engineering

Thermal subsidence

Thermal subsidence 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 Thermal subsidence rather than just read about it. In short: In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal expansion: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes denser than the surrounding material.

Key takeaways

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

Reference excerpt

In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal expansion: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes denser than the surrounding material. Additional material added to the lithosphere thickens it and further causes a buoyant decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming a sedimentary basin.

Causes Thermal subsidence can occur anywhere in which a temperature differential exists between a section of the lithosphere and its surroundings. There are a variety of contributing factors that can initiate thermal subsidence or affect the process as it is ongoing.

Delamination As endogenous and exogenous processes cause denudation of the earth's surface, lower, warmer sections of the lithosphere are exposed to relative differences in weight and density. This relative difference creates buoyancy. Isostatic uplift can then further expose the lithosphere to conductive cooling, causing a “rise and fall” phenomenon as warmer, less dense rock layers are pushed or buoyed up, then cooled, causing it to contract and sink back down.

Conduction The conditions to create thermal subsidence can be initiated by various forms of uplift and denudation, but the actual process of thermal subsidence is governed by the loss of heat via thermal conduction. Contact with surrounding rock or the surface causes heat to leach out of a section of the lithosphere. As the lithosphere cools, it causes the rock to contract.

Isostasy When conduction causes a section of the lithosphere to contract and increase in density, it does not directly add mass to the rock. Instead, it causes the volume to decrease, increasing the mass of the section for a given area. The lithosphere is isostatic with the mantle; its weight is supported by the relative density of the surrounding rock. When a section cools and its density increases, it sinks, causing the relative elevation to decrease. This can create a basin in which sediments are deposited, which adds weight on top of the sinking section of the lithosphere and increase the total mass of the section per unit area, causing it to sink further.

Effects Thermal subsidence can have an effect on island formation. Isostatic uplift can be balanced with thermal subsidence in response to erosion on islands without barrier reefs, which sink only when subjected to wave erosion. However, volcanic islands and seamounts with barrier reefs are shielded from wave and stream erosion, and thus the countervailing isostatic uplift is eliminated, causing them to subside and create an atoll.

Metamorphism Thermal subsidence can drive metamorphism in rocks. The conduction of heat out of a section of lithosphere causes the rock to thicken and become more insulated to heat flowing in from the mantle; as this thicker section is buried by the descending column of the lithosphere, it descends into surrounding rock layers with a higher relative geothermal gradient. This gradient can cause metamorphism in rocks, as seen in South Australia.

Eustasy Eustasy refers to a change in the relative sea level. It can have effects on the thermal subsidence during the formation of geological features such as mountain ranges. Sea level often changes in response to the formation of glaciers on land; the weight of these glaciers or the absence thereof can influence the overall rate of thermal subsidence.

Petroleum formation As the lithosphere cools and subsides, a sedimentary basin can be formed on top of the subsiding mass. The characteristics of the basement sediments can produce conditions conducive to the conversion of kerogen into petroleum. The gigantic Wilmington Oil Field in the Los Angeles basin was formed as a result of this process.

References

Worked examples

Example 1 — a first encounter with Thermal subsidence

Start with the simplest possible case. Write down what Thermal subsidence 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 Thermal subsidence 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 Thermal subsidence 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 Thermal subsidence

In research: Thermal subsidence 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 Thermal subsidence 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: Thermal subsidence is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geodynamics, so understanding it makes those chapters shorter.
In everyday life: Look for Thermal subsidence 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 Thermal subsidence in 20 minutes

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

Frequently asked questions

What is Thermal subsidence in simple terms?

Why does Thermal subsidence 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 Thermal subsidence?

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 Thermal subsidence.