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Magma Formation and Behavior Questions

• What causes mantle rock to melt, resulting in magma formation?

• Why is magma formation restricted to specific plate tectonic settings?

• Why are mafic (basaltic) magmas erupted along mid-ocean ridges if they form by the melting of ultramafic rock (peridotite)

• Why do granitic (felsic) magmas usually crystallize before reaching Earth’s surface? (Most intrusive rocks are granite)

• Why are mafic (basaltic) magmas usually erupted as lavas at Earth’s surface? (Most volcanic rocks are basalt)

• According to plate tectonic theory, most

magmas form in the following settings:

– Subduction zones, along convergent plate

boundaries

– Rift zones, along divergent plate boundaries

– Mantle plumes (hot spots)

Subduction Zones

Rift Zone Mantle Plume

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How can magma form by melting of

mantle rock?

• Increase Temperature:

– Least important cause of melting within the

Earth

• There is no source of heat to increase

temperature of mantle/lower crustal rocks

to their melting temperatures.

How can magma form by melting of

mantle rock?

• Decrease Pressure

– High pressures – atoms in minerals are so

tightly compacted that chemical bonds cannot

be broken to transform the solid minerals to a

liquid (a magma)

– Low pressure – atoms in solid minerals are

spaced apart so that atoms have room to

vibrate and chemical bonds between them

can be broken to form a liquid (a magma)

At high pressures deep within the mantle, atoms in

minerals are so tightly compacted that chemical

bonds cannot be broken to transform the solid

minerals to a liquid (a magma).

At low pressures, atoms in solid minerals are

spaced apart so that atoms have room to vibrate

and chemical bonds between them can be broken

to form a liquid (a magma)

Melting by Decrease in Pressure Occurs

Along Mid-Ocean RidgesMelting by Decrease in Pressure Occurs

Within Mantle Plumes

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Peridotite – ultramafic rock which makes up

Earth’s mantle

Phase Diagram – A graph which shows the various

physical conditions under which a phase (such as a solid,

liquid, gas) can exist

Phase Diagram for Melting of Mantle Rock

(peridotite) by Decrease in Pressure

solidus

Why do magmas form along

subduction zones?

• Rocks within the ‘mantle wedge’ are

‘squeezed’, resulting in an increase in

pressure.

– Therefore, ‘decompression melting’

cannot explain the formation of magmas

within a subduction zone

What unique feature of a subduction zone might

cause melting of mantle rock in the mantle wedge?

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How can magma form by melting of mantle

rock?

• Addition of Water

– Small amount of water acts as a catalyst to lower the melting temperature of most minerals.

• Unequal distribution of electrons around the water molecule (electrical polarization) results in decreased anion-cation attraction within minerals, so that chemical bonds in the mineral structure are more easily broken

Melting by Addition of Water Occurs Within

Subduction Zones

Phase Diagram for Melting of Mantle Rock

by Addition of Water

solidus

Why does cold beer sometimes partially freeze

when the can is opened?

Why are mafic (basalt) lavas

erupted along mid-ocean ridges if

the rock being melted to form the

magma is ultramafic (peridotite)?

Partial Melting

• Minerals with highest Si content melt at the lower temperatures than minerals with low Si content (reverse of Bowen’s Reaction Series)– Silicate rocks melt over a range of

temperatures.

– Magmas produced by partial melting of a rock will have a higher Si content than the ‘parent rock’ which underwent melting.

• Example: partial melting of an ultramafic rock forms a mafic magma

• Explains that mafic lavas (basalts) erupted along MOR’s form by partial melting of ultramafic mantle rock (peridotite)

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Low temperature minerals at the ‘bottom’ of BRS melt

first, producing a ‘partial melt’ with a higher Si content

than that of the rock being melted.

Partial Melting

– Silicate rocks melt over a range of temperatures.

– Magmas produced by partial melting of a rock will have a higher Si content than the ‘parent rock’ which underwent melting.• Partial melting of an ultramafic rock forms a

mafic magma

• Explains that mafic lavas (basalts) erupted along MOR’s form by partial melting of ultramafic mantle rock (peridotite)

Partial melting of a mafic rock produces a magma

of what composition?

Why are most mafic (basaltic) magmas

erupted at volcanoes, while most felsic

(granitic) magmas crystallize beneath the

surface?

Felsic Magma vs. Mafic Magma

• Felsic Magmas – most crystallize as intrusive rocks (granite) beneath Earth’s surface

– High Si content – results in greater viscosity (resistance to flow); flows to surface very slowly

– High dissolved water content – water escapes as steam as magma ascends, increasingtemperatures at which minerals will start to crystallize

• Higher crystallization temperatures increase the likelihood that complete solidification will occur before reaching the surface

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Why does cold beer sometimes partially freeze

when the can is opened?

‘CO2 – Rich’ Beer

(liquid)

‘CO2 – Dry’ Beer

(solid)

Felsic Magma vs. Mafic Magma

• Mafic Magmas – most are erupted as

basaltic lavas at Earth’s surface

– Low Si Content – results in lower viscosity

(flows to surface quickly)

– Low dissolved water content – loss of water

as magma ascends has little effect on mineral

crystallization temperatures

Summary

• To Crystallize a Magma:

– Decrease temperature

– Increase pressure

– Remove water

• To Melt a Rock:

– Increase temperature

– Decrease pressure

– Add Water

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Fractional Crystallization

• Involves removal of crystals from the

surrounding melt

– Chemical reactions between crystals and

surrounding melt cannot occur

– Results in solid rock of entirely different

chemical composition than that of the melt

from which it formed

Equilibrium Crystallization

• Occurs when crystal remain in contact with

the surrounding melt

– Chemical reactions between solid crystals

and surrounding melt can occur

– Results in solid rock of the same chemical

composition as that of the melt from which it

crystallized

Fractional Crystallization

Equilibrium Crystallization