engineering

Lower mantle

Lower mantle is a engineering 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 Lower mantle rather than just read about it. In short: The lower mantle, historically also called the mesosphere, occupies about 56% of the total volume of Earth, and is the region from 660 to 2,890 km (410 to 1,800 mi) below Earth's surface; between the transition zone and the outer core. The preliminary reference Earth model (PREM) separates the lower mantle into three sections, the uppermost (660–770 km (410–480 mi)), mid-lower mantle (770–2,700 km (480–1,680 mi)), a…

Key takeaways

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

Reference excerpt

The lower mantle, historically also called the mesosphere, occupies about 56% of the total volume of Earth, and is the region from 660 to 2,890 km (410 to 1,800 mi) below Earth's surface; between the transition zone and the outer core. The preliminary reference Earth model (PREM) separates the lower mantle into three sections, the uppermost (660–770 km (410–480 mi)), mid-lower mantle (770–2,700 km (480–1,680 mi)), and the D layer (2,700–2,890 km (1,680–1,800 mi)). Pressure and temperature in the lower mantle range from 24–127 GPa (3,500,000–18,400,000 psi) and 1,900–2,600 K (1,630–2,330 °C; 2,960–4,220 °F). It has been proposed that the composition of the lower mantle is pyrolitic, containing three major phases of bridgmanite, ferropericlase, and calcium-silicate perovskite. The high pressure in the lower mantle has been shown to induce a spin transition of iron-bearing bridgmanite and ferropericlase, which may affect both mantle plume dynamics and lower mantle chemistry. The mantle moves at about 1 cm (0.39 in) per year. The upper boundary is defined by the sharp increase in seismic wave velocities and density at a depth of 660 km (410 mi). At a depth of 660 km (410 mi), ringwoodite γ-((Mg,Fe)2SiO4) decomposes into Mg-Si perovskite and magnesiowüstite. This reaction marks the boundary between the upper mantle and lower mantle. This measurement is estimated from seismic data and high-pressure laboratory experiments. The base of the mesosphere includes the D″ zone which lies just above the mantle–core boundary at around 2,700–2,890 km (1,678–1,796 mi).

Physical properties The lower mantle was initially labelled as the D-layer in Bullen's spherically symmetric model of the Earth. The PREM seismic model of the Earth's interior separated the D-layer into three distinctive layers defined by the discontinuity in seismic wave velocities:

660–770 km: A discontinuity in compression wave velocity (6–11%) followed by a steep gradient is indicative of the transformation of the mineral ringwoodite to bridgmanite and ferropericlase and the transition between the transition zone layer to the lower mantle. 770–2700 km: A gradual increase in velocity indicative of the adiabatic compression of the mineral phases in the lower mantle. 2700–2900 km: The D-layer is considered the transition from the lower mantle to the outer core. The temperature of the lower mantle ranges from 1,960 K (1,690 °C; 3,070 °F) at the topmost layer to 2,630 K (2,360 °C; 4,270 °F) at a depth of 2,700 kilometres (1,700 mi). Models of the temperature of the lower mantle approximate convection as the primary heat transport contribution, while conduction and radiative heat transfer are considered negligible. As a result, the lower mantle's temperature gradient as a function of depth is approximately adiabatic. Calculation of the geothermal gradient observed a decrease from 0.47 kelvins per kilometre (0.47 °C/km; 1.4 °F/mi) at the uppermost lower mantle to 0.24 kelvins per kilometre (0.24 °C/km; 0.70 °F/mi) at 2,600 kilometres (1,600 mi).

Composition The lower mantle is mainly composed of three components, bridgmanite, ferropericlase, and calcium-silicate perovskite (CaSiO3-perovskite). The proportion of each component has been a subject of discussion historically where the bulk composition is suggested to be,

Pyrolitic: derived from petrological composition trends from upper mantle peridotite suggesting homogeneity between the upper and lower mantle with a Mg/Si ratio of 1.27. This model implies that the lower mantle is composed of 75% bridgmanite, 17% ferropericlase, and 8% CaSiO3-perovskite by volume. Chondritic: suggests that the Earth's lower mantle was accreted from the composition of chondritic meteorite suggesting a Mg/Si ratio of approximately 1. This infers that bridgmanite and CaSiO3-perovskites are major components. Laboratory multi-anvil compression experiments of pyrolite simulated conditions of the adiabatic geotherm and measured the density using in situ X-ray diffraction. It was shown that the density profile along the geotherm is in agreement with the PREM model. The first principle calculation of the density and velocity profile across the lower mantle geotherm of varying bridgmanite and ferropericlase proportion observed a match to the PREM model at an 8:2 proportion. This proportion is consistent with the pyrolitic bulk composition at the lower mantle. Furthermore, shear wave velocity calculations of pyrolitic lower mantle compositions considering minor elements resulted in a match with the PREM shear velocity profile within 1%. On the other hand, Brillouin spectroscopic studies at relevant pressures and temperatures revealed that a lower mantle composed of greater than 93% bridgmanite phase has corresponding shear-wave velocities to measured seismic velocities. The suggested composition is consistent with a chondritic lower mantle. Thus, the bulk composition of the lower mantle is currently a subject of discussion.

Spin transition zone The electronic environment of two iron-bearing minerals in the lower mantle (bridgmanite, ferropericlase) transitions from a high-spin (HS) to a low-spin (LS) state. Fe2+ in ferropericlase undergoes the transition between 50–90 GPa. Bridgmanite contains both Fe3+ and Fe2+ in the structure, the Fe2+ occupy the A-site and transition to a LS state at 120 GPa. While Fe3+ occupies both A- and B-sites, the B-site Fe3+ undergoes HS to LS transition at 30–70 GPa while the A-site Fe3+ exchanges with the B-site Al3+ cation and becomes LS. This spin transition of the iron cation results in the increase in partition coefficient between ferropericlase and bridgmanite to 10–14 depleting bridgmanite and enriching ferropericlase of Fe2+. The HS to LS transition are reported to affect the physical properties of the iron bearing minerals. For example, the density and incompressibility was reported to increase from HS to LS state in ferropericlase. The effects of the spin transition on the transport properties and rheology of the lower mantle is currently being investigated and discussed using numerical simulations.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Lower mantle

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

In research: Lower mantle appears in engineering 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 Lower mantle 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: Lower mantle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Earth's mantle, Structure of the Earth, so understanding it makes those chapters shorter.
In everyday life: Look for Lower mantle 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 Lower mantle in 20 minutes

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

Frequently asked questions

What is Lower mantle in simple terms?

Why does Lower mantle matter?

Because it connects several engineering 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 Lower mantle?

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 Lower mantle.