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Phacolith

Phacolith 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 Phacolith rather than just read about it. In short: A phacolith is a pluton of igneous rock parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline.

Phacolith — main illustration
Phacolith — illustration

Key takeaways

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

Reference excerpt

A phacolith is a pluton of igneous rock parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline. In rare cases the body may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that in cross section it has an S shape. In intensely folded terrain the hinge of folds would be areas of reduced pressure and thus potential sites for magma migration and emplacement. The term was coined and initially defined by Alfred Harker in his The Natural History of Igneous Rocks in 1909.

Examples in the Franklin and Hamburg areas of Sussex County, New Jersey, United States the Omey pluton in Ireland near Bayalan, Ajmer district, Rajasthan in India Corndon Hill, in Shropshire, England, United Kingdom

See also Laccolith Lopolith Batholith

References

Davis A. Young (2003) Mind Over Magma: The Story of Igneous Petrology, page 335, Princeton University Press. ISBN 0-691-10279-1 American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.

Illustrations

Phacolith: Cross-sectional diagram of phacoliths (red) in older folded rocks
Cross-sectional diagram of phacoliths (red) in older folded rocks

Worked examples

Example 1 — a first encounter with Phacolith

Start with the simplest possible case. Write down what Phacolith 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 Phacolith 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 Phacolith 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 Phacolith

In research
Phacolith 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 Phacolith 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
Phacolith is common in secondary-school and first-year university syllabi. It links to neighbouring topics Igneous petrology, Igneous rocks, Petrology stubs, so understanding it makes those chapters shorter.
In everyday life
Look for Phacolith 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 Phacolith in 20 minutes

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

Frequently asked questions

What is Phacolith in simple terms?

A phacolith is a pluton of igneous rock parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline.

Why does Phacolith 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 Phacolith?

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 Phacolith.

Tags

  • Igneous petrology
  • Igneous rocks
  • Petrology stubs

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