TAS Diagram (Total Alkali-Silica)

TAS Diagram (Total Alkali-Silica)

Plot geochemical assays to officially classify fine-grained and glassy volcanic rocks.

Geochemistry (wt%)

Input XRF weight percentages. Ensure your data is normalized on a volatile-free basis.

Formal IUGS Name

Basalt
Total Alkalis: 3.30 wt%

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 1)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 2)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 3)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 4)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 5)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 6)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

The Total Alkali-Silica (TAS) Diagram

When an effusive volcano erupts, the magma often cools so rapidly upon reaching the surface that it forms glass (like obsidian) or a microscopic, fine-grained groundmass (like aphyric basalt). Under a microscope, you cannot see individual crystals. This renders the traditional mineral-based QAPF classification diagram completely useless. To classify these volcanic rocks, volcanologists turn to geochemistry and the TAS Diagram.

The TAS (Total Alkali-Silica) diagram classifies volcanic rocks based on their bulk chemical composition—specifically, the weight percentages of Silica (SiO₂) and Total Alkalis (Na₂O + K₂O). Adopted by the International Union of Geological Sciences (IUGS) (Le Maitre et al., 1989), the TAS diagram is the absolute global standard for naming fine-grained and glassy volcanic rocks.

Our interactive TAS Calculator utilizes the Plotly.js library to give you a fully interactive, zoomable, and touch-friendly experience. You can input your XRF geochemical assay data, instantly plot it against the IUGS official polygons, and determine both the rock name and its magmatic series.

Important Data Preparation: Before plotting on the TAS diagram, your geochemical data must be recalculated to 100% on a volatile-free basis. If your rock suffered weathering (high Loss On Ignition, LOI) or contains significant H₂O or CO₂, plotting the raw data will artificially depress the SiO₂ and Alkali values, leading to an incorrect classification!

1. The Two Axes of Magma Evolution (Part 7)

The geometry of the TAS diagram is not arbitrary; it beautifully displays the two primary vectors of magma evolution: Fractional Crystallization and Partial Melting.

  • X-Axis (Silica, SiO₂): As a magma chamber cools, mafic minerals (olivine, pyroxene) crystallize and sink to the bottom. Because these minerals are poor in silica, the remaining liquid magma becomes progressively enriched in silica. Thus, rocks move from the left side of the diagram (Basalt, ~50% SiO₂) to the right side (Rhyolite, >70% SiO₂) as they evolve.
  • Y-Axis (Total Alkalis, Na₂O + K₂O): Alkalis are "incompatible elements." Because their ions are very large, they do not easily fit into the crystal lattices of early-forming minerals. Therefore, as magma evolves, the alkalis are concentrated in the remaining liquid, driving the composition upward on the diagram.

2. Alkalic vs. Subalkalic Magma Series

You will notice a dashed red curve slicing diagonally through the TAS diagram. This is the famous Irvine and Baragar (1971) dividing line. It separates two fundamentally different types of volcanism on Earth.

Rocks plotting below the curve belong to the Subalkalic Series (Tholeiitic or Calc-Alkaline). These are the basalts, andesites, and dacites erupted at Mid-Ocean Ridges (divergent boundaries) and Volcanic Arcs (subduction zones like the Andes or Cascades).

Rocks plotting above the curve belong to the Alkalic Series. These magmas are highly enriched in sodium and potassium, forming exotic rocks like Trachytes and Phonolites. Alkalic magmas typically erupt at Continental Rift zones (like the East African Rift) or deep-mantle Hotspots (like Hawaii). They result from very small degrees of partial melting of the deep mantle.

3. Navigating the Polygons

The TAS diagram contains 15 main fields. Here is a breakdown of the most common pathways:

  • The Standard Pathway (Subalkalic): Basalt → Basaltic Andesite → Andesite → Dacite → Rhyolite. This is the classic progression seen at stratovolcanoes like Mount St. Helens.
  • The Alkalic Pathway: Basalt → Trachybasalt → Trachyandesite → Trachyte. This pathway involves a steep increase in alkalis with a moderate increase in silica.
  • The Highly Undersaturated Pathway: Basanite → Tephrite → Phonolite. These rocks are so starved of silica that they crystallize feldspathoids (like nepheline) instead of quartz.

4. Limitations of the TAS Diagram

While the TAS diagram is incredibly robust, it has a few limitations that every petrologist must be aware of.

First, it assumes the rock has not undergone significant chemical alteration since eruption. Alkalis (Sodium and Potassium) are highly mobile elements. If a basalt erupts underwater and undergoes hydrothermal alteration (spilitization), the seawater will strip calcium out of the rock and pump sodium in. This will artificially drive the rock upward on the TAS diagram, making a standard mid-ocean ridge basalt masquerade as a Trachybasalt! For altered rocks, geochemists must use immobile trace elements (like Zirconium, Titanium, and Yttrium) on the Winchester and Floyd (1977) diagram instead.

5. Frequently Asked Questions (FAQ)

Can I plot plutonic (intrusive) rocks on the TAS diagram? ▼
Technically, yes, but you shouldn't. The TAS diagram was specifically calibrated by the IUGS for volcanic rocks. Plutonic rocks should always be classified using the QAPF diagram since their slow cooling allows for accurate point-counting of minerals. If you plot a plutonic rock's chemistry on the TAS diagram, it will give you the volcanic equivalent name (e.g., it will say Rhyolite instead of Granite).
What do the letters in the fields mean (e.g., O1, S2, U3)? ▼
These are the official IUGS shorthand codes for the fields. 'O' stands for Oversaturated (the subalkalic basalt-rhyolite trend). 'S' stands for Saturated (the alkalic trachyte trend). 'U' stands for Undersaturated (the highly alkalic phonolite trend). 'Pc', 'B', 'R', 'T', and 'F' are abbreviations for specific rock names (Picrobasalt, Basalt, Rhyolite, Trachyte, Foidite).

6. Authoritative References and Outbound Resources

  • International Union of Geological Sciences (IUGS): The official governing body for rock classification. Visit IUGS.
  • USGS Volcano Hazards Program: To see TAS chemistry applied to active monitoring of volcanoes like Kilauea, visit the USGS VHP.
  • Geochemical Earth Reference Model (GERM): For massive datasets of volcanic rock geochemistry, consult EarthRef.