1 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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How do we know if a rock is intrusive or extrusive?mtweb.mtsu.edu/cribb/1030igneousrocks.pdf · volcanic glass) 3 Glassy Texture Glassy, Vesicular Texture Vesicular Texture Fragmental
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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
• 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