Factors affecting Magma and Evolution of Magma, Study Guides, Projects, Research of Geology

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J. N. V. U. JODHPUR

Geology Deptt.

Topic- Factors Affecting Magma And

Evolution of Magma

Submitted to -

Dr. S.C. Mathur

Submitted by-

BABLU

Class-

M.Sc. previous

General characteristics of magma

General characteristics of magma

• Igneous rocks form as molten rock cools and solidifies

General characteristics of magma:

• Parent material of igneous rocks
• Forms from partial melting of rocks
• Magma at surface is called lava
• Rocks formed from lava = extrusive, or volcanic rocks
• Rocks formed from magma at depth = intrusive, or plutonic rocks

The nature of magma

• Consists of three components: - Liquid portion = melt - Solids, if any, are silicate minerals - Volatiles = dissolved gases in the melt, including water vapor (H

2

O), carbon

dioxide (CO

2

), and sulfur dioxide (SO

2

)

Crystallization of magma

• Cooling of magma results in the systematic arrangement of ions into orderly

patterns

• The silicate minerals resulting from crystallization form in a predictable order
• Texture - size and arrangement of mineral grains



The above three factors control the “viscosity”

of a magma, which in turn controls the nature

of an eruption.



Viscosity –

is the measure of a material's resistance to flow.

The more viscous the material, the greater its resistance to

flow. Ex: syrup is more viscous than water.

Factors affecting

Factors affecting

viscosity

viscosity

1. Temperature - hotter magmas

are less viscous

2. Composition-

( silica (SiO

2

) content )

A. High silica- high viscosity

• Higher silica content magmas are

more viscous.

(e.g.- rhyolitic and andesitic magmas)

B. Low silica—more fluid

• Lower silica content magmas are less

viscous.

(e.g.- basaltic lavas)

A single volcano may extrude lavas exhibiting

A single volcano may extrude lavas exhibiting

very different compositions

very different compositions

Each volcanic eruption tends to exhibit a

Each volcanic eruption tends to exhibit a

unique geochemical fingerprint, defined by

unique geochemical fingerprint, defined by

trace element percentages

trace element percentages

Bowen’s reaction series

Bowen’s reaction series

Minerals crystallize in a systematic fashion

Minerals crystallize in a systematic fashion

based on their melting points

based on their melting points

During crystallization, the composition of the

During crystallization, the composition of the

liquid portion of the magma continually changes

liquid portion of the magma continually changes

Evolution of Magma

Evolution of Magma

Bowen’s reaction series

Bowen’s reaction series

C. Formation of granitic magmas -

• Most likely form as the end product of crystallization of andesitic magma
• Granitic magmas are more viscous than other magmas so they tend to

lose their mobility before reaching the surface

• Tend to produce large plutonic structures

The major processes which are responsible for the

The major processes which are responsible for the

formation of secondary or derivative magmas out of

formation of secondary or derivative magmas out of

primary magmas are ;

primary magmas are ;

A.

A.

Magmatic differentiation -

Magmatic differentiation -

Separation of a melt from earlier formed crystals.

Separation of a melt from earlier formed crystals.

• Magmatic differentiation refers to the process whereby an

originally homogeneous magma changes it composition or

becomes heterogeneous via three main mechanisms:

i. Crystal Fractionation- Crystal fractionation is likely to be the most

important in controlling magmatic differentiation.

ii.Liquid Immiscibility

iii.Liquid Fractionation

B.

B.

Magmatic Assimilation ( Magma

Magmatic Assimilation ( Magma

Contamination) –

Contamination) –

Changing a magma’s composition by the incorporation of

Changing a magma’s composition by the incorporation of

surrounding rock bodies into a magma.

surrounding rock bodies into a magma.

• the mechanism may be viewed as mixing of goreign
• the mechanism may be viewed as mixing of goreign

rock materialswith invading magma.

rock materialswith invading magma.

C.

C.

Mixing of Magma -

Mixing of Magma -

Two chemically distinct magmas may produce a

Two chemically distinct magmas may produce a

composition quite different from either original magma

composition quite different from either original magma

• mixing is very likely to take place in basaltic magmas
• mixing is very likely to take place in basaltic magmas

which in general have similar compositions but have

which in general have similar compositions but have

low viscosities.

low viscosities.

Magma evolves as the

Magma evolves as the

hotter minerals crystallize

hotter minerals crystallize

and settle to the bottom of

and settle to the bottom of

the magma chamber

the magma chamber