The Rock Cycle
• the evolution of Earth Materials

• interaction of internal and surface processes

• - Rock Types

• Igneous

• Sedimentary

• Metamorphic

IGNEOUS ROCKS
•  Igneous Rocks = Earth’s “new” rocks
 
= from magma (molten rock) => crystalline

=> the result of Earth’s Internal Processes

Origin of Magma
 
=> What causes melting?

 1. Geothermal Gradient (30°C/km)
 => from radioactive decay -> Earth’s Internal Heat Engine.....

Heat from Earth’s Internal Engine can melt materials in two ways

1. Heat brought up towards the surface
•  Convection => divergent boundaries & hot spots
• - pressure relief melting
•  melt upper mantle rocks (partial melting)

2. Take the rocks closer to the Earth’s  Heat Engine

•  Subduction  => convergent boundaries
•  Melting of oceanic crust (partial melting)

Other Heat =>  Friction

Crystallization of Magma

=> how does crystallization occur?

=> 2 factors important…

composition of melt -> affects sequence of crystallization = Composition of rock

(2) cooling history -> affects  texture....

Volcanic vs. plutonic

Sequence of Crystallization
(a) compositions affects melting; also affects crystallization
(b) occurs over a range of T’s :  600 - 1200°C

(c) relationship modeled by Bowen’s Reaction Series

•  order in which silicate minerals crystallize as cooling occurs.…
•  Mafic, Intermediate, & Felsic

Bowen’s Reaction Series

•   3 parts to diagram

a. continuous reaction series => plagioclase (Ca to Na)
    => a solid solution series

b. discontinuous reaction series => mafic silicates (Fe&Mg)

c. silica-rich residual series => melt dominated by silica

• as T goes down, melt depleted in Mg&Fe, but enriched in Si....…
•  where series starts and ends - determine rock type…

-> BRS also illustrates mineral stability at surface P&T
 

2. Texture of Igneous Rocks => depends on rate of cooling

  => texture = crystal size...........

(a.) slow cooling -> deep within crust

•  crystals coarse (visible) =  phaneritic texture

•  rocks = intrusive or plutonic
 

(b.) fast cooling -> at surface  (in air or water)

• fine crystals (invisible) =  aphanitic texture

• rocks = extrusive or volcanic
 

Other Textures
- Very Rapid Cooling

 = Glassy (obsidian)
• mixed cooling history ->
• slow at depth, then fast
• near surface.......
• => coarse crystals  = (phenocrysts)

 => porphyritic texture..........

Classification of Igneous Rocks
•  based on composition and texture

• these characteristics can be used to infer the history of the rock = genetic classification...
Classification of Igneous Rocks
• rocks listed in pairs => same composition but different texture

Important Properties

 1. Texture

 2. Composition
 

 - clues about

  - origins

  - processes