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Tholeiitic magma series

Tholeiitic magma series 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 Tholeiitic magma series rather than just read about it. In short: The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member.

Tholeiitic magma series — main illustration
Tholeiitic magma series — illustration

Key takeaways

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

Reference excerpt

The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series include tholeiitic basalt, ferro-basalt, tholeiitic basaltic andesite, tholeiitic andesite, dacite and rhyolite. The variety of basalt in the series was originally called tholeiite but the International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to that term. Tholeiitic rock types tend to be more enriched in iron and less enriched in magnesium and aluminium than calc-alkaline rock types. They are thought to form in a less oxidized environment than calc-alkaline rocks. Tholeiitic basalt is formed at mid-ocean ridges and makes up much of the oceanic crust. Almost all the basalt found on the Moon is tholeiitic basalt.

Geochemical characterization

Rocks in the tholeiitic magma series are classified as subalkaline (they contain less sodium than some other basalts) and are distinguished from rocks in the calc-alkaline magma series by the redox state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized ). When the parent magmas of basalts crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine and pyroxene, causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. However, a calc-alkaline magma is oxidized enough to precipitate significant amounts of the iron oxide magnetite, causing the iron content of the magma to remain more steady as it cools than with a tholeiitic magma. The difference between these two magma series can be seen on an AFM diagram, a ternary diagram showing the relative proportions of the oxides Na2O + K2O (A), FeO + Fe2O3 (F), and MgO (M). As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing the magmas to move towards the alkali corner as they cool. In the tholeiitic magma, magnesium-rich crystals are produced preferentially, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and simply moves towards the alkali corner as it loses iron and any remaining magnesium. With the calc-alkaline series, however, the precipitation of magnetite causes the iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram. The AFM plot distinguishes the intermediate members of the tholeiitic and calc-alkali magma series quite well. However, the felsic end members of the two series are nearly indistinguishable, so granitic rocks are generally assigned to the calc-alkali magma series. The mafic end members may be distinguished by the aluminium content, with tholeiitic basalts containing 12% to 16% Al2O3 versus 16% to 20% Al2O3 for calc-alkali basalts.

Petrography

Like all basalt, the tholeiitic type is dominated by olivine, clinopyroxene and plagioclase, with minor iron-titanium oxides. Orthopyroxene or pigeonite may also be present in tholeiitic basalt, and olivine, if present, may be rimmed by either of these calcium-poor pyroxenes. Tridymite or quartz may be present in the fine-grained groundmass of tholeiitic basalt, and feldspathoids are absent. Tholeiitic rocks may have a fine, glassy groundmass, as may other types of basalt.

Geologic context Tholeiitic rocks are the most common igneous rocks in Earth's crust, produced by submarine volcanism at mid-ocean ridges and make up much of the ocean crust. Tholeiitic basaltic magmas are initially generated as partial melts of lherzolite (olivine, enstatite and diopside) produced by decompression melting of the Earth's mantle. Tholeiitic basalt constituting the oceanic crust is termed MORB: mid-ocean-ridge basalt. Throughout the process of igneous differentiation, the oceanic crust acts to reduce the magma, producing the tholeiitic trend. In contrast, alkali basalts are not typical of ocean ridges, but are erupted on some oceanic islands and on continents, as also is tholeiitic basalt. Because the Moon is extremely reduced, all of its basalts are tholeiitic.

Type locality Tholeiite is named for its type locality near the municipality of Tholey in Saarland, Germany.

See also Alkaline magma series Petrology

References

Citations

Sources

Illustrations

Tholeiitic magma series: TAS diagram showing chemical composition range of sub-alkaline volcanic rocks including tholeiitic rocks (yellow area) and alkaline volcanic rocks (blue area)
TAS diagram showing chemical composition range of sub-alkaline volcanic rocks including tholeiitic rocks (yellow area) and alkaline volcanic rocks (blue area)
Tholeiitic magma series: AFM diagram showing the relative proportions of the oxides of alkalis (A), iron (F), and magnesium (M), with arrows showing the compositional change path of the magmas in the tholeiitic and the calc-alkaline magma series (BT=tholeiitic basalt, FB=ferro-basalt, ABT=tholeiitic basaltic andesite, AT=tholeiitic andesite, D=dacite, R=rhyolite, B=basalt, AB=basaltic andesite, A=andesite; dashed line=boundary between tholeiitic and calc-alkaline compositions)
AFM diagram showing the relative proportions of the oxides of alkalis (A), iron (F), and magnesium (M), with arrows showing the compositional change path of the magmas in the tholeiitic and the calc-alkaline magma series (BT=tholeiitic basalt, FB=ferro-basalt, ABT=tholeiitic basaltic andesite, AT=tholeiitic andesite, D=dacite, R=rhyolite, B=basalt, AB=basaltic andesite, A=andesite; dashed line=boundary between tholeiitic and calc-alkaline compositions)
Tholeiitic magma series: Photomicrograph of thin section of tholeiitic basalt (in plane polarized light)
Photomicrograph of thin section of tholeiitic basalt (in plane polarized light)
Tholeiitic magma series: Photomicrograph of thin section of tholeiitic basalt (in cross polarized light)
Photomicrograph of thin section of tholeiitic basalt (in cross polarized light)

Worked examples

Example 1 — a first encounter with Tholeiitic magma series

Start with the simplest possible case. Write down what Tholeiitic magma series 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 Tholeiitic magma series 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 Tholeiitic magma series 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 Tholeiitic magma series

In research
Tholeiitic magma series 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 Tholeiitic magma series 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
Tholeiitic magma series is common in secondary-school and first-year university syllabi. It links to neighbouring topics Basalt, Igneous petrology, Industrial minerals, so understanding it makes those chapters shorter.
In everyday life
Look for Tholeiitic magma series 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 Tholeiitic magma series in 20 minutes

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

Frequently asked questions

What is Tholeiitic magma series in simple terms?

The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member.

Why does Tholeiitic magma series 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 Tholeiitic magma series?

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 Tholeiitic magma series.

Tags

  • Basalt
  • Igneous petrology
  • Industrial minerals

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