Welcome to GLG101C Introduction to Geology Fall 2004 Professor James Tyburczy
Department of Geological Sciences
	Class Notes Chapter 5

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Chapter 5. IGNEOUS ROCKS: SOLIDS FROM MELTS

Formed from the cooling and solidification of magma - molten silicate, high temperature (1200 deg C)

Classification of igneous rocks:

• Textures: fine-grained, coarse-grained, mixed grain sizes, glassy, vesicular, fragmental (textures tell us whether rock cooled at the Earth's surface (extrusive) or cooled slowly in the Earth's interior (intrusive))
• Composition: felsic (rich in SiO2), intermediate, mafic (Mg and Fe rich), ultramafic (very Fe and Mg rich)

How do we know about cooling rates of rocks?

• Observations of lava flows
• Laboratory experiments
• Field studies of ancient intrusions - chilled fine-grained margins, coarse grained interiors, baked zones surrounding

Intrusive rocks - Evidence for crystallization below Earth's surface

• Mineralogy same as extrusive rocks, but minerals show evidence (in details of chemical composition) of crystallizing at high P and T
• Igneous contacts (chilled margin, baked zones) and structures
• Xenoliths

Chemical classification of igneous rocks - (see Table 5.2 and Figure 5.4) - chemical composition ranges (SiO2 content 70 % for granite, 50% for basalt), characteristic minerals, plagioclase feldspar composition, temperature, viscosity, intrusive and extrusive rock names

• Peridotite, Gabbro, Diorite, Granite, Basalt, Andesite, Rhyolite, Pegmatite

Origin of magma

• Partial melting of mantle rocks-> Separation -> Transport -> Emplacement/Eruption
  • Melting temperature - increases with depth, is lower with water present, is lower for more SiO2-rich rocks
    
  1. Geothermal gradient - temperature is higher deeper in the Earth
  2. Effects of water in source region - water lowers melting T of rocks
  3. Bring material from deep Earth up to shallower level (i.e. by mantle flow)
  4. Partial Melting - melt has different composition than whole rock that melts

Magmatic Differentiation - why do magmas have such a wide range of compositions?

• Tectonic setting - magmas in continental settings range from mafic to felsic, but magmas in oceanic settings tend to be mostly mafic
• Magma chamber processes - crystal settling, magma mixing, ... can change magma composition

Controls on types of igneous rocks

• Plate tectonic settings, depth of crystallization, cooling history, magma chamber history (sinking of crystals,...) eruptive history

Intrusive bodies - large or small, characteristic shape, deep or shallow, structural relation to surrounding rocks - dike, sill, pluton, batholith

Other rock textures:

• Pegmatite - very coarse-grained - slow cooling with a lot of water present
• Mixed grain sizes (Porphyry) - two grain sizes - large & small - indicates two-stage cooling history

Global Distribution of Plutonic Rocks

• Granites - underlay cores of continents
• Basalt, gabbro - oceanic crust
• Andesite - most young volcanic mountain chains (subduction zones), especially those on continents
• Ultramafic rocks (peridotite) - the mantle

Controls on types of igneous rocks - plate tectonic settings - divergent margins, convergent margins, intra-plate (hot spots)

• Plate tectonic settings, depth of crystallization, cooling history, magma chamber history (sinking of crystals,...), eruptive history
• Basalts, ultramafic rocks - form at Mid-ocean ridges (spreading centers) from melting of mantle rock - ophiolite sequence - represents a section of the oceanic lithosphere
• Andesites - form in subduction zones - from melting in the presence of water of subducted basaltic crust and/or upper mantle above the subducted plate
• Granites - form above subducted plate by partial melting of lower continental crust