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ESCI 101: LectureESCI 101: LectureThe Rock Cycle The Rock Cycle & Igneous Rocks& Igneous Rocks
February 23, 2007February 23, 2007
Copy of this lecture will be found at:http://www.owlnet.rice.edu/~esci101
alternate pathway of the rock cycle: (1) igneous rocks are formed, (2) become metamorphosed, (3) exposed, and (4) evolve into sandstone. Describe the main processes and events that might take place at each step along the way, and the plate tectonic settings in which they are likely to occur. (Your answer should not exceed one page.)
- See the accompanying plot on the following page.
Pop-HW#4Pop-HW#4 An An
Alternate Alternate Rock Rock CycleCycle
-Melting & Intrusion-Solidification of melt-???-???-???-???-???--> Sandstone
Fig 4.9
X
XX
(a) The Rock (a) The Rock CycleCycle
Fig 4.9
E.g., Convergent
Plate Boundary-Subducting
slab-Mantle melting-Bouyant rise
of melt
(b) The Rock (b) The Rock CycleCycle
Fig 4.9
E.g., Convergent Plate Boundary- Solidification
of melt- Volcanic activity
Intrusive rocks
Extrusive
rocks
Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks
Granite Gabbro
• IntrusiveIntrusive: crystallized from slowly cooling magma intruded within the Earth’s crust; e.g. granite, gabbro.
Rock Rock TexturTextur
eeFig. 5.1
Intrusive: Course-grained,
interlocking
Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks
Rhyolite Basalt
• ExtrusiveExtrusive: crystallized from rapidly cooling magma extruded on the surface of the Earth as lava, …
Rock Rock TexturTextur
eeFig. 5.1
Intrusive: Course-grained,
interlocking
Extrusive: Fine-grained,
discrete crystals, often
glassy
Genetic Classification of Genetic Classification of Igneous RocksIgneous Rocks• ExtrusiveExtrusive: … or erupted as
pyroclastic material, i.e., fragmented pieces of magma ejected and cooled in the air.
Pumice
Scoria
Ash
Formation of Igneous Formation of Igneous RocksRocks
Pyroclasts
Extrusive
Intrusive
Porphyry: partially crystalline
Fig. 5.3
Process of Process of IntrusionIntrusion• Intrusive rocksIntrusive rocks fracture and cross-
cut the “country rock”. • Igneous rocks cool quickly near the intrusive contacts, and cause contact metamorphism.
Fig. 5.2
Composition and Composition and Classification of Classification of Igneous RocksIgneous 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)
Felsic Igneous Rocks:Felsic Igneous Rocks:
Igneous rocks rich in mineralsIgneous rocks rich in minerals highhigh in silicain silica andand low inlow in iron and magnesium. They iron and magnesium. They
include:include:
GraniteGranite RhyoliteRhyolite
Mafic Igneous Rocks:Mafic Igneous Rocks:
Igneous rocks rich in mineralsIgneous rocks rich in minerals lowlow in silicain silica andand high inhigh in iron and magnesium. They iron and magnesium. They
• Water ContentWater Content: Increased water content lowers melting points
The Formation The Formation of Magma of Magma ChambersChambers
Partial meltingPartial melting
Less dense magmaLess dense magma
Magma risesMagma rises
Magma pools in Magma pools in magma chambermagma chamber
Some minerals melt before others. Results in mixture of melt and solid.
Melt is less dense than solid. Low density minerals tend to melt first.
Buoyant melt migrates through rock pores and fractures.
- Covers 15,400 mi2 !! - Composition
- Granite- Monzonite- Granodiorite- Diorite- Even Gneiss
- All in one “magma chamber”
Why do magmas have such Why do magmas have such different compositions different compositions - - even when derived from a even when derived from a single “parent magma”single “parent magma”
e.g., granite, granodiorite, diorite
Magma DifferentiationMagma 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