earth science

Trachyandesite

Trachyandesite 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 Trachyandesite rather than just read about it. In short: Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar.

Key takeaways

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

Reference excerpt

Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar. It is formed from the cooling of lava enriched in alkali metals and with an intermediate content of silica. The term trachyandesite had begun to fall into disfavor by 1985 but was revived to describe extrusive igneous rocks falling into the S3 field of the TAS classification. These are divided into sodium-rich benmoreite and potassium-rich latite. Trachyandesitic magma can produce explosive Plinian eruptions, such as happened at Tambora in 1815. The Eyjafjallajökull 2010 eruption (VEI-4), which disrupted European and transatlantic air travel from 15-20 April 2010, for some time was dominated by trachyandesite.

Petrology Trachyandesite is characterized by a silica content near 58% and a total alkali oxide content near 9%. This places trachyandesite in the S3 field of the TAS diagram. When it is possible to identify the minerals present, trachyandesite is characterized by a high content of sodic plagioclase, typically andesine, and contains at least 10% alkali feldspar. Common mafic accessory minerals are amphibole, biotite or pyroxene. Small amounts of nepheline may be present and apatite is a common accessory mineral. Trachyandesite is not a recognized rock type in the QAPF classification, which is based on the actual mineral content. However, latite is recognized in this classification, while benmoreite would likely fall into either the latite or the andesite fields. Trachyandesite magmas can have a relatively high sulfur content, and their eruption can inject great quantities of sulfur into the stratosphere. The sulfur may take the form of anhydrite phenocrysts in the magma. The 1982 El Chichón eruption produced trachyandesite pumice rich in anhydrite, and released 2.2 × 107 metric tons of sulfur.

Varieties Sodium-rich trachyandesite (with %Na2O > %K2O + 2) is called benmoreite, while the more potassic form is called latite. Feldspathoid-bearing latite is sometimes referred to as tristanite. Basaltic trachyandesite is transitional to basalt and likewise comes in two varieties, mugearite (sodium-rich) and shoshonite (potassium-rich).

Occurrence Trachyandesite is a member of the alkaline magma series, in which alkaline basaltic magma experiences fractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma. As a result, trachyandesite is common wherever alkali magma is erupted, including late eruptions of oceanic islands and in continental rift valleys and mantle plumes. Trachyandesite is found in the Yellowstone area as part of the Absaroka Volcanic Supergroup, and has been erupted in arc volcanism in Mesoamerica and at Mount Tambora.

References

External links Photomicrograph of a thin section of trachyandesite from France, in crossed-polarised light

Illustrations

Worked examples

Example 1 — a first encounter with Trachyandesite

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

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

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

Frequently asked questions

What is Trachyandesite in simple terms?

Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar.

Why does Trachyandesite 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 Trachyandesite?

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 Trachyandesite.