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Igneous Rocks Igneous Rocks

Why Study Igneous Rocks? Why study igneous rocks?

• All igneous rocks form by cooling and crystallization of molten rock. Molten rock forms by melting of rocks in Earth’s crust and mantle. So – the study of igneous rocks helps us understand:– The mineralogical/chemical composition of Earth’s

interior– Processes involved in the formation of magmas– How magmas change as they rise toward Earth’s

surface– Internal processes of volcanoes from which lavas are

erupted

Important Definitions

• Magma – molten rock beneath Earth’s surface

• Lava – molten rock above Earth’s surface• Extrusive (Volcanic) Rock – forms when

lava solidifies on Earth’s surface– Pyroclastic Rock – extrusive rock made of

material explosively ejected from a volcano

• Intrusive (Plutonic) Rock – forms when magma solidifies beneath Earth’s surface

How do we know if a rock is intrusive or extrusive?

• Rock Texture – a description of the size, shape and arrangement of the mineral grains making up the rock– Intrusive Rocks – are typically coarse grained– Extrusive Rocks – are typically fine grained

• Pyroclastic Rocks – are typically made of volcanic glass and/or pieces of pre-existing rocks

• So – grain size is an excellent clue!

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Terms Describing Rock Texture

• Phaneritic – all grains can be seen with the naked eye– Results from slow cooling, slow crystallization

(intrusive rocks)

• Aphanitic - all grains cannot be seen with the naked eye (need hand lens, microscope, etc.)– Results from fast cooling, fast crystallization

(extrusive rocks)

Phaneritic Texture

Aphanitic TextureTerms Describing Rock Texture

• Porphyritic – two distinctly different crystal sizes are present (reflecting two distinctly different cooling/crystallization rates)– Phenocrysts – large crystals (slow

cooling/crystallization)– Groundmass – microscopic crystals (fast

cooling/crystallization)

• Question: What can be learned from an igneous rock with an aphanitic, porphyritic texture?

Porphyritic TextureTerms Describing Rock Texture

• Glassy – no crystals form due to very rapid cooling (forms volcanic glass)

• Vesicular – abundant holes (vesicles), formed by expanding gas bubbles in lava

• Fragmental – consists of fragments of pre-existing rock (typical of pyroclastic rocks) and volcanic ash (microscopic pieces of volcanic glass)

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Glassy Texture Glassy, Vesicular Texture

Vesicular TextureFragmental Texture

Igneous Rock Classification

• The classification of igneous rocks is largely based upon:

–Whether the rock is intrusive or extrusive• If extrusive, is the rock crystallized

lava or a pyroclastic rock?

–Chemical and mineralogical composition

Classification of Igneous Rocks

High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na

Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca

Low Temperature >>>>>>>>>>>> High Temperature

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Mafic Rocks

Extrusive Intrusive

Classification of Igneous Rocks

High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na

Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca

Low Temperature >>>>>>>>>>>> High Temperature

Intermediate Rocks

Andesite Diorite

Extrusive Intrusive

Classification of Igneous Rocks

High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na

Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca

Low Temperature >>>>>>>>>>>> High Temperature

Felsic Rocks

Rhyolite Granite

Extrusive Intrusive

Classification of Igneous Rocks

High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na

Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca

Low Temperature >>>>>>>>>>>> High Temperature

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Obsidian Pumice

Important Question

• If all igneous rocks form by cooling and crystallization of magma/lava, then:

–Why do igneous rocks exhibit such a wide range of mineralogical/chemical compositions?

–Why do igneous rocks exhibit such a wide range of colors?

Bowen’s Reaction Series

• Norman Bowen (early 20th century) –performed laboratory experiments to simulate crystallization of a mafic magma over a range of temperatures.

• Bowen’s objective: to reproduce the naturally occurring results of the cooling and crystallization of mafic magma.

• Results are summarized by ‘Bowen’s Reaction Series’

Bowen’s Reaction SeriesAs mafic magmas undergo progressive cooling

and crystallization, more Si-rich magmas are produced– There is a regular sequence of silicate mineral

crystallization• Highest temperatures – crystallization of

minerals common to mafic rocks• Lowest temperatures – crystallization of

minerals common to felsic rocks– Mineral crystals undergo chemical reactions

with the surrounding melt to produce the next lower temperature mineral in the crystallization sequence

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Discontinuous vs. Continuous ?

• Discontinuous Series – minerals have differing arrangement of Si-O tetrahedra– Degree of O sharing among tetrahedra

increases with decreasing temperature• Highest temperature – independent tetrahedra (no

O sharing)• Lowest temperature – framework silicates (all 4 O

shared)

Discontinuous vs. Continuous ?

• Continuous Series – All minerals are members of the plagioclase (Ca, Na) feldspar group– All framework silicates

– Highest temperatures – Ca-rich plagioclase (anorthite)

– Lowest temperatures – Na-rich plagioclase (albite)

Importance of Bowen’s Reaction Series

• Explains how rocks with a variety of mineralogical/chemical compositions can evolve from a single magma.– Fractionational Crystalization – the

progressive change in the chemical/mineralogical composition of a magma as the crystallization series proceeds

– Crystal settling and volcanic eruption may stop the crystallization sequence before it ‘completes’

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Importance of Bowen’s Reaction Series

• Allows interpretation of an igneous rock’s crystallization temperature based upon its mineralogical composition– Highest temperature crystallization produces

rocks ultramafic in composition

– Lowest temperature crstyallization produces rocks felsic in composition

Classification of Igneous Rocks

High Si, K, Na <<<<<<<<<<<<<<<<<<<< Low Si, K, Na

Low Fe, Mg, Ca >>>>>>>>>>>>>>>>High Fe, Mg, Ca

Low Temperature >>>>>>>>>>>> High Temperature