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

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

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

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.

   
   

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

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