Intrusives
How do oceans and continents differ? Chapter 4 The average ocean depth is 3000m.
If the Earth’s surface is 30% land and 70% ocean, why is IGNEOUS ROCKS there any land at all?
Why are there not just a few little islands?
How do oceans and continents How do oceans and continents differ? differ?
How do oceans and continents How do oceans and continents differ? differ?
Ocean crust is thin, dense, and young.
Continent crust is thick, light, and old.
1 Polymerization of Silica • Each Si atom is surrounded by 4 oxygens. • Polymerization is the degree to which the oxygens are shared with other Si atoms. • Polymerization is the number of Si-O-Si bonds per tetrahedron.
– Olivine has none: isolated SiO4 groups
– Pyroxene has 50% shared: SiO3 chains
– Mica has has 75% shared: Si4O10 sheets
– Quartz and feldspar have 100% (Si,Al)O2 framework.
Polymerization of Silica Polymerization of Silica Pyroxene
Tetrahedron MgSiO3 Olivine (Mg2SiO4) Tetrahedron
Polymerization of Silica MAGMA • Magma is the term for any molten Mica (Sheet) Quartz (Framework) silicate material, whether below the surface or on top. • Volcanic rocks are erupted on the surface. – Volcanic rocks are fine-grained (<1 mm) • Intrusive igneous rocks crystallize from magma below the surface. – Intrusive igneous rocks are coarse- grained (> 1mm) – Pegmatites are very coarse-grained (> 1cm)
2 Igneous Rocks: Intrusive Igneous Rocks Learning Goals • Composition • What does ‘igneous’ mean? • Mineralogy • Composition • Geologic Setting • Mineralogy – Mantle – Mantle – Oceanic Crust – Oceanic Crust – Continental Crust – Continental Crust • Igneous Fractionation • Igneous Fractionation – How the chemistry evolves – How the chemistry evolves
Igneous Rock Compositions Igneous Rock Compositions
• Rock compositions are described • Igneous rocks vary in composition
in weight percents of oxides: (SiO2 content)
• SiO2, MgO, FeO, Al2O3, etc – ultramafic (~40 wt%) (peridotite) • The principal variation in igneous – mafic (45-55%) (gabbro / basalt) – intermediate (55-65%) (diorite/andesite) rock compositions is silica (SiO2) content. – silicic (65-75 wt %) (rhyolite/granite) • The degree of polymerization of • The mantle is peridotite (ultramafic). silica increases with silica content. • The ocean crust is gabbro (mafic). (in both crystals and melt). • The continents are granite (silicic)
Igneous Rock Names Igneous Rock Mineralogy and Compositions • Peridotite (Ultramafic) • Gabbro (Mafic) • Composition • Intrusive • Volcanic Name Rock Name Rock Name – Olivine ((Mg,Fe)2SiO4) – Feldspar – Pyroxene (CaAl2Si2O8) • Ultramafic • Peridotite • (Komatiite) ((Mg,Fe,Ca)SiO3) – Pyroxene •Mafic • Gabbro • Basalt – Garnet (Mg3Al2Si3O12) or ((Mg,Fe,Ca)SiO3) – Spinel (MgAl O ) • Intermediate • Diorite • Andesite 2 4 – Olivine • Silicic (felsic) • Granite • Rhyolite ((Mg,Fe)2SiO4)
3 Igneous Rock Mineralogy Igneous Rock Names and Compositions • Diorite (Intermediate) • Granite (Silicic) • Composition • Intrusive • Volcanic – Feldspar (CaAl2Si2O8) – Quartz (SiO2) Name – Feldspar (NaAlSi3O8) – Feldspar (NaAlSi3O8) Rock Name Rock Name
– Pyroxene ((Mg,Fe,Ca)SiO3) – Feldspar (KAlSi3O8) • Ultramafic • Peridotite • (Komatiite) – Mica – Mica (biotite) •Mafic • Gabbro • Basalt K(Mg,Fe,) (Si,Al) O (OH) (K(Mg,Fe,) (Si,Al) O (OH) 3 2 10 2 3 4 10 2 • Intermediate • Diorite • Andesite – Mica (muscovite) • Silicic (felsic) • Granite • Rhyolite – (KAl2(Si,Al)4O10(OH)2
Geologic Settings: Mantle Geologic Settings Mid-ocean ridges Oceanic Islands Island arcs Continental • Compositions • Where? • Ultramafic • Mantle • Mafic • Oceanic • Intermediate • Subduction Zones • Silicic (felsic) • Continental Mass
There appear to be 13 major plates that cover the globe. Igneous Fractionation: Peridotite-basalt-granite • Partial Melting – Rocks have a melting interval of several hundred degrees C. – First melt is rich in silica, water and trace elements. • Fractional Crystallization – First-formed crystals low in silica settle to the bottom.
4 Peridotite Peridotite Partial Melting
Peridotite Partial Melting First melt at 3-mineral junctions
Garnet
Orthopyroxene Olivine
Clinopyroxene
Melt composition is different Melt is less dense from host
5 Melt flows when pockets touch Peridotite + Basalt Melt
Igneous Fractionation:
6 Layered Mafic Intrusion Granite Outcrop
Dikes: Intrusive forms Tabular discordant
• Pluton: Any large discordant intrusion at depth – Batholith: large > 100 km2 – Stock: small < 100 km2 • Dike: Tabular non-conformable • Sill: Tabular conformable
Dikes: Sills: Tabular discordant Tabular conformable
7 Sills: Tabular conformable
How do oceans and continents How do oceans and continents differ? differ? • Oceanic crust is: – Thin (~7 km), – Dense (~3.1g/cm3)
– Mafic (45-50% SiO2) – Young (< 200 my) • Continental Crust is: – Thick (>30 km), – Light (~2.7 g/cm3)
– Silicic (> 60% SiO2) – Old (> 1000 my)
How do oceans and continents Terms differ? • Polymerization • Silicic • Difference is result of partial • Magma • Peridotite melting and fractional • Pegmatitie • Gabbro crystallization. • Igneous • Diorite Fractionation • Granite – Mid ocean ridge makes basalt from • Partial melting • Pluton peridotite. • Fractional • Stock Crystallization – Partial melting at subduction zones • Batholith makes andesite from basalt. • Ultramafic • Dike • Mafic • Sill • Intermediate
8 Assignment Clicker Question • What is the most likely •Grotzinger Chapter 5 geologic setting in which conglomerate was deposited? •Sedimentary Rocks – A. Continental alluvial fans – B. Continental deserts and beaches – C. Shallow-water marine – D. Deep-water marine – E. Reefs
Clicker Question Clicker Question • What is the most likely • What is the most likely geologic setting in which geologic setting in which sandstone was deposited? shale was deposited? – A. Continental alluvial fans – A. Continental alluvial fans – B. Continental deserts and beaches – B. Continental deserts and beaches – C. Shallow-water marine – C. Shallow-water marine – D. Deep-water marine – D. Deep-water marine – E. Reefs – E. Reefs (Tropical Shallow Marine)
Crust and Mantle Clicker Question 1 Lithosphere and Asthenosphere • The term for any molten silicate material on or below the Earth’s surface is:
– A. Granite – B. Basalt – C. Magma – D. Pegmatite – E. Lava
9 Clicker Question 1 Clicker Question 2 • The term for any molten silicate • The most abundant element in material on or below the Earth’s the Earth is: surface is: – A. Hydrogen – A. Granite – B. Oxygen – B. Basalt – C. Magnesium –C. Magma – D. Silicon – D. Pegmatite – E. Iron – E. Lava
Clicker Question 2 Clicker Question 3 • The most abundant element in • The composition of the mantle is the Earth is: said to be:
– A. Hydrogen – A. Ultramafic –B. Oxygen – B. Mafic – C. Magnesium – C. Intermediate – D. Silicon – D. Silicic – E. Iron – E. Sedimentary
Clicker Question 3 • The composition of the mantle is Clicker Question said to be: • Any small or large discordant intrusion at depth is known as a –A. Ultramafic – A. Pluton – B. Mafic – B. Batholith – C. Intermediate – C. Stock: – D. Silicic – D. Dike – E. Sedimentary – E. Sill
10 Clicker Question Clicker Question
• Any small or large discordant • A small conformable intrusion intrusion at depth is known as a between and parallel to – A. Pluton sedimentary layers is known as a – B. Batholith – A. Pluton – C. Stock: – B. Batholith – D. Dike – C. Stock: – E. Sill – D. Dike – E. Sill
Clicker Question Clicker Question
• A small conformable intrusion • A small discordant intrusion that between and parallel to cuts across sedimentary layers is sedimentary layers is known as a known as a – A. Pluton – A. Pluton – B. Batholith – B. Batholith – C. Stock: – C. Stock: – D. Dike – D. Dike –E. Sill – E. Sill
Clicker Question
• A small discordant intrusion that cuts across sedimentary layers is known as a – A. Pluton – B. Batholith – C. Stock: –D. Dike – E. Sill
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