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Why is the Challenger Deep so deep?

In 2003, the journal "Earth and Planetary Science Letters" published an article by six authors, entitled "Why is the Challenger Deep so deep?" The Challenger Deep is the deepest part - as far as known - of the Mariana Trench.

Abstracts are available on the web, for instance, here (at Ingenta - a large literature database) and here (at Elsevier, the publishing company of the article). I cannot show the article or post these abstracts here, as I would be breaking copyright law if I did.

What I can do without breaking the law, is talk about this article and explain it. I have gone through each section of the paper, more or less summarized it for you and added something here and there.

The paper has six authors. Nowadays, the person who is mentioned first - in this case, Patricia Fryer - has usually done most of the work. In the old days, the person mentioned first was the most "senior" author. In this case, the authors all worked at the University of Hawaii at Manoa at the time when the paper was written. Often, however, the authors of a paper are from different institutions.

The paper is set up as follows (red blocks in the images):

1. Introduction
2. Data
3. Regional geologic and tectonic observations
4. Interpretation
5. Conclusions
   
   Acknowledgements
   References

1. Introduction

There is a page at the University of Delaware, which shows the shape and location of the Mariana Trench on a map. Yes, it is that banana shape!

The part of the east (on the left) is the Mariana Trench. The other side of the banana is the West Mariana Ridge (and in between is the Mariana Arc: small islands). Guam is located near the lower end of the banana shape.

The Mariana Trench is a so-called convergent plate margin, where plates (pieces of the earth's crust) move toward each other and one plate ducks under the other: subduction.

In the above image, the two slices of cake represent the two crustal plates and the cheese stands for the earth's mantle, the zone under the earth's crust. The fruitcake plate is the subducting under the other plate. As you can see, that creates a natural low: a trench, in this case.

The Pacific plate started subducting under the Philippine Plate fifty million years or more ago. Since then, the trench has moved westward and its shape became more and more bow-like. You can see that bow shape on this page at the University of Delaware.

This particular convergent plate margin is nonaccretionary, which means that for some reason, no softer materials such as clays are being scraped off the subducting plate and collecting - or accreting - in the trench. That makes this type of trench - nonaccretionary - deeper, as a rule.

Also, the older seafloor is, the deeper it is (and older seafloor therefore ducks under younger seafloor).

The parts of the Mariana Trench with the oldest seafloor, however, are not the deepest parts. The authors then say that in this paper they are going to explain why a different part - the southern part - of the Mariana Trench is deeper than expected.

2. Data

The data were collected during two research cruises. The first one took place with the Research Vessel Moana Wave in 1997 and the second with the RV Melville in 2001. The authors state which equipment they used (the HAWAII MR1 seafloor mapping system), how they used it, and why.

3. Regional geologic and tectonic observations

The first part of this section looks at the subducting plate. South of the trench are faults that run (= "strike" in earth science jargon) more or less parallel to the trench. There are up and down movements along these faults and this has created grabens (low) and horsts (high) in block of up to 10 km wide. The vertical movement (throw: the vertical component of the rock movement) on these faults is at most 400 m and is largest near the trench.

They don't know precisely how old the rocks are here because there are no magnetic lineation data (those tend to be created when ocean floor rocks form and show the influence of the earth's magnetic field) and there are very few data from drillings for this location. The rocks might be from the Oligocene, which would be a little bit younger than the Jurassic rocks farther to the east.

The various depth estimates for the Challenger Deep differ by more than 500 meters. The first one dates back to 1957 and was 10,990 m, later corrected to 11,034 meters.

This section continues with brief descriptions of some geological structures, referring to other publications.

4. Interpretation

Some authors have concluded that most of the plate motion at the Challenger Deep area is that of rock masses sliding past each other, not over and under one another: a so-called transform fault. The authors of the 2003 paper by Fryer and coworkers say that they found clear indications that subduction is going on, at this the plate boundary. One such indication is the occurrence of earthquakes at certain depths: Benioff zone.

Moreover, they see that the downgoing plate has faults (breaks, tears) in it.

Now, to cut a long story short - and most scientists will not like things explained this way at all - the authors of the paper think that there is a tear in the downgoing plate. Not only does that make the location of the tear a bit deeper than the rest, it also has made the section on one side of that tear a bit deeper than the rest.

The latter has to do with something called "rollback". It means that as result of a particular combination of forces (think of gravity, friction and the pushing movement of the plates), it is like the subducting plate really wants to fall into the mantle. And as a result of the tear, the part on one side of the tear does that a little bit more than the part on the other side of the tear. That makes that part a little bit deeper.

The above is a plan view - so you are looking down on both crustal plates, as from an airplane flying along the length of the trench - with the fruitcake as the subducting Pacific plate. See the tear?

The above is a plan view - again as if seen from an airplane but this time flying along with the subducting plate - with the fruitcake as the subducting plate.

You may get an idea of what rollback is like if you imagine the fruitcake in the above photo as a very heavy sheet and the yellow part as not so heavy and also very softy (maybe like lemonade with a sheet of plastic or a slice of cheese on top or vanilla yogurt with something heavier on top). If you have some intuition for how these things work, you will probably get a sense of the fruitcake plate "wanting" to sink so much that you will probably understand why this "rollback" would make the trench deeper (and also a bit wider).

If you want another silly example of a situation that resembles rollback - with a similar combination of forces - try picturing standing in such a strong wind that the upper part of your body starts to fall back. There is gravity, there is force of the wind, there is friction between your shoes and the ground. At your feet, the friction with the ground keeps your feet where they are. There is no friction with the ground near your head and torso. If the wind is strong enough, it will start blowing you over while gravity start pulling that part of your body toward the ground. Your body "wants" to fall (but your muscles are trying to prevent that).

5. Conclusions

The Conclusions are not very interesting because they mainly summarize the paper again. This is a phenomenon you see more and more often. The Conclusions are supposed to place the research within a larger framework, talk about its implications and address future research.

Acknowledgements

This is where the authors say who paid for the research and who helped them.

References

This is the section where the authors give all the information about the publications they have mentioned in the paper.

LAST BUT NOT LEAST

This is theory! It is not a fact! Scientific papers often contain theories and opinions. It depends on the subject, but for some subjects, theories and opinions are the mainstay of science.