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IGNEOUS ACTIVITY

( IGNE = FIRE )

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ROCK CYCLE

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MAGMA

1. MAGMA origina) Melting of the Lithosphere : Subduction zone, Ridge,

Mantle plumes and Hot spots

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°  Maximum at center 6500° CRadioactive decay : Heat accumulates from decay, this

Friction : Heat is generated in subduction zones

c) Heat transfer : Heat is transferred outwards mainly by 3 processes :

 – – 

Heat flow is measured by special instruments. Heat flow is foundhighest at the mid-ocean ridges, lowest in subduction zones.

d) Effect of water content :

Presence of water decreases melting point of the rocks

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 Temperature C°

0 200 400 600 800 1000 1200

1050 Cº750 Cº

    )

0

   h   i  n   K  m 20

  e   (   D  e

  p40

   P

  r  e  s  s  u

60

80

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  ,while others have low heat conductivity. Thus, heat waves fromEarth’s core, begin to accumulate heat in the “LOW” zones,

, ,forming a MAGMA liquid. This liquid fills the Magma chamber

  c  o  r  e

Convection currents form

   E  a  r   t   h   ’  s

MAGMA

  s

   f  r  o  m

  a   t  w  a  v

   H

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f Movement of Ma ma : Ma ma has less densit than countr

rocks; therefore it moves upwards, by melting the rocks above,which is the roof of magma chamber. This process is called

STOPING .

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Subcrus

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2. Magma composition : Depending on the plate where magma

forms by melting rocks, there are mainly 3 types of magmas.

: u u z -

b) FELSIC magma : Continental plates

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Comparison of three main MAGMA types

MAFIC MAGMAINTERMEDIATE MAGMAFELSIC MAGMA

High fluidity (Fast flow)IntermediateHigh viscosity (Slow flow)

Dark colored Intermediate colored Light colored 

Much ases ke t in ma maIntermediateMuch ases are lost to air 

High temperature (12000 )Intermediate ( 10000 C)Low temperature ( 8000 C)

ow s ca -nterme ate -g s ca content >

In addition to SiO2 magma

is rich in the following

oxides:Intermediate

In addition to SiO2 magma

is rich in the following

oxides:

Fe2O3, FeO

MgO, CaO

Al2O3

Al2O3

 Na2O3

K 2O3

Ca, Fe, Mg

Na, K, Al

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3. Coolin of ma ma : Ma ma loses heat b conduction and 

convection. If magma reaches to Earth surface, further loss of heat occurs by radiation. Thus magma gradually cools down.

4. Crystallization of minerals from liquid magma : As magma

cools down, ions of various elements in the hot liquid begin to, .

 The earliest forming minerals are OLIVINE ( Fe, Mg silicate ).

 They are followed by PYROXENE, AMPHIBOLE and BIOTITEminerals. Parallel to them are PLAGIOCLASE FELDSPARS.

Latest forming minerals are MUSCOVITE, ORTHOCLASE andQUARTZ.

 This sequence of crystallization was first discovered by a famousmineralogist / petrologist called BOWEN. Therefore it is knownas the BOWEN’s Reaction Series.

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 Tem eratures C° 

1100

High 

Intermediate

700

Low

500

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5. Changes in magma composition : In several ways theoriginal magma composition may be altered.

a) Crystal settl ing and differentiation : Early formedcrystals may settle to the lower part of the magma chamber,

,liquid with a different composition.

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b) Stoping and assimilation of inclusions : Assimilated,

original composition. Unassimilated fragments arecalled XENOLITH.

Country rock

Xenolith

Assimilated country rocks

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about 900° C , melting will occur only of minerals whichhave a low melting point. Thus, this magma will be

.

d) Mixing (mingling) of magmas : As two differentma mas rise u wards b sto in , the ma meet andget mixed. Intermediate magmas are mostly formed inthis way.

6. Texture (grain size) due to cooling time :  The length of cooling time of magma affects the size of the mineral grains.Long time (several millions of years) produces large crystals.Short time (several hundreds of 1000 years) produces smallgrains. Very fast cooling (several hours) produces glass, with no

crystals.

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a aner c ex ureCoarse: > 1 mm

- .

b) Aphanitic texture

Fine : < 0.01 mm

Glassy : no crystals

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 grain sizes. It indicates 2 stages of cooling history.

x u x y y(several cm to several m in sizes). They develop under very

special physico-chemical conditions.

f) Pyroclastic texture is made of mineral and rock fragmentswelded to ether. This texture indicates a volcanic eru tion.

g) Vesicular texture occurs as many spherical or oblong.

bubbles in the liquid stage of the rock. The gasses laterescape into atmosphere.

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.

parameters : Mineral composition and grain size.

FELSIC MAFIC U L T R A M A F I C

(acidic)Intermediate

(basic) ( u l t r a b a s I c )

P roxenite Peridotite Dunite

QuartzPyroxene, Amphibole, Biotite Mafic

Feldspar Olivine minerals

Ferromagnesian minerals

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Practical classification of igneous rocks

M A G M A T Y P E S

Grain

size Felsic Intermediate Mafic U l t r a m a f i c

---

DunitePeridotitePyroxiniteGABBRODIORITEGRANITECoarse

>1 mm

---

Dolerite(Diabase)MicrodioriteMicrogranite

Medium

1 – 0.01

mm

---

BasaltAndesiteRhyolite<0.01 mm

PalagoniteFelsite, Obsidianass

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Practical classification of igneous rocks

M A G M A T Y P E S

Grain

size Felsic Intermediate Mafic U l t r a m a f i c

---

DunitePeridotitePyroxiniteGABBRODIORITEGRANITECoarse

>1 mm

---

Dolerite(Diabase)MicrodioriteMicrogranite

Medium

1 – 0.01

mm

---

BasaltAndesiteRhyolite<0.01 mm

PalagoniteFelsite, Obsidianass

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8. Intrusive i neous rocks :  The are formed b coolin 

and crystallization of magma, injected inside the Earth’scrust, without reaching the surface.

Extrusivei neous rocks

Intrusiveigneous rocksinjected intocountry rock

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EXTRUSIVEIGNEOUSROCKS

INTRUSIVEIGNEOUSROCKS

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Dike : When magma is injected into a fracture in the country rock, it

orms a thin sheet o igneous rock. This body is called a DIKE.

Dike

Dike

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a e zone

Baked zone

Columnar jointing

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S I L L

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COLUMNAR JOINTS

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Metamorphosed(baked) zones

S I L L

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How do columnar joints develop ?

During cooling of magma, contraction towards centers causeoints to develo due to shrinkin 

Upper part of a sill

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How do columnar joints develop ?

During cooling of magma, contraction towards centers causeoints to develo due to shrinkin 

Upper part of a sill

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