earth science

Unconformity

Unconformity is a earth 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 Unconformity rather than just read about it. In short: An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record.

Key takeaways

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

Reference excerpt

An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record. The significance of angular unconformity (see below) was shown by James Hutton, who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland. The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region or were subsequently eroded before the next deposition. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus. It is a kind of relative dating.

Types

Disconformity

A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in the rock record.

Nonconformity

A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below the break is igneous or has lost its bedding due to metamorphism, then the plane of juncture is a nonconformity.

Angular unconformity

An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic. Angular unconformities can occur in ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions. In these cases, the hiatus in deposition represented by the unconformity may be geologically very short – hours, days or weeks.

Paraconformity

A paraconformity is a type of unconformity in which the sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity. Short paraconformities are called diastems.

Buttress unconformity A buttress unconformity, also known as onlap unconformity, occurs when younger bedding is deposited against older strata thus influencing its bedding structure.

Blended unconformity A blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, paleosols, or beds of pebbles derived from the underlying rock.

Gallery

Illustrations

Worked examples

Example 1 — a first encounter with Unconformity

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

In research: Unconformity appears in earth 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 Unconformity 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: Unconformity is common in secondary-school and first-year university syllabi. It links to neighbouring topics Historical geology, Stratigraphy, Unconformities, so understanding it makes those chapters shorter.
In everyday life: Look for Unconformity 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 Unconformity in 20 minutes

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

Frequently asked questions

What is Unconformity in simple terms?

Why does Unconformity matter?

Because it connects several earth 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 Unconformity?

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 Unconformity.