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Magma is a hot, molten or partially molten mixture of rock material beneath the Earth’s surface, consisting of:

Liquid melt

Solid crystals

Dissolved gases (volatiles like H₂O, CO₂)

It forms deep within the Earth (mainly in the mantle and lower crust) and can rise toward the surface. When magma erupts, it is called lava.

Key Characteristics of Magma

Temperature: ~700°C to 1300°C

Composition: Mainly silicate minerals (SiO₂-rich)

Phases:

Liquid (melt)

Solid (crystals)

Gas (volatiles)

Magma composition controls:

Viscosity (how thick/runny it is)

Type of volcanic eruption

Type of igneous rock formed

Formation of Magma

Magma forms through:

Partial melting of rocks

Caused by:

Increase in temperature

Decrease in pressure (decompression melting)

Addition of volatiles (e.g., water lowers melting point) (Encyclopaedia Britannica, n.d.)

Reference

Encyclopaedia Britannica (n.d.) Magma. Available at: https://www.britannica.com/science/magma-rock

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Parental magma is the original magma from which a suite of related igneous rocks is derived through processes such as fractional crystallization, assimilation, or magma mixing.

In other words, it is the starting composition of a magma before it evolves into different rock types.

Key Idea

One parental magma → can produce many different igneous rocks

Evolution happens through:

Fractional crystallization

Crustal contamination (assimilation)

Magma mixing

Geochemical Context

Geochemistry helps identify parental magmas by analyzing:

major and trace elements

isotopic compositions

Thermodynamics can predict how minerals crystallize from magma and how composition changes during cooling

References

Winter, J.D. (2010) Principles of Igneous and Metamorphic Petrology. 2nd edn. Pearson.

Rollinson, H.R. (1993) Using Geochemical Data: Evaluation, Presentation, Interpretation. Longman.

White, W.M. (2013) Geochemistry.

Relevant concepts on magma evolution and thermodynamics

Encyclopaedia Britannica (n.d.) Magma. Available at: https://www.britannica.com/science/magma-rock

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Primary magma is a magma that forms directly by partial melting of its source rock (usually in the mantle) and has not undergone significant chemical modification (such as fractional crystallization or contamination).

Key Points

Forms directly from the source (e.g., mantle peridotite)

Represents the initial melt composition

Has not evolved through magmatic processes

Often difficult to find at the surface because most magmas change during ascent

Geochemical Context

Primary magmas are important because they:

Reflect the composition of the mantle source

Help scientists understand melting conditions (temperature, pressure)

Thermodynamics helps predict:

When melting occurs

Composition of the melt formed

References

1. Winter, J.D. (2010) Principles of Igneous and Metamorphic Petrology. 2nd edn. Pearson.

2. Rollinson, H.R. (1993) Using Geochemical Data: Evaluation, Presentation, Interpretation. Longman.

3. White, W.M. (2013) Geochemistry.

Thermodynamic basis for melting and magma formation

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Partial melting is the process in which only a portion of a solid rock melts while the rest remains solid, producing a magma that is different in composition from the original rock.

Key Idea

Rocks are made of different minerals with different melting points

When heating occurs:

Low melting-point minerals melt first

High melting-point minerals remain solid

➡️ Result: The melt (magma) is chemically different from the source rock

Geological Importance

Produces most magmas in the Earth (especially in the mantle and crust)

Controls:

Composition of magma

Formation of igneous rocks

Explains why continental crust is more silica-rich than mantle rocks

Geochemical Context

Thermodynamics allows us to:

Predict when melting begins

Determine composition of the melt and remaining solid

References

1. Winter, J.D. (2010) Principles of Igneous and Metamorphic Petrology. 2nd edn. Pearson.

2. Rollinson, H.R. (1993) Using Geochemical Data: Evaluation, Presentation, Interpretation. Longman.

3. White, W.M. (2013) Geochemistry.

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Basalt is a dark-coloured, fine-grained volcanic (extrusive) igneous rock that forms from the rapid cooling of mafic lava at or near the Earth’s surface.

Key Characteristics

Colour: Dark grey to black

Texture: Fine-grained (crystals too small to see easily)

Composition: Rich in iron (Fe) and magnesium (Mg), low in silica

Origin: Forms from basaltic magma erupted as lava

Formation

Produced by partial melting of the mantle

Magma rises and erupts as lava

Rapid cooling → small crystals form

Geological Importance

Most common rock in:

Oceanic crust

Mid-ocean ridges

Forms features like:

Lava flows

Volcanic islands (e.g., Hawaii)

Geochemical Context

Basalt composition reflects mantle source material

Thermodynamics helps predict:

Melting conditions

Crystallization sequence of basaltic magma

Reference

Encyclopaedia Britannica (n.d.) Basalt. Available at: https://www.britannica.com/science/basalt-rock

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Basalt is a rock, not a mineral.

Why?

A rock is a mixture (aggregate) of one or more minerals

A mineral is a single naturally occurring substance with a fixed chemical composition

Basalt specifically:

It is an igneous rock

Made up of minerals such as:

Pyroxene

Plagioclase feldspar

Sometimes olivine

Reference

Encyclopaedia Britannica (n.d.) Basalt. Available at: https://www.britannica.com/science/basalt-rock

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