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Journey to Center of Earth Rocks at the Earth Surface Igneous Rocks

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Journey to Center of Earth Rocks at the Earth Surface Igneous Rocks Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Journey to Center of Earth Rocks at the Earth Surface • Types of rocks Garrison Fig. 3.5 p. 59 • Igneous: that make up the – Cooled directly from liquid rock (magma) Earth • Sedimentary • Internal layers of – Formed from small particles (sediments) the Earth that sink to the sea floor • Ocean • Compressed to form rock (lithified) sediments • Metamorphic – Igneous or sedimentary rocks altered in shape and chemistry by pressure, heat, and bending 1 2 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Igneous Rocks What Does Color Tell Us? • Liquid rock is called: • Density – Magma below the Earth’s surface – g/cm3 (water = 1) – Lava on the Earth’s surface – Denser rocks are darker in color • Classified by two properties • Chemical composition – Color – Inclusion of heavier elements makes rock • Indicates density denser – Size of crystal grains – Heavier elements are darker in color • Indicates where it formed • Online study guide to igneous rocks: – http://volcano.und.edu/vwdocs/vwlessons/lessons/Igrocks/Igrocks1.html 3 4 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Color, Density, & Chemistry Color, Density, & Chemistry • Dense and dark color • A denser, darker – More heavy, dark-colored elements: iron rock: Basalt (Fe) and manganese (Mn) – Referred to as “Basic,” “Sima” and “Mafic.” • Less dense and lighter color • A less dense, lighter – More light & light-colored elements: Si, rock: Granite aluminum (Al), and magnesium (Mg) – Referred to as “Acidic,” “Sial” and “Felsic.” • Both are mostly silicon (Si) 5 6 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Grain Size Grain Size • Reveals where rock cooled from magma • Fine grains = invisible to naked eye • Coarse grains = visible to naked eye – Rapid cooling—no time for crystals to grow – Slow cooling—time for large crystals to – At Earth grow surface – Deep in Earth – Exposed to – Insulated from air or water air & water – Extrusive atop – Intrusive into existing rock existing rock • Basalt layers • Granite 7 8 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Pillow Basalt Pillow Basalt Examples of Basalt Fine-Grained “Felsic” Rock • Lava flow under water • Lava with same composition as granite • Rapid cooling exploded into air • Light color • Very rapid cooling • Frozen gas bubbles • Pumice Sheet flow “Pillow lava” 9 10 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington The Rock Families The “Hybrid” Rock Family • “Granite family” • Mixtures of Granite & Basalt families – Granite = coarse; Rhyolite = fine – Basaltic magma erupting through granitic – Density = 2.8 g/cm3 rock • “Basalt family” – Intermediate in density and color – Basalt is fine; Gabbro = coarse – Occurs under specific geologic conditions – Density = 3.0 g/cm3 • Andesite—fine-grained • Very dense “Ultramafic”/“Ultrabasic” – Mt. St. Helens rocks – Explosive combination – Olivine, pyroxene below Earth’s surface • Diorite—coarse-grained 11 12 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Categories of Igneous Rocks Internal Structure of Earth Continental Intermediate & Mixtures Oceanic Crust: • How do we Crust: Mafic, Sima Felsic, Sial (Silicon- know what (Silicon- Magnesium) Aluminum) is inside the Density ! 2.8 ! 2.9 ! 3.0 Earth? Silica (SiO2) content 70-78% 62-70% 54-62% 45-54% Color Lighter............................................................................Darker Coarse-grained Granite Granodiorite Diorite Gabbro (intrusive, slow subsurface cooling) Fine-grained Rhyolite Dacite Andesite Basalt (extrusive, rapid surface cooling) Figure 2.3 13 14 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Probing the Earth’s Interior Probing the Earth’s Interior • Seismic waves generated by • P=primary (push- earthquakes pull or pressure) – P=primary (push-pull or pressure) waves waves refract with • Travel through solid and liquid but bend when changes in density passing through changes in density • S=secondary S=secondary (snake) waves – (snake) waves • Cannot pass through liquid pass only through – Both waves travel slowly through soft (warm) layers & faster through hard (cold) solid layers layers “Shadow zones” reveal inner layers 15 16 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington The Three-layered Earth The Three-layered Earth • If Earth was a • Seismic waves candy, what and other data would it be? tell us: – Brittle candy – Crust shell • 2 Sublayers – Toffee middle – Mantle layer • 3 sublayers – Liquid inner – Core layer • 2 sublayers – Hard candy or nut center Figure 2.3 Figure 2.3 17 18 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington The 2-Layered Core The 3-layered Mantle • Inner (solid) core • Upper (rigid) mantle – Density 13 g/cm3 attached to crust – Density of liquid mercury • Middle (plastic, is 13 ductile or “putty- • Outer (liquid) core like”) – Density 11 g/cm3 asthenosphere – Density of solid lead is • Lower (solid) mantle 11.5 (Mesosphere) • Composed mainly of iron • Mg-Fe silicates (Fe) & nickel (Ni) (“ultramafic”) 19 20 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Layers of the Earth's Interior The Lithosphere Layer Depth (km) State Composition Density (g/cm3) • Solid outer layer Lithosphere Crust 0-10, 0-65 Solid – Crust + Upper Continental 0-65 Solid Al silicates 2.7 mantle Oceanic 0-10 Solid Mg silicates 3.0 • Moho is Moho boundary Mantle 10/65-2900 Mg-Fe silicates (4.5) between upper Upper 10/65-70/150 Solid Mg-Fe silicates 3.2 Asthenosphere 70/150-250/700 Ductile Mg-Fe silicates 3.4 mantle and crust Mesosphere 250/700-2900 Solid Mg-Fe silicates 5.6 – Abrupt change in Core 2900-6370 Fe & Ni density & Outer 2900-5300 Liquid Fe & Ni 11.5 chemical composition 21 Inner 5300-6370 Solid Fe & Ni 13.0 22 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington The 2-layered Crust The 2-layered Crust • Continental crust • Oceanic crust – Light-colored, less-dense (2.7) – Dark-colored, denser (3.0) – Granite-family rocks (Sial) – Basalt-family rocks (Sima) – Relatively thick (up to 65 km) – Relatively thin (10 km) 23 24 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington The 2-layered Crust The process of Isostasy • Continental crust extends underwater • A form of buoyancy – Includes continental margin • The depth at which an object floats on a – Rise or trench is geologic boundary with denser fluid oceanic crust – I.e. wood on water, iceberg on the ocean – But chunks of oceanic crust found on “Ice-ostasy”: like an iceberg continents • – Most of a floating object is below the fluid surface. – Removing or adding weight causes the object to float higher or lower. 25 26 Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington Oceanography 101, Richard Strickland Lecture 3 © 2006 University of Washington The process of Isostasy The process of Isostasy • Added load on crust causes it to sink • The higher a • New volcano sinks over time as the lithosphere feature rises above gradually subsides atop asthenosphere the Earth’s surface, the lower its “roots” will extend below the Earth’s surface. Figure 2.6 – Continental crust is thickest & deepest where it is highest above the surface 27 – Like an iceberg 28.
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