Internal Structure of the Earth Lecture by Dr. Ken Galli, Boston College EESC Environmental Issues and Resources July 7, 2016 Please do not distribute beyond the EESC Class. Topic 3 A

Case History: Two Major CA Cities San Andreas fault: a transform plate boundary between the North American and the Pacific plates Two major cities on the opposite sides of the fault: Los Angeles and San Francisco Many major earthquakes related to the fault system Loss of many lives and billions of property damages due to earthquakes New construction and retrofitting of infrastructures has become more expensive When will be the next “big one” and what to do? How to deal with the potential consequence? North America Plate Pacific Plate

Internal Structure of Earth The Earth is layered and dynamic: Interior differentiation and concentric layers Chemical model by composition and density (heavy or light): Crust, mantle, core, and Moho discontinuity between the crust and mantle Physical property model (solid or liquid, weak or strong): Lithosphere (crust and upper rigid mantle), asthenosphere, mesosphere, liquid outer core, inner solid core

Theory of Plate Tectonics The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other Most (!) deformation of Earth’s crust occurs at plate boundaries

3 main layers defined by composition: Crust - Outer Mantle - Middle Core - Center Earth’s Layers

Study of Earth’s Interior Structure Knowledge primarily through the study of seismology Seismology: Study of earthquakes and seismic waves Examining the paths and speeds of seismic waves through reflection and refraction Magma likely generated in the asthenosphere Slabs of lithosphere have apparently sunk deep into the mantle Variability of lithosphere thickness reflects changes in its age and history

Composition - How Do We Know? Best Guess! Whole Earth Meteorites - Fe, Ni (same age as Earth) Information from velocities of seismic waves indicate material Crust (5-40 Km) Samples (mountain building helps!) Mantle (5/40 to 2885 Km) Kimberlite pipes - intrusive igneous rock from the mantle Lava / volcanic rock Mountain building Core (2885 to 6371 Km) Inference –Earth’s mean density = 5.5 g/cm 3 –Crust 2.5 to 3 g/cm 3 ; mantle 3.3 g/cm 3 to 5.5 g/cm 3 –Density of core at least 10 to 11 g/cm 3 (iron and nickel)

Seismic P Wave Primary or push-pull wave, travels like sound wave Direction of rock particle vibration parallel to that of wave propagation Fastest rates of propagation, first arrival to the seismograph Body wave travels through Earth interior and all media—solid and liquid

Seismic S-Wave Secondary or shear waves The direction of particle vibration perpendicular to that of propagation Propagates slower than P waves Body wave, propagating through Earth’s interior, but not its liquid layers

Seismic Waves and Internal Structures Earth’s interior boundaries: Sudden changes in the speed of seismic waves Different characteristics: Different rates and paths of wave propagation Asthenosphere: Low velocity zone, major source of Earth magma Outer Core: Liquid, no S wave transmits through it

Figure 2.2b Model of Earth’s Interior ————— Moho Crust Mantle

Crust Our deepest hole: 9 Kilometers ….. we have a long way to go! Two types of crust: –Continental 30% of crust Granites and Diorites - rich in silicates and feldspars (lighter materials) Km thick Oldest is 3.8 billion years (90% solar system age; missing ~700 m.y.) –Oceanic crust Basalt - Mg, Fe (heavier materials) 5-10 Km thick 200 Ma oldest; 100 Ma average

Mantle MOHO - Mohorovicic Discontinuity Core mantle boundary - change in mineralogy Density - getting heavier g/cm 3 Probably material such as peridotite (lots of heavy olivine - Fe, Mg) Samples from kimberlites, xenoliths in volcanic eruptions, basalt composition; lab experiments Our deepest hole: 9 Kilometers ….. we have a long way to go!

Core Outer core –Molten, near solid point (does not transmit certain seismic waves) –Density of pure iron or nickel/iron –Includes ~ half of diameter of Earth –2x density of mantle Inner core –Solid (higher pressure than outer core) –Density of pure iron or nickel/iron –~ Size of moon Our deepest hole: 9 Kilometers ….. we have a long way to go!

3 layers defined by mechanical properties (strength): Lithosphere Asthenosphere Mesosphere Earth’s Layers

Lithosphere –PLATES in Plate Tectonics –Upper 100 km –Crust and upper mantle –Rigid Asthenosphere –100 km to ~350 km (to 660km in places) –Upper mantle –Near melting point; little strength; ductile - NOT A LIQUID! –Plates moving on this –Magma generation Mesosphere –Extends to core –Also hot; strong due to pressure

Internal Dynamics of Earth Evidence –Earth’s landscape –Dynamic phenomena: earthquakes, volcanoes Plate Tectonics: Hypothesis and Theory –Continental drift –Seafloor spreading –Plate tectonics – a unifying theory

Dynamic Earth—Evidence Mountain belts (continental mountain ranges and oceanic ridges) Earthquake distribution: Concentrated zones Earthquake occurrences over time Volcanism in space: Concentrated zones Volcanism over time