science

Mantle wedge

Mantle wedge is a science 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 Mantle wedge rather than just read about it. In short: A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. A wedge can be identified using seismic velocity imaging as well as earthquake maps.

Key takeaways

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

Reference excerpt

A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. A wedge can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic slabs carry large amounts of water; this water lowers the melting temperature of the above mantle wedge. Melting of the wedge can also be contributed to depressurization due to the flow in the wedge. This melt gives rise to associated volcanism on the Earth's surface. This volcanism can be seen around the world in places such as Japan and Indonesia.

Charateristics

Structure The forearc mantle extends from where the subducting slab meets the cold nose of the mantle wedge, at depths from 10 to 40 km. Low seismic attenuation and high seismic velocities characterize this region. There is a boundary between this low attenuation region and a high attenuation region on the forearc side of the arc volcanoes. To image the mantle wedge region below volcanic arcs P-wave, S-wave and seismic attenuation images should be used in coordination. These tomographic images show a low velocity, high attenuation region above the subducting slab. The slowest velocities in these volcanic arc regions are Vp= 7.4 km·s−1 and Vs= 4 km·s−1. Mantle wedge regions that do not have associated arc volcanism do not show such low velocities. This can be attributed to the melt production in the mantle wedge. Studies have shown that magmas that produce island arcs are more oxidized than the magmas that are produced at mid-ocean ridges. This relative degree of oxidation has been determined by the iron oxidation state of fluid inclusions in glassy volcanic rocks. It has been determined that this state of oxidation is correlated with the water content of the mantle wedge. Water is a poor oxidant, and therefore the oxidizing agent must be transported as a dissolved ion in the subducting slab. Magmas produced in subduction zone regions have high volatile contents. This water is derived from the breakdown of hydrous minerals in the subducting slab, as well as water in the oceanic plate from percolation of seawater. This water rises from the subducting slab to the overriding mantle wedge. The water lowers the melting temperature of the wedge and leaves behind melt inclusions that can be measured in the associated arc volcanic rocks.

Flow Flow has important effects on the thermal structure, overall mantle circulation, and melt within the wedge. Minerals are anisotropic and have the ability to align themselves within the mantle when exposed to strain. These mineral alignments can be seen using seismic imaging, as waves will travel through different orientations of a mineral at different speeds. Shear strain associated with mantle flow will align the fast direction of pyroxene and olivine grains in the direction of flow. This is the most common theory on flow within the mantle, although opposing theories do exist. Flow within the mantle wedge is parallel to the crust until it reaches the relatively cooler nose of the wedge, then is overturned and is parallel to the subducting slab. The nose of the wedge is generally isolated from the overall mantle flow.

References

Illustrations

Worked examples

Example 1 — a first encounter with Mantle wedge

Start with the simplest possible case. Write down what Mantle wedge claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Mantle wedge 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 Mantle wedge 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 Mantle wedge

In research: Mantle wedge appears in science 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 Mantle wedge 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: Mantle wedge is common in secondary-school and first-year university syllabi. It links to neighbouring topics Subduction, Tectonics, so understanding it makes those chapters shorter.
In everyday life: Look for Mantle wedge 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 Mantle wedge in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mantle wedge 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 Mantle wedge out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mantle wedge in simple terms?

Why does Mantle wedge matter?

Because it connects several science 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 Mantle wedge?

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 Mantle wedge.