TAS Rock Classification Calculator
Plot SiO₂ against total alkalis (Na₂O + K₂O) on the TAS diagram and read the volcanic-rock field — an interactive classification with the sample point shown on the standard Le Bas et al. (1986) fields.
Major oxides (wt%)
TAS applies to volcanic rocks and is not a universal classifier. Consider normalising the major oxides first.
TAS diagram
Runs entirely in your browser. Your assay data is not uploaded or stored by Zerdly.
- Enter SiO₂, Na₂O and K₂O.
Assumptions & basis
- Field boundaries: Le Bas et al. (1986), J. Petrology 27, 745–750 (derived)
- Applicability: Volcanic rocks; not a universal classifier (user)
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter SiO₂, Na₂O and K₂O in wt%.
- Read the total alkalis, the plotted point on the TAS diagram and the field name.
- Normalise the major oxides first if you want a comparable anhydrous basis.
How it works
The TAS (total alkali–silica) diagram classifies volcanic rocks by plotting SiO₂ (x) against Na₂O + K₂O (y). The tool computes the total alkalis and tests which of the Le Bas et al. (1986) field polygons the point lies inside, using a point-in-polygon test. The field boundaries are stored as versioned static data and are regression-tested with interior points for every field.
TAS is not a universal classifier: it is designed for volcanic rocks and assumes an appropriate analytical basis. A point on or near a boundary, or one that falls outside the polygon set, is flagged rather than force-classified — normalising the analysis and checking the rock type is sensible before relying on the field name.
Worked example
SiO₂ 60, Na₂O 4, K₂O 3 wt%. Total alkalis = 4 + 3 = 7 wt%. Plotting (60, 7) on the TAS diagram places the sample in the trachyandesite field.
Common mistakes
- Applying TAS to plutonic or altered rocks, which it is not designed for.
- Using a non-normalised analysis so the SiO₂ position shifts.
- Over-trusting a classification when the point sits on a field boundary.
Frequently asked questions
Which boundaries does it use?
The Le Bas et al. (1986) TAS fields, stored as versioned polygons and validated with interior-point tests for every field.
Why is my point outside the diagram?
The analysis may be un-normalised, altered, or not a volcanic rock. TAS covers a defined SiO₂/alkali range; points outside it are flagged rather than classified.
Related tools
- Major Oxide Normalization Calculator
- Shand Index (A/CNK & A/NK) Calculator
- REE Normalisation & Spider Plot
- Elements to Oxide Calculator
- Feldspar An-Ab-Or Calculator
- Total Rare Earth Oxide (TREO) Calculator
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Cite this tool
Zerdly. (2026). TAS Rock Classification Calculator. Retrieved from https://www.zerdly.com/tools/tas-rock-classification-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.