ClassNotes: Geo 1330: Dr Dupre´

Igneous Rocks

_______________: formed from the crystallization from a magma

_____________: Usually a hot (650 to 1200°C). silicate melt (liquid), most of which consists of ions of eight elements, mixed with gases and suspended crystals.

______________: The process by which crystals precipitate from the melt, usually as the melt(magma) cools. It can be thought of the as opposite of melting. The exact sequence in which minerals crystallize from a magma is highly variable (more later).

Magma extruded at the surface is called: ___________________

Fragments of volcanic material ejected into the air are called: ________________________

Genetic Igneous Rock Classifications

1)_________________: extruded at the earth’s surface as lava or pyroclastic material

2)_________________: intruded within the earth’s crust

II. Descriptive Igneous Rock Classifications

1)_______________: size, shape, arrangement of crystals

2)_______________: chemical or mineral components

Factors Controlling Texture of Igneous Rocks

1) ____________________________ (most important)

2) Amount of ____________ present

3) Amount of __________________

Effect ofCooling Rate on Crystal Size:

__________ cooling: results in few nucleation sites and large crystals.

__________cooling: results in many nucleation sites and many small crystals.

______________cooling: results in an amorphous glass.

TYPES OF IGNEOUSTEXTURES:

1) ___________________

2) ___________________

3) ___________________

4) ___________________

5) ___________________

IGNEOUS TEXTURE=

Individual crystals can’t be seen by the unaided eye.

IGNEOUS TEXTURE=

Individual crystals can be seen by the unaided eye.

IGNEOUS TEXTURE=

large crystals(Phenocrysts) in a finer-grained matrix (groundmass).

IGNEOUS TEXTURE=

lacks a crystalline structure because of rapid cooling.

IGNEOUS TEXTURE=

Contains abundant gas bubbles “frothy”

Examples

Major Rock-Forming Silicates

Olivine (Mg,Fe) [SiO]

Pyroxene (Mg,Fe) Ca [Si O]

Amphibole (Mg,Fe) Ca [Si O] (OH)

Biotite (Mg,Fe) KAl [Si O] (OH)

Muscovite KAl [Si O] (OH)

Orthoclase KAl [Si O]

Plagioclase (Ca,Na)Al [Si O]

Quartz SiO2

Mafic Minerals =

Felsic Minerals=

Felsic, Mafic, and Intermediate Rocks:

% “dark minerals” ?

Chemical variation between mafic, intermediate, and felsic rocks

Some Changes from Felsic to Mafic Rocks

Mineral composition of major igneous rock types:

1) Ultramafic =

--45% SiO----

2) Mafic =

--52% SiO----

3) Intermediate =

--65% SiO----

4) Felsic =

Mineralogy of Common Igneous Rocks (fig 4.7)

Classification of Igneous Rocks (fig 4.8)

Chemical composition	Felsic	Intermediate	Mafic	Ultramafic
Dominant minerals
Color
Phaneritic
Aphanitic
Porphyritic

Examples:

Rhyolite

Granite

Gabbro

Basalt

Peridotite:

Igneous rocks you should know include:

1)

2)

3)

4)

5)

6)

7)

8)

9)

How to Generate Magma from solid rock:

1) Role of Heat

What is decompression melting and where might it occur?

What is the role of volatiles (mainly water) in controlling the melting of rocks, and where might that occur?

Factors affecting melting temperatures

Typical Geothermal Gradient (fig. 4.18)

Idealized Melting Curves (fig. 4.19)

Melting begins at the intersection of the melting curve and the Geothermal Gradient

Q. Would you expect basaltic magma to form at a deeper or shallower depth than a granitic magma?

A. Why?

Possible mechanisms for changing composition of magmas include:

1)

2)

3)

Fig. 4.25

Q. What are xenoliths and what are they evidence of?

A.

Q. Where does crystal settling occur, and what is the evidence that it has occurred?

A.

_______________:

Ideally, the opposite of melting. The process is complicated, however, because rocks are complex aggregates of many minerals with different melting (crystallization) temperatures.

_____________________:

examples: ______________

When a melt reaches the crystallization temperature of a mineral, the mineral crystallizes and undergoes no further changes with subsequent cooling,

_____________________:

examples: _______________________

Crystals react with melt to produce new minerals

_____________________:

example: _______________________

Crystals change their composition continually as they grow due to changes in the composition of the magma.

_____________________:

The modification of magma by the removal of early formed crystals. Because only certain elements will go into a given mineral, this will tend to change the composition of the remaining liquid.

Early Partial Crystallization and Crystal Settling:

Liquids squeezed from crystal “mush”

______________________ (figure 4.23):

Sequence of crystallization from a cooling basaltic magma:

_______________ Reaction Series:

Olivine -> Pyroxene ->Amphibole -> Biotite

_______________ Reaction Series:

Ca-rich Plagioclase -> Na-rich Plagioclase

Q. What is the significance of a zoned plagioclase crystal?

A.

More on Bowen’s Reaction Series:

e.g. different mineral assemblages at different stages in the cooling of a fractionating magma create our main classes of magmas and igneous rocks!.

Q. What’s so interesting about the Palisades Sill in NY?

A.

Q. What the problem with generating granite by fractional crystallization?

A.

___________________:

The opposite of fractional crystallization. Last minerals to form will melt at lowest temperature.

Q. What happens when you partially melt a Peridotite?

A.

Evidence?

Tectonic Controls on Igneous Activity

Where do most magmas form?

1)

2)

3)

At _________________ plate margins:

Partially melted asthenosphere (_________________) rises at spreading centers, causing __________________ of up to 15% of the rock to form

_________ magma.

Q. What type of magma is typically found in the magma chambers at divergent plate margins? (figure 4.20)

A.

Q. How does it form?

A.

At _________________ plate margins:

________________ drags oceanic lithosphere (including a veneer of “wet” sediments) beneath the adjacent plate.

The release of __________ lowers the melting point of the adjacent mantle

causing ________________ to form a magma which becomes

more ______________ in composition as it rises through the overlying crust.

Q. What type of magma forms in a subduction zone at a convergent plate margin?

A.

Q. How does it form?

A.

Q. What type of magmas forms the volcanoes on the Indonesian island of Java? They form along what type of plate boundary?

A.

Q. What type of magma forms Mt Ranier in Washington State. It is forming along what type of plate boundary?

A.

Q. The Hawaiian volcanoes are forming along what type of plate boundary? (trick question)

A.

Mafic magmas are generated by:

Intermediate Magmas are generated by:

Felsic Magmas are generated by:

Partial Melting of:

Upper Mantle--------------- > ?

Oceanic Crust --------------> ?

Arc material ----------------> ?

Partial Melting of:

Peridotite------------------- > ?

Basalt --------> ?

Andesite ----------------> ?

Geometry of Igneous Intrusions:

Most magmas are intruded at depth, where they cool to form: __________________

These bodies are classified on the basis of:

A. ____________________:

________________ vs ____________________

C. ____________________:

________________ vs ____________________

_______________________: massive, discordant bodies with a surface area > 100 km²

_______________________: discordant, tabular bodies

_______________________: concordant, tabular bodies

See figure 6.28