Fig. 03.01

Fig. 03.02

Magma: molten rock + volatiles beneath the

Earth’s surface.

Lava: molten rock on the Earth’s surface

that has lost at least some of its volatiles.

Magma is usually silicate in composition:

~50-70 wt% SiO2

The rest is made up of Al2O3, Fe2O3, FeO,

Na2O MgO, K2O, CaO, P2O5, TiO2

A continuous range of magma chemistries

is observed.

Partial Melting

Rocks rarely melt to 100%.

Rocks contain several minerals – the one

with the lowest melting point melts first and

that with the highest melts last.

Liquid may be removed along grain

boundaries.

This can create a magma of a different

composition than the source material.

Fig. 03.06

Table 03.01

Felsic ~70 wt% SiO2

Intermediate ~60 wt% SiO2

Mafic ~50 wt% SiO2

Ultramafic <50 wt% SiO2

(Komatiite)

Fig. 03.07a

Fig. 03.07b

Fig. 03.07c

Fig. 03.07e

Fig. 03.07f

General Rules

Felsic = light-colored (Si-, Al-, alkali-rich).

Ultramafic and Mafic = dark-colored (Fe, Mg,

Ca rich, ferromagnesian mineral-rich).

Magmatic temperatures range from ~600˚C to

>1,500˚C.

The Exception that Proves the Rule - Obsidian

Obsidian = felsic (quenched glass), but black.

General Textures & Crystallization

Rate of crystallization dictates mineral size:

slow cooling = large crystals; fast cooling =

small crystals or a quench glass.

General Textures

Extruded or erupted magmas or shallowly emplaced magma

bodies cool quickly many small, interlocking crystals with

some interstitial quench glass.

Intrusions of magma deeper within the crust are insulated.

Although the margin of the magma next to the wallrock is chilled

(small grain size) and the wallrock is baked, the interior of the

magma cools much more slowly = large grain size (phaneritic).

Deep intrusive igneous rocks are give the generic term “plutonic”.

Fig. 03.05a

Plutonic Rock (Granite)

Fig. 03.05b

Plutonic Rock (Granite) in Thin Section

Fig. 03.03

Plutonic Rock (Granite) in the Field

Textural Terms APHANITIC: individual crystals are so small they cannot be

discerned with the naked eye. Common in eruptive and shallow

intrusive igneous rocks.

PHANERITIC: crystals can be seen, common in interiors

of thick flows and deeper intrusions.

Textural Terms

PORPHYRITIC:

large crystal

(PHENOCRYSTS) in

a fine-grained

groundmass. Reflects

two cooling stages –

slow cooling (in a

magma chamber) to

form the phenocrysts,

followed by rapid

cooling as would occur

upon eruption.

Textural Terms

Textural Terms

PEGMATITIC: contains very large

(cm-size) crystals.

Crystal Shapes

EUHEDRAL: well-

formed crystals with

many crystal faces

developed.

Crystal Shapes

SUBHEDRAL: crystals with only a few

crystal faces developed.

Crystal Shapes ANHEDRAL: crystals with no crystal faces

developed.

Magma Viscosity

Magma viscosity is dependent upon

composition and temperature:

Mafic << Felsic

Hot << Cool

Silica affects viscosity because of poly-

merization or linking of SiO2 tetrahedra.

The more silica-rich the more felsic the

magma is and the stickier it is.

Magma Viscosity All magmas contain gases, but felsic magmas are

generally more gas-rich - explosive.

When gases escape, can form vesicular lavas,

e.g., vesicular basalt:

Pumice = “froth” = felsic.

Intrusive Rocks Intrude older rocks (generically known as

“countyrock” – can be igneous, sedimentary, or

metamorphic) and bake them = contact

metamorphism.

Intrusive rocks recognized by:

Intrusive Rocks

Coarse grain size, interlocking crystals, typically

lacking a fabric (oriented texture).

Baked contacts (country rock) and chill zone

(finer grain size) at the edges of the intrusion.

Inclusions of country rock = “xenoliths”.

They cross-cut features in the country rock.

Veins protrude outward into country rock.

Fig. 03.04

Ship Rock, New Mexico

Fig. 03.08b

Fig. 03.13

Fig. 03.12

Fig. 03.14

Granite Batholith, Sierra Nevada

Fig. 03.10

How to distinguish a sill from a flow:

• baked contacts above and below;

• vesicles (gas bubbles) in flow often

filled in with flows;

• smaller dikes present above sill

intruding the overlying countryrock.

Intrusion Types

Changing a Magma Composition

Fractional crystallization via crystal settling:

Changing a Magma Composition

Mixing of two compositionally distinct magmas:

Changing a Magma Composition

Assimilation of country rock:

Changing a Magma Composition

Partial melting of the source rock to different

degrees.

Plate Tectonics &

Igneous Rocks

Divergent Plate Margins –

lithosphere and asthenosphere;

pressure-release, almost all

mafic magmatism.

Plate Tectonics & Igneous Rocks

Intraplate: Hawaii,

Yellowstone, LIPs

– mantle plume

activity.

Fig. 03.24

Plate Tectonics &

Igneous Rocks

Convergent Plate Margins: more

intermediate and felsic

magmatism, especially if

volcanoes are built on continents.

Fig. 03.26

Origin of Granite: magmatic underplating.