CH. 3 – IGNEOUS ROCKS

Igneous Rocks

Ignis = “fire”

Form from cooling & crystallization of magma (molten rock)

Lava = molten rock at surface

Igneous Rocks

Classified by texture & composition

Texture = crystal size of minerals

Composition = minerals present

Changes of State of Matter

Add heat (melting):

Rx Magma (atoms faster)

Remove heat (crystallize):

Rx (atoms slower) Magma

Texture of Igneous Rocks

Depends on magma’s cooling rate

1) Cools slowly

Atoms can move through liquid

- crystals grow (large, small)

Magma’s Cooling Rate

2) Cools quickly

Forms a solid faster than atoms can move around

- crystals are (large, small)

Texture (Crystal Size)

Two environments where magma cools:

1) At surface or shallow burial

2) Deep underground

Types of Texture

1) Aphanitic

- crystals invisible to naked eye

- magma cooled ___________

- extrusive/volcanic

Types of Texture

2) Vesicular

- aphanitic rock w/many holes (vesicles) in it

- gas trapped as magma cools very quick

Types of Texture

3) Phaneritic

- crystals easily visible

- magma cooled ___________

- intrusive/plutonic

Types of Texture

4) Porphyritic

- two size crystals in same rock

Large crystals = phenocrysts

Small crystals = groundmass

Porphyritic Texture

2-stage cooling process:

1st stage = slow cooling at depth

_______________ are formed

2nd stage = rapid cooling of remaining magma

_______________ are formed

Types of Texture

5) Glassy

- no crystalline structure

Ex: Obsidian; pumice

Types of Texture

6) Pegmatic

All minerals larger than 1 cm

Forms during later stage of crystallization

- large percentage of liquid allows ions to migrate easier

Types of Texture

7) Pyroclastic

“pyro” = fire; “clastic” = pieces

Consolidated rock fragments ejected during violent volcanic eruptions

Pyroclastic texture

Fragments can be volcanic ash, blobs of magma, or angular blocks

- not composed of interlocking crystals!

Igneous Rock Composition

The most abundant elements of Earth’s crust are _____ & ______

Magma is mostly silica with varying amounts of other six elements

Composition

Other elements collect in water at top of magma chamber

Creates hydrothermal solution

- cools in fractures as highly concentrated ore deposits

Igneous Compositions

Mafic

- 45-50% silica (“silica-poor”)- rich in magnesium, iron (Fe), &

calcium

- dark-colored: black, dark gray, brownish-red

Igneous Compositions

Felsic (feldspar & silica)

- 60-75% silica (“silica rich”)- rich in aluminum, potassium, &

sodium

- light-colored: light gray, pinkish

Igneous Compositions

Intermediate

Aphanitic: medium gray

Phaneritic: black & white crystals

Importance of Igneous Rocks

Compose 90% of Earth’s crust

Two types of crust:

oceanic crust = basalt

continental crust = granite

Question:

Why different igneous rock compositions if only 8 elements?

N.L. Bowen experimented with mafic magmas

Results:

** Same sequence of minerals crystallized from magma

Referred to as Bowen’s Reaction Series (Fig. 3.16)

Bowen’s Reaction Series

Discontinuous series (left) vs. continuous series (right)

Discontinuous series:

- each mineral has a unique silicate structure

Continuous series

Different compositions of plagioclase feldspars

- ions of Ca2+ and Na+ can interchange within crystalline structure

Bowen’s Reaction Series

As magma crystallizes:

First mineral to form = mafic

Composition of remaining magma changes

Bowen’s Reaction Series

Biotite uses last mafic elements in magma

- remaining magma becomes more “silica rich”

Bowen’s Reaction Series

Minerals that form in same temp. conditions are found together in same igneous rock

QUESTION: Which mineral would you expect to form phenocrysts in a porphyritic basalt?

Phenocryst Mineral:

a) Quartz

b) Potassium feldspar

c) Biotite

d) Calcium-rich plagioclase f-spar

e) Sodium-rich plagioclase f-spar

Felsic magma

Felsic minerals melt at cooler temp’s

- magma not as hot as mafic magma

- felsic magma not as buoyant

Felsic magma

Magma doesn’t reach surface

Ex: Continental crust = granite

Important Concepts

A mineral is stable if the environmental conditions are similar to where it formed

Ex: temperature, pressure, water- content

Example:

Mafic minerals are stable in ____ temperatures

Felsic minerals are stable in ____ temperatures

Important Concepts

If a mineral is put under different environmental conditions, it becomes unstable

- mineral has to react to new environment by changing

New Environmental Conditions:

If at the surface, weathering occurs

If within the crust, metamorphism occurs