Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 4 Igneous Rocks: Solids from Melts

Igneous Rocks

Igneous rocks Formed from the cooling and consolidation of magma plutonic (intrusive) — cooled below the surface volcanic (extrusive) — cooled on the surface

Coarsely Crystalline Granite Fig. 4.1

Photomicrograph of Granite Fig. 4.1

Finely Crystalline Basalt Fig. 4.1

Photomicrograph of Basalt Fig. 4.1

Igneous textures Glassyno minerals present Crystallinerocks made of mineral grains Porphyriticmixture of coarse and fine Vesicularwith bubble holes

Composition of melts affects behavior while still fluid More SiO 2 will increase viscosity, making strong temporary bonds in magma.

Factors controlling the viscosity of magmas Composition: higher SiO 2 ; higher viscosity lower volatiles; higher viscosity Temperature: lower temperature; higher viscosity

Fig. 4.2 Granite (g) Intruding Metamorphic (m) Rocks g m

Magma Usually a silicate melt (liquid) at high temperatures (650 to 1200°C). Mixture of all the elements that make up minerals plus volatile components: H 2 O, CO 2, Cl, F, S These components form gases and will boil off when pressure is released.

Texture of Igneous Rocks Controlled by cooling rate Grain size Degree of crystallinity Vesicularity

Classification of Igneous Rocks Defined by texture: Fine-grained: extrusive or volcanic Coarse-grained: intrusive or plutonic

Pyroclastic Igneous Rocks Fig. 4.3 Obsidian Pumice Ash

Quartz-rich Felsic Porphyry Fig. 4.4

Classification by composition and texture ExtrusiveIntrusive basaltgabbro andesitediorite rhyolitegranite

Fig. 4.5 ExtrusiveIntrusive Basalt Gabbro RhyoliteGranite

Classification of Igneous Rocks Determined by composition (both chemical and mineralogical): magnesium (Mg) + iron (Fe) = mafic feldspar + quartz (Si) = felsic

Classification of Igneous Rocks When we talk about the chemical composition of a rock we usually speak in terms of the oxides, e.g., Typical basaltTypical granite SiO 2 50% 70% Al 2 O 3 15% 12% FeO+MgO 15% 3% CaO 8% 2% K 2 O+Na 2 O 5% 8%

Classification of Igneous Rocks Fig. 4.6

The process of complete melting of a rock or complete crystallization of a magma will not change the bulk composition of the system, but if either of these processes goes only part way, the composition of the solid and the liquid can be very different (especially for small ƒ).

Partial melting Opposite of fractional crystallization Last minerals to form will melt at lowest temperature Biggest changes will be for small degrees of melting

Factors Affecting Melting Temperatures Fig. 4.7

Where do magmas come from? BasaltsBasalts: Broadly speaking, we know that mantle rocks (45% SiO 2 ) partially melt (10 to 15%) to produce basalts (50% SiO 2 ). AndesiteWith the addition of some water, basalts will partially melt to produce Andesite (60% SiO 2 ).

Where do magmas come from? Granites Granites may also be produced by fractional crystallization of a basaltic magma. But this works only for a dry granite. However, most granites contain many hydrous phases (micas), and some granites are associated with extensive hydrothermal ore deposits.

Tectonic Settings of Igneous Activity Fig. 4.8

Volcanic Island Arc, Indonesia Fig. 4.8

Oceanic Hot Spot Hawaii Fig. 4.8

Continental Volcanic Arc N. Cascades Fig. 4.8

Volcanic Island Arc Java, Indonesia

Lava flow at Volcanoes National Park, Hawaii

Continental Volcanic Arc North Cascades, Washington

Fractional crystallization The modification of magma by crystallization and removal of mineral phases. Because only certain elements will go into a given mineral, this will tend to change the composition of the remaining liquid.

Fig. 4.9a Early Crystallization

Liquids Squeezed from Crystals Fig. 4.9b

The Palisades Sill Fig. 4.10

Crystallization Ideally, crystallization is the opposite of melting. In fact, the process is more complicated than that because rocks are complex aggregates of many minerals with different melting (crystallization) points.

Simple crystallization Example: Quartz When melt reaches the crystallization temperature of a mineral, the mineral forms and undergoes no further changes with subsequent cooling.

Continuous crystallization Example: Plagioclase feldspar When a mineral begins to crystallize it takes on a given composition but the composition of the crystallizing exterior (and therefore the entire crystal) changes due to changes in the composition of the magma.

Continuous Crystallization Plagioclase Feldspar

Discontinuous crystallization Examples: Olivine and Pyroxenes Crystals previously formed react with melt to produce new minerals.

Discontinuous crystallization Olivine  Pyroxene

Bowen’s reaction series Series of chemical reactions that take place in silicate magmas as they cool First investigated in the 1920s and 1930s by N. L. Bowen Important experiments that help us understand the evolution of magmas

Fig Bowen’s Reaction Series

Magma Differentiation Fig. 4.12

Forms of intrusive igneous masses Plutons can be divided into two groups: 1) Concordant 2) Discordant

Forms of intrusive igneous masses Concordant:Discordant: SillsDikes LaccolithsNecks

Forms of intrusive igneous masses Batholith: Any deep-seated pluton of coarse-grained rocks that has a surface exposure of over 100 sq. km that is mostly granitic. Examples include Sierra Nevada, Coast Range, Idaho batholiths. Stock: Same as a batholith, only smaller.

Fig Types of Igneous Structures

Fig Methods of Intruding Magma

Fig Sill

Fig Dike

Fig Pegmatite Dike

Magma Chamber Beneath Mid-ocean Spreading Ridge Fig. 4.18

Volcanism Due to Partial Melting in a Subduction Zone Fig. 4.19

Fig Mt. Rainier