I.    Introduction

Igneous rocks are formed either from volcanic activity (lava) or crystallization of molten rock material (magma) forming plutons.


This chapter deals with

(1)  origin, composition, texture, classification of igneous rocks (volcanic and plutonic)

(2) origin, significance, and types of plutons.


II. Magma and Lava


-magma- molten rock below Earth's surface (made mostly of silica); may also have dissolved gases and solid minerals; less dense than cooler surrounding rock so it rises.


-lava- magma at the Earth's surface


-plutonic (intrusive)- igneous rock emplaced into rock beneath earth's surface (see only after erosion)

-volcanic (extrusive)- igneous rock formed at earth's surface (solidifies from lava flow or is explosively ejected from a volcano as pyroclastic material).



Composition of Magma (table 3-1)


-mafic- silica content low (45-52%); other ~50% Mg, Fe, Ca, Al ions; crystallizes to form ferromagnesian minerals.


-intermediate- composition between mafic and felsic.


-felsic- high silica content (>65%); other 35% Na, K and Al ions; form nonferromagnesian minerals.


Temperature of magma is not known, but erupting lavas have temperatures ranging from ~1000 to 1350˚C (fig. 3.2).


Viscosity (resistance to flow)

   Viscosity of magma is controlled by temperature and silica content:

-high temp. = low viscosity & low temp. = high viscosity.


-high silica content (felsic magma) = high viscosity (many silica tetrahedra bonds to break) & low silica content (mafic magma) = low viscosity.



III.      Origin and Evolution of Magma


-magma rises from depths of 100 to 300 km, but most of it forms at much shallower depths in the crust and upper mantle


-magma usually accumulates in magma chambers beneath spreading centers (few kms deep) or convergent plate boundaries (few tens of kms)


Bowens Reaction Series (fig. 3.3)- N.L. Bowen studied the order in which minerals crystallize from a cooling magma; he believed all magma types came from a single mafic 'parent ' magma (believed mafic magmas can become intermediate or felsic).


-minerals crystallize from a magma in a known sequence; the composition of the magma changes over time as different minerals crystallize.


-found that those minerals with the highest melting temps. (those at top) crystallize first from the magma.


-these early formed mins. react with remaining magma ('dissolve') to form the next mins. as the magma cools.




Origin of Magma at Spreading Ridges (divergent plate boundaries)


-geothermal gradient- increase in temperature with depth in the Earth (~25˚C/km).


-also an increase in pressure with depth (lithoststic pressure) that usually keeps rocks in Earth from melting (melting temp. increases with depth) (fig. 3.4).


-at spreading ridges, geothermal gradient is high therefore get high temperatures at shallow depths (and therefore low pressures) resulting in melting (fig. 3.4).


-presence of water at spreading centers may also contribute to melting (fig. 3.4).


-mantle plumes- cylindrical plumes of hot mantle material rises beneath the ridges.


-magmas formed at spreading ridges are mafic and originate from partial melting of ultramafic mantle rocks -only minerals with lower melting temperatures (more silica rich minerals at bottom of Bowens) melt and form magma.


Origin of Magma at Subduction Zones (convergent plate boundaries)


-at subduction zones, a belt of volcanoes and plutons are formed near edge of overriding plate. (Fig 3.5)


-magmas formed at subduction zones are intermediate (53-65% silica) to felsic (>65% silica).


-partial melting of mafic rock (basalt) of the subducting plate (oceanic crust) to form more felsic magma; assimilation and melting silica-rich sediments also a factor.


Chemical Changes of Magma


-The same magma can give rise to different igneous rocks (magma changes composition).  How ?


1- crystal settling-  a type of differentiation in which denser, early formed minerals sink and separate from main magma (can't react with remaining magma) (fig. 3.6).


Bowens Reaction Series is useful because:

1) gives the sequence of crystallization of minerals from cooling magma (sequence of melting is reverse).

2) provides a mean (along with differentiation and settling) for yielding magmas of different composition.

3) a great tool for learning igneous minerals and igneous rock compositions.


2- assimilation- magma melts surrounding rock ("country rock") and assimilates rock into magma making magma intermediate in composition between rock comp. and original magma comp. (fig. 3.6).


3- magma mixing-magmas meet in the Earth's crust and mix; get comp. between the two (fig. 3.7).


*partial melting- create a felsic magma from a more mafic "parent" rock by melting only minerals with the lower melting temperatures (those towards the bottom of Bowens reaction series).



IV.      Igneous Rocks

Form when minerals crystallize from a magma.


-plutonic (intrusive) and volcanic (extrusive) igneous rocks come from same magma types; major difference in 2 rock types is the grain size.


Igneous Textures (fig. 3.8)

Texture refers to size, shape, and arrangement of mineral grains.


Rapid cooling-> large number of crystal nuclei->less growth

Slow cooling->small number of crystal nuclei->more growth


   -aphanitic- fine grained; result of rapid cooling of a lava at surface (volcanic).


   -phaneritic- coarse grained; result of slow cooling of magma deep within earth (plutonic).


   -porphyritic- large and small grains(mixed); 2 stage or complex cooling history. Phenocrysts (large) and ground mass (small)


   -vesicular- holes (vesicles) in rock; form as water vapor and gas came out of magma near earth's surface.


   -pyroclastic- solidified volcanic ash and rock fragments exploded from a volcano.






Compsition of Igneous Rocks


Magma Composition          Volcanic         Plutonic

mafic- (~45%-52% silica)             basalt        gabbro

intermediate- (~53%-65% silica)             andesite           diorite

felsic- (> ~65% silica)                    rhyolite           granite



Classification of Igneous Rock

Classification usually based on textural features (aphanitic/phaneritic) and composition (minerals present; ferromagnesian vs. nonferromagnesian) (figs. 3.10, 3.16).


ultramafic->  (<45% silica)  ferromagnesian minerals

peridotite (olivine)

pyroxenite (pyroxene)


basalt/gabbro-> ( mafic magma 45-52% silica) Dark color

basalt - fine-grained (extrusive rock)

gabbro - coarse-grained (intrusive)


andesite/diorite->  (intermediate 53-65% silica)

andesite - fine-grained (extrusive rock)

diorite - coarse-grained (intrusive)


rhyolite/granite -> (felsic >65% silica) light colored

rhyolite - fine-grained (extrusive rock)

granite - coarse-grained (intrusive)


Pegmatite-> (close to granite)  Textural rather than composition.  Characterize by their large size of minerals.


other igneous rocks

tuff- consolidated pyroclastic material (ash)

obsidian- volcanic glass

pumice- vesicular volcanic glass.


V.           Intrusive Igneous Bodies: Plutons (fig. 3.19)


-pluton- general term for a variably shaped igneous body resulting from crystallization of magma at depth.

Observed after erosion (Figure)


-could be tabular (table like), massive (irregularly shaped), concordant (parallels layering), or discordant (cuts across layering).


-dikes- tabular discordant pluton (enters fractures in the Earth) (fig. 3.20).


-sills- tabular concordant pluton (most parallel bedding in sedimentary rock [weak]).


-laccoliths- concordant, mushroom-shaped igneous body (lifts overlying rock layers).  Usually form at shallow depths.


-volcanic pipes and necks- cylindrical conduit connecting volcanic crater with magma called volcanic pipe.  Volcanic neck is the remnant of volcanic pipe.


-stocks- small discordant pluton (surface area of < 100 km2); may be small exposed part of larger pluton.


-batholiths- large discordant pluton (>100 km2); usually multiple intrusions; almost always granitic rocks.





VI- Emplacement of Batholiths


-Granitization; solid state phenomenon (metamorphism); expected gradual changes.  Only small amount may be account for this process.


-forceful injection; Magma deforms and shoulders aside the country rock, and as it rises further, some of the country rock fills the space beneath the magma (Figure 3.23).

The process is analogous to the formation of salt domes (Figure 3.24).


Usually takes place in the deeper part of the crust (material is hot and easily deformed)


-stoping;  Magma moves along fractures and planes of the country rock.  Pieces of the country rock are detached and settle into the magma (Figure 3.25).