island arc opposite the Pacific plate is called Micronesia. Guam, the biggest of these beautiful islands is the peak of the large mountain created by the Pacific plate subduction. The tropical paradise of Micronesia hosted some of the bloodiest fighting of WWII. Had the U.S. and its allies lost the war, the British would never have had the liberty to cruise around the Pacific Rim. The trench may have never been discovered.

Image courtesy of:

Also, subduction creates much friction as the sinking plate slides past the overriding plate. This friction is what creates earthquakes. However, this activity occurs many kilometers below the earth's surface and the residents of Micronesia live further still from the surface and so are not at great risk of earthquake catastrophes.


Diagrams courtesy of:

The above diagram illustrates perfectly the processes which result in the creation of a trench. Note that the Volcanic Island Arc occurs on the side opposite the subducting plate. Were this actually the Marianas trench we would be facing South with the Pacific plate on our left.

"Enraged at having to bow to its enemy plate and sink to the fiery depths of the earth, the subducting plate takes revenge by melting the lithosphere of its nemesis and scars its surface forever" -Rory O'Shaugnessey, 1998

. The diagram below shows the progression of the mountain building caused by lava accumulation at a subduction zone.

Literature Cited

1. The Earth's Crust and Upper Mantle. Pembroke, J. Hart.William Byrd Press. Richmond, VA. 1969.

2. Plate Tectonics and Crustal Evolution. Condie, Kent. Pergamon Press. New York. 1989.

3. The Earth's Mantle: Composition, Structure and Evolution, Jackson, Ian. Cambridge University Press.Cambridge. 1998.


Shinkai 6500, it's a bad motha!

Original photo by Ron Parker

Check it Out!

History of Trieste



Last Revised: 4/8/2003

Creator: Ruairi K. Rhodes, Junior Spanish Major

Earlham College in Richmond, IN

Designed for Physical Geology 211, Professor Ron Parker


Marianas Trench

The Marianas Trench is the deepest point in the earth's surface!

Located in the South Pacific, the trench is deeper than Mt. Everest is high, reaching reaching about 7 kilometers down into the earth's crust. In fact, if Mt. Everest were relocated to the bottom of the trench, its peak would lie about a mile below the water's surface. It is the fountainhead of geologic progress, at once a living example of the most intense geologic activities and the most miraculous creations of biochemistry.

Image courtesy of:

The Eastern Pacific Rim, Psychadelic Jimi Hendrix style. Trench indicated by Purple Haze

Brief Human History of Trench

The trench was discovered in 1951 by a British vessel named the Challenger. At that time the depth was determined using sonar and the deepest part of the trench was named the Challenger Deep. After centuries of human wonder, myths and dieties, the deepest part of the world's vast ocean was found. In 1960 the Trieste reached the trench's bottom with a soft thud and recorded a depth of 35,797 feet. This voyage was made by the U.S. Navy and a Swedish scientist and was the only time man has ever reached the bottom. Kaiko, a Japanese submersible also reached bottom in 1995 but with no humans aboard. The depth recorded by this mission is considered by some to be the most accurate but I would personally be more inclined to believe the men who felt the thud! The Japanese submersible Shinkai 6500 is currently the deepest diving manned submarine in the world. Yet it cannot handle the trench.

Image courtesy of:

A map of the plate boundaries found in the Pacific Rim. Note that my website is the Shiznit!

Image courtesy of:

Image of the trench facing north with the Pacific plate on the right and the volcanos occuring on the Philippine side. Note that only some volcanos are tall enough to rise above the water's surface.

What Is a Back Arc Basin?

Note that in the image above there exists a large basin in between the inhabited Marianas island arc and the arc directly to their west. What are these? These are extinct volcanos which have been removed from a magma source. Their magma flow stopped long ago, so no new lava accumulates. They are not large enough to rise above the water's surface and are therefore uninhabited. This chain is called a remnant arc.

When melting occurs due to plate subduction, the magma which rises to the surface creates a volcano which begins to rift. The magma essentially splits the volcano in two and eventually their distance grows large enough to create a large basin. Since the volcanos of the back arc are cut off from their magma supply they go extinct. This process spans millions of years.


A three dimensional model of the trench and a deep ocean vent similar to those which most likely exist at the bottom of the trench. Sources Unknown.

Ya Mama's Ashtray? No, a colony of tube worms found in the ocean's deepest depths.

Da Trench: A Way of Life

The trench is...... extremely frigid as sunlight can only reach a depth of 300 meters below the water's surface. Strange looking creatures do exist in the trench, feeding on smaller animals that find their way into the trench. These creatures are highly adapted to the harsh trench life.

1,554 miles long

44 miles wide

35,797 feet deep

8 tons of pressure per square inch

To put things into perspective, the deepest dive ever recorded was by an American woman to a depth of 1,000 feet where the pressure would already be dangerous.



In Loving Memory of John B. Rhodes and in Dedication to Donald Mooney Sr.

who fought on the beautiful islands of Micronesia less than a decade before the Marianas trench was discovered.



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Earlham · Geosciences Department · Geociences 211: Physical Geology