earth science

Rhyodacite

Rhyodacite 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 Rhyodacite rather than just read about it. In short: Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite.

Key takeaways

Rhyodacite 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 Rhyodacite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Rhyodacite from memory before moving on to harder problems.

Reference excerpt

Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.

Description

Under IUGS guidelines, rhyodacites are not formally defined in either the QAPF classification, used to classify igneous rocks by their mineral content, or the TAS classification, used to classify volcanic rocks chemically. However, the IUGS allows the use of the term to describe rocks close to the boundary between the rhyolite and dacite fields in each classification scheme. Rhyodacite then describes a fine-grained igneous rock containing between 20% and 60% quartz and in which plagioclase makes up about two-thirds of the total feldspar content. Such a rock will contain between 69% and 72% silica by weight. The U.S. Geological Survey defines rhyodacite as volcanic rock containing 20% to 60% quartz and with plagioclase making up 35% to 65% of the total feldspar content. This makes rhyodacite the extrusive equivalent of monzogranite. In addition to its quartz and feldspar content, rhyodacite commonly contains phenocrysts of andesine, biotite, hornblende, and pyroxene. Quartz and sanidine phenocrysts are less common than in rhyolite, and rhyolite has only scant mafic phenocrysts.

Occurrence

Like other silica-rich compositions, rhyodacites are more abundant as pyroclastic rock than as lava flows. These include extensive ash flows and fallout sheets that are important stratigraphic markers. Rhyodacite lava flows occur in northwestern Ferry County (Washington), and at An Sgùrr on the island of Eigg in Scotland.

References

Illustrations

Worked examples

Example 1 — a first encounter with Rhyodacite

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

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

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

Frequently asked questions

What is Rhyodacite in simple terms?

Why does Rhyodacite 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 Rhyodacite?

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 Rhyodacite.