physics

Outer trench swell

Outer trench swell is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Outer trench swell rather than just read about it. In short: The outer trench swell, outer trench high, or outer rise is a subtle ridge on the seafloor near an oceanic trench, where a descending plate begins to flex and fault in preparation for its descent into the mantle at a subduction zone. The lithosphere is bent upwards by plate stresses, and is not in isostatic equilibrium (distinguish from the "outer ridge" of a forearc).

Key takeaways

Outer trench swell belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Outer trench swell to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Outer trench swell from memory before moving on to harder problems.

Reference excerpt

The outer trench swell, outer trench high, or outer rise is a subtle ridge on the seafloor near an oceanic trench, where a descending plate begins to flex and fault in preparation for its descent into the mantle at a subduction zone. The lithosphere is bent upwards by plate stresses, and is not in isostatic equilibrium (distinguish from the "outer ridge" of a forearc).

Characteristics Typically, the gravity field over the outer swell is about 50 mGal (0.5 mm/s²) higher than expected from isostasy, while gravity over the trench is about 200 mGal (2 mm/s²) less than that expected from isostatic considerations. The bending of the plate is associated with tension in the upper 20 km, and shallow earthquakes, caused by tensional failure induced by the downward bending of the oceanic plate are common; about 20 extensional outer rise earthquakes with magnitude 5 or greater occur annually. Most tension axes are perpendicular to the trench, independent of the direction of relative motion between the two plates, indicating that failure is controlled by bending stresses in the plate. Plate bending also causes deeper (down to 50 km) earthquakes due to compression. The wavelength and amplitude of this flexure can be used to constrain the state of stress across the plate boundary. The width of the outer rise is directly related to the flexural rigidity of the lithosphere. The thickness of the elastic lithosphere varies between 20 and 30 km for most trench profiles. Faulting related to plate bending and stair-stepping of the descending slab into the trench may allow seawater to infiltrate deep into the crust and perhaps upper mantle. This may lead to large scale formation of serpentinite in the upper mantle of the downgoing plate (Ranero et al., 2003). Faulting of the downgoing plate results in a horst and graben structure that allows sediment that reaches the trench to be deposited in graben and carried downward. This faulting also breaks up seamounts as they approach the trench. The principal mechanism of frontal erosion may reflect combined effects of seamount tunneling, mass wasting and transport to the trench, deposition in a graben on the downgoing plate, and descent into the mantle. Outer trench swells are geoscientific frontiers and much remains to be learned about them. Recent volcanoes have been discovered on ~135-million-year-old Pacific plate east of Japan (Hirano et al., 2006). These small alkalic volcanoes are small percent melts of asthenosphere that exploit bending-related lithospheric faults to reach the seafloor. Hirano et al., (2006) proposed that these small volcanoes erupted along lithospheric fractures in response to plate flexure during subduction. If bending-related faulting and serpentinization is an important process beneath outer trench swells, there are probably also abundant low-temperature hydrothermal vents on the swells, similar to those of the Lost City (hydrothermal field).

References Hirano, N., Takahashi, E., Yamamoto, J., Abe, N., Ingle, S.P., Kaneoka, I., Hirata, T., Kimura, J.-I., Ishii, T., Ogawa, Y., Machida, S., and Suyehiro, K., 2006. "Volcanism in Response to Plate Flexure: Science 313, 1426, doi:10.1126/science.1128235 Ranero, C., Morgan, J. P., McIntosh, K., and Reichert, C., 2003. "Bending-related faulting and mantle serpentinization at the Middle America trench". Nature 425, 367–373 Stern, R.J., 2002. "Subduction Zones" Reviews of Geophysics, 40, doi:10.1029/2001RG000108

Illustrations

Worked examples

Example 1 — a first encounter with Outer trench swell

Start with the simplest possible case. Write down what Outer trench swell claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Outer trench swell before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Outer trench swell ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Outer trench swell

In research: Outer trench swell appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Outer trench swell in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Outer trench swell is common in secondary-school and first-year university syllabi. It links to neighbouring topics Physical oceanography, Plate tectonics, so understanding it makes those chapters shorter.
In everyday life: Look for Outer trench swell outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Outer trench swell in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Outer trench swell means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Outer trench swell out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Outer trench swell in simple terms?

Why does Outer trench swell matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Outer trench swell?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Outer trench swell.