1) Classification of igneous rocks (texture, composition) 2) Naming of igneous rocks 3) Origin of magma 4) Igneous diversity 5) Magma recipes Chapter 5:

1) Classification of igneous rocks

(texture, composition)

2) Naming of igneous rocks

3) Origin of magma

4) Igneous diversity

5) Magma recipes

Chapter 5: Chapter 5: Igneous rocksIgneous rocks

Today:

1) Igneous textures

As magma cools, atoms arrange in anorderly crystal structure

crystallizationcrystallization

1) Igneous textures

Note: different crystal sizes

1) Igneous textures

Close up of crystals Note the variety of crystal sizes

Texture

1) Igneous textures

Texture

a. size

b. shape

c. arrangement

of interlocking crystals

1) Igneous textures

–Factors affecting crystal size

–Rate of cooling

–Amount of silica (SiO2) present

–Amount of dissolved gases

1) Igneous textures

crystal sizecrystal size

Slow cooling

cooling ratecooling rate

larger crystals

Fast cooling small or no crystals

1) Igneous compositions

•mainly silicate minerals

•determined by composition of magma from

which it crystallized

•magma mainly 8 elements:

Si, O, Al, Ca, Na, K, Mg, Fe

Most abundant constituents

1) Igneous compositions

Magma

cools

solidifies

forms silicate minerals

Two major silicate mineral groups:

DARK silicates LIGHT silicates

1) Igneous compositions

DARK“ferromagnesian”

LIGHTLIGHT“non-

ferromagnesian”

Rich in

Fe and/or Mg

Si, K, Na, CaSi, K, Na, Ca

Poor in

Si

Fe and/or MgFe and/or Mg

Silicate group

olivine

pyroxene

amphibole

biotite

quartzquartz

muscovitemuscovite

FeldsparsFeldspars

(40% of most (40% of most

igneous rocks)igneous rocks)

Examples

See Table 5.1

1) Igneous compositions

Fig. 5.4, see Table 5.2

The classification model of igneous rock.

2) Naming igneous rocks

granitegranite

rhyoliterhyolite

zooming inzooming in

f

e l s

i c

f

e l s

i c

plu

ton

icvolc

an

ic

See Fig. 5.3

2) Naming igneous rocks

obsidianobsidian

pumicepumice

f

e l s

i c

f

e l s

i c

volc

an

icvolc

an

ic

•Obsidian–Dark colored–Glassy texture

•PumiceVolcanicGlassy textureFrothy appearance with numerous voids

See Fig. 5.3

2) Naming igneous rocks

Another type of igneous rock

pyroclastic rock

from fragments ejected during eruption

most common: “tuff” (ash-sized fragments, cemented together

called volcanic breccia if it mostly contains

fragments larger than ash (rocks from vent

wall,crystals, glass fragments…)

2) Naming igneous rocks

pyroclastic rock Ash and pumice layers

Common in western US

which had high volcanic

Activity millions of years

Ago

2) Naming igneous rocks

andesiteandesite

dioritediorite

i n

t e

r m

e d

i a

t e

i n

t e

r m

e d

i a

t e

zooming inzooming in

volc

an

icp

luto

nic

See Fig. 5.3

2) Naming igneous rocks

basaltbasalt

gabbrogabbro

m

a f

i c

m

a f

i c

zooming inzooming in

volc

an

icp

luto

nic

See Fig. 5.3

RecallRecall

igneousigneous rock =

cooled & solidified magma or lava

magmamagma =from melting solid rock in crust and/or upper mantle

…but how do we melt rock???

3) Origin of magma

Depends on: composition

pressure

water content

““melting temperature”melting temperature”

= the temperature at which something melts

1st, let’s define

Temperature scale digression…Temperature scale digression…

Celsius (centigrade) scale Fahrenheit scale

Water freezesWater freezes

Water boilsWater boils

0oC

100oC

32oF

212oF

oC = (oF - 32)59

Pressure

Tem

p.

SOLIDSOLID

LIQUIDLIQUID

Melting temperature curve

Melting temperature decreases with increasing

waterwater content.

SOLIDSOLID

LIQUIDLIQUID

Melting temperature curve

Melting temperature increases at higher pressure.

3 ways to melt rock3 ways to melt rock3 ways to melt rock3 ways to melt rock

raise the temperature

lower the pressure

add volatiles (water)

Inside Earth examples:

Rocks in the lower crust and Rocks in the lower crust and upper mantle are near their upper mantle are near their melting pointsmelting points.. Rising magma Rising magma heats rocks furtherheats rocks further

Rocks in the lower crust and Rocks in the lower crust and upper mantle are near their upper mantle are near their melting pointsmelting points.. Rising magma Rising magma heats rocks furtherheats rocks further

Rock ascends to shallowerRock ascends to shallowerdepth in convective flow.depth in convective flow.When confining pressures drop, When confining pressures drop, decompression melting occursdecompression melting occurswithout additional heatwithout additional heat

Rock ascends to shallowerRock ascends to shallowerdepth in convective flow.depth in convective flow.When confining pressures drop, When confining pressures drop, decompression melting occursdecompression melting occurswithout additional heatwithout additional heat

Water percolates up fromWater percolates up fromsubducting oceanic subducting oceanic lithosphere into overlyinglithosphere into overlyingmantle rocksmantle rocks

Water percolates up fromWater percolates up fromsubducting oceanic subducting oceanic lithosphere into overlyinglithosphere into overlyingmantle rocksmantle rocks

See Fig. 5.15

Decompression meltingHot mantle rock

ascends and moves

into zones of lower

pressure. This drop

in confining pressure

may trigger melting.

Ascend rates of magma are 0.3 to 50 m/yr. Magma chambers (several km3 big) form in the cavities of the lithosphere as magma rises.

Water vapor mixed with molten sedimentary rocks from subducting plate rise and lower the melting temperature of mantle and lithospheric rock.

= fluid-induced melting

Effect of water in melting mantle rock:

5.15

4) Igneous diversity

magmatic differentiationmagmatic differentiation

New…

Mechanisms to account for the great

diversity of igneous rock

assimilationassimilation

magma mixingmagma mixing

4) Igneous diversity

magmatic differentiationmagmatic differentiation

In a magma body:

crystals form as magma cools

not all at the same time! = fractional crystallization

heavy crystals sink to bottom =crystal settling

more than 1 type of magma from same parent

variety of igneous rocks from same source

Mechanisms to account for the great diversity of igneous rock

See Figure story 5.5

4) Igneous diversity

assimilationassimilation

As magma migrates upward:•may incorporate surrounding host rock

•In deeper layers magma may melt surrounding host rock

magma mixingmagma mixing

If magma body collides into another:

•magmas can mix

(see Fig 5.6)

4) Igneous diversity

magmatic differentiationmagmatic differentiation

Mechanisms to account for the great

diversity of igneous rock

assimilationassimilation

magma mixingmagma mixing

Fig. 5.6Mixing of two magmas=andesitic magma

basaltic

andesitic

granitic

“felsic”

“mafic”

mantle rock:peridotite

Mantle peridotites+ felsic crustal rocks

Si-rich continentalcrust rocks

Partialmelt of:

buoyant rockascends/reductionin pressure, largeoutpourings

basalt ascends,melts/assimilatescrustal rocks

Basalt heats &melts crust frombelow (assoc. w/subduction)It is more viscous And stays subsurface.

How/process:

“felsic”“felsic”

5. Magma recipes5. Magma recipes 3 main types