1. The Rock Cycle & Igneous Rocks November 11, 2008
Copy of this lecture will be found at:
With Some Graphics from Press et al.,
Understanding Earth, 4th Ed.
(Copyright © 2004 by W. H. Freeman & Company)‏

2. The Rock Cycle Melting & Intrusion Solidification of melt
Mountain Building
Uplift & Exposure
Weathering
Erosion & Transport
Deposition & Burial
Metamorphism
Fig 4.9

3. Genetic Classification of Igneous Rocks
Intrusive: crystallized from slowly cooling magma intruded within the Earth’s crust; e.g. granite, gabbro.
Granite
Gabbro

4. Course-grained, interlocking
Rock Texture
Intrusive:
Course-grained, interlocking
Fig. 5.1

5. Genetic Classification of Igneous Rocks
Extrusive: crystallized from rapidly cooling magma extruded on the surface of the Earth as lava, …
Rhyolite
Basalt

6. Rock Texture Intrusive: Course-grained, interlocking Extrusive:
Fine-grained, discrete crystals, often glassy
Fig. 5.1

7. Genetic Classification of Igneous Rocks
Extrusive: … or erupted as pyroclastic material, i.e., fragmented pieces of magma ejected and cooled in the air.
Ash
Pumice
Scoria

8. Formation of Igneous Rocks
Pyroclasts
Extrusive
Intrusive
Porphyry:
partially
crystalline
Fig. 5.3

9. Process of Intrusion
Intrusive rocks fracture and cross-cut the “country rock”.
Igneous rocks cool quickly near the intrusive contacts, and cause contact metamorphism.
Fig. 5.2

10. Composition and Classification of Igneous Rocks
Chemistry: e.g. % SiO2
Mineralogy: e.g.
Felsic - high silica
Intermediate - intermediate silica
Mafic - low silica
Ultramafic - very low silica
(continental crust)‏
(oceanic crust)‏
(mantle)‏

11. Felsic Igneous Rocks: Igneous rocks rich in minerals high in silica and low in iron and magnesium. They include:
Granite
Rhyolite

12. Mafic Igneous Rocks: Igneous rocks rich in minerals low in silica and high in iron and magnesium. They include:
Gabbro
Basalt

13. Common Minerals

14. Intermediate Igneous Rocks: Igneous rocks intermediate in composition between felsic and mafic igneous rocks. They include:
Granodiorite (Dacite)‏
Diorite (Andesite)‏

15. Ultramafic Igneous Rocks: Igneous rocks with very low silica content, consisting dominantly of mafic minerals. The most common ultramafic rock is:
Peridotite (no extrusive equivalent)‏

16. Fig. 5.4

17. Compositional Classification
Granite
Compositional Classification
Quartz
Orthoclase
Biotite
Granite
Plagioclase

18. Compositional Classification
Granite
Compositional Classification
Granodiorite
Quartz
Granite
Amphibole
Plagioclase

19. Compositional Classification
Granite
Compositional Classification
Granodiorite
Diorite
Granite
Plagioclase
Amphibole

20. Compositional Classification
Granite
Compositional Classification
Granodiorite
Diorite
Granite
Gabbro
Plagioclase
Pyroxene

21. Compositional Classification
Granite
Compositional Classification
Granodiorite
Diorite
Granite
Gabbro
Pyroxene
Olivine
Peridotite

22. Felsic Intermediate Mafic Table. 5.2 Granite Granodiorite Diorite
Gabbro
Rhyolite
Dacite
Andesite
Basalt
Viscosity
Melting Temperature

23. When rocks melt (or partially melt).
How do magmas form?
When rocks melt (or partially melt).
When do rocks melt?
When the temperature exceeds the melting point of the rock or some minerals within the rock.

24. Factors that Affect Melting of Minerals (and Rocks)
Composition: Felsic minerals melt at lower temperatures than mafic minerals
Pressure: Increased pressures raises melting points
Water Content: Increased water content lowers melting points

25. The Formation of Magma Chambers
Magma pools in
magma chamber
Magma rises
Buoyant melt migrates through rock pores and fractures.
Less dense magma
Melt is less dense than solid. Low density minerals tend to melt first.
Partial melting
Some minerals melt before others. Results in mixture of melt and solid.

26. Covers 15,400 mi2 !!
Composition
Granite
Monzonite
Granodiorite
Diorite
Even Gneiss
All in one “magma chamber”

27. e.g., granite, granodiorite, diorite
Why do magmas have such different compositions - even when derived from a single “parent magma”
e.g., granite, granodiorite, diorite

28. Magma Differentiation
The process by which rocks of various compositions can arise from a uniform parent magma
Occurs because different minerals crystallize at different temperatures
(i.e., the opposite of partial melting)
Tune in Wednesday for the exciting conclusion!