Igneous Rocks



Melting Within the Earth	


Magma - molten silicate-rich liquid that may 
contain solids and gases



Geothermal Gradient - rate of increase in 
temperature with increasing depth



Near the surface temperature rises at a rate of 
about 30 degrees Centigrade per kilometer - 
cannot continue to rise at this rate with 
increasing depth






Pressure increases at a rate of about 333 bars 
per kilometer - one bar equals one atmosphere 
- about 14.7 pounds per square inch.



Lithostatic pressure - due to mass of 
overlying material  - assumed to be equal in 
all directions
Partial Melting	



Rocks are mixtures of two or more minerals


The melting behavior of mixtures differs 
from that of single substances



Mixtures have a melting range - that is, a 
rock does not have a unique melting point 
but a melting range.



Melting Behavior of H2O


Melting of a Mixture

Partial Melting
1000 C		Liquid (100%)

900 C		Solid + Liquid (75%)

800 C		Solid + Liquid (25%)

700 C		Solid + Liquid  ( 5%)

650 C		Solid (100%)

600 C		Solid (100%)





Magmas and Lavas


Magmas migrate upwards driven by the 
density contrast between solids and liquids



Magmas INTRUDE into older rocks as they 
migrate





Magmas that cool at depth form INTRUSIVE 
rocks




A Magma is called LAVA when it reaches the 
surface - forms EXTRUSIVE rocks




Cooling Rate



Heat flows from bodies at high temperature 
into cooler bodies - until the temperature is 
identical in both.




The temperature contrast (DT) strongly 
influences the rate of heat flow.




Lavas are in contact with the atmosphere and 
DT is large....cooling is rapid and solids tend 
to be small -APHANATIC TEXTURE




For Magmas DT may be much smaller - 
function of the temperature of the 
COUNTRY ROCK.




With a low cooling rate crystals may grow 
quite large - PHANERITIC TEXTURE.


Mixed sizes - POPRHYRITIC TEXTURE .. slow 
cooling to develop some large crystals (the 
PHENOCRYSTS).  Cooling rate increases 
(extrusion to the surface?) and remaining 
liquid cools more quickly. 







Mineralogy - Variation in Temperature




In general, minerals with abundant Fe, Mg 
and Ca crystallize at higher temperatures.




As the temperature declines, minerals with a 
higher percentage of Si and O begin to 
crystallize.





The relationship between temperature and 
mineral assemblage is depicted in BOWEN¹s 
REACTION SERIES

 


DISCONTINUOUS - Olivine begins to 
crystallize at high temperatures 




At some lower temperature olivine reacts 
with the liquid and a pyroxene forms




CONTINUOUS - there is a continuous 
variation in composition within the 
Plagioclase Solid Solution Series




At much lower temperatures Alkali Feldspar 
and Quartz crystallize from the melt






VISCOSITY - the resistance a liquid offers to 
flowing : high viscosity - very sticky and liquid 
flows with difficulty



 
Mineral Assemblages



Olivine & Pyroxene : high temperatures - 
mantle/asthenosphere




Olivine, Pyroxene and Ca-rich Plagioclase : 
high temperatures - oceanic crust - 
spreading centers




Amphibole and Na-Rich Plagioclase : 
intermediate temperatures - subduction 
zones




Mineral Assemblages




Quartz and Alkali Feldspar : low 
temperatures - continental crust - 
continent/continent collision zones




Consider a high temperature liquid that will 
crystallize Olivine and Ca-Plagioclase.




Rapid cooling produces Aphanatic texture




Slow cooling produces Phaneritic texture
Classification of Igneous Rocks




TEXTURE - size, shape and arrangement of 
grains - interpreted as measuring Rate Of 
Cooling





MINERALOGY - minerals present - 
interpreted as measuring the Temperature of 
the liquid : the type of Feldspar is a good 
index for estimating temperature






Names of Igneous Rocks



Texture			Alkali		Na-rich		Ca-rich
Phaneritic		Granite		Diorite		Gabbro
Aphanatic		Rhyolite	Andesite	 Basalt

CONTINENTAL CRUST - GRANITES



OCEANIC CRUST - BASALTS



SUBDUCTION - ANDESITE


Evolution of Magmas




Migration of melts upwards




Crystal settling or floating



Mixing of two magmas




Assimilation of country rock(s)




SHAPES OF INTRUSIVE BODIES


Tabular - flows along cracks - low viscosity



Recall that the higher temperature melts 
tend to have lower viscosities



Massive - melt does not easily flow .. 
produces ³blobs² - stocks or batholiths




Recall that lower temperature melts tend to 
have high viscosities



The presence of water will reduce viscosity













SHAPES OF EXTRUSIVE FlOWS



AA - blocky flow - high viscosity




Pahoehoe - smooth, fluid - low viscosity



The higher the viscosity, the more likely that 
the flow will be explosive - Strato Volcanoes





The lower the viscosity, the more likely that 
the flow will be ³gentle² - Shield Volcanoes




Volcanic Deposits



Flood Basalts - low viscosity



Pahoehoe (smooth, ropy)


Aa (blocky)


Pillow Lavas





Pyroclastics - ³fire broken² 

Eruptive Sytles and Landforms	


Central Euptions


Shield Volcanoes



Volcanic Domes



Cinder Cones



Composite Vlocanoes



Craters



Calderas