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Unformatted text preview: World's first geological map, William "Strata" Smith, 1815 Lecture 2: Structure of the Earth Prof Steven Glaser CE 70 Introduction to Engineering Geology Fall 2010 Distances from Earth The dimensions of the Universe are staggering! We must consider huge expanses of space and :me. The speed of light (c) is 186,000 miles/s (300,000 km/s). The Moon is 1.3 light seconds (~237,000 miles) away. The Sun is 8.3 light minutes (~93 million miles) away. A light year measures a distance of 5.87 trillion miles. Alpha Centauri, the closest star, is 4.3 light years away. The visible Universe is > 13 billion light years away Human percep:ons have changed. Early history Planets as moving lights. 1600s 1st telescopes saw hazy spheres. Today A complex, evolving system. Structure History The Solar System Space probes have photographed and analyzed planets. Scien:sts have hypothesized likely origins of the solar system. Earth's Circumference Eratosthenes calculated ~25,000 miles in ~ 200 B.C. He measured shadows in deep wells 800 km apart. Measurement taken at noon on the same day. Syene Shadow absent (directly overhead). Alexandria Shadow at 7.2o. He calculated that 800 km was 1/50th of Earth's circumference. He was correct!! Earth's Rota:on How do we know that Earth rotates about Polaris? Clearly visible in a :me-lapse photograph of the night sky. Foucault's pendulum (1851) proved Earth's rota:on. Earth and the Solar System What would solar system visitors no:ce? Magne:c field. Atmosphere. Surface features. Con:nents. Oceans. Polar ice caps. Evidence of humanity? Structures. Dams. Great Wall of China. Ci:es. Roads / canals. Electric lights. Interstellar space: a vacuum with a virtual absence of maeer. The amount of maeer greatly increases approaching the Sun. The Sun ejects maeer outward into space as the solar wind. Solar wind: Magne:cally charged par:cles. Stream outward in all direc:ons. Consists of... Protons (+ charge). Electrons ( charge). The Celes:al Neighborhood A small percentage of the solar wind impinges upon Earth. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 2: Journey to the Center of the Earth p.42-43bc Aurorae Some ions escape Van Allen belts. These ions are pulled to the magne:c poles. The ions create light in the upper atmosphere. Spectacular aurora follow solar flares. Aurora borealis Northern lights. Aurora australis Southern lights. The Atmosphere Earth has a well developed atmosphere. Thus, Earth is unique among the terrestrial planets. Densest at sea-level, the atmosphere thins upward. The Atmosphere The atmosphere is mostly nitrogen. Oxygen was absent from the atmosphere before 2.5 Ga. Other gases include: The Atmosphere 99% of atmosphere is below 50 km, the rest is between 50 and 500 km. The atmosphere is layered. Troposphere (0-11 km). Mixing layer. Weather is limited to this layer. Tropopause (11-12 km). Stratosphere (12-30 km). Sea-level atmospheric pressure. 14.7 pounds per square inch (psi). 1.0 bar. Fig. 20.05 W. W. Norton Earth's Surface Our experience with Earth is limited to its surface. Yet Earth has a complicated interior. Earth is characterized by... An internally generated magne:c field. Solid and liquid surfaces. A gaseous envelope. A layered interior. Surface Features Earth's surface is dominated by water and land. Water is a part of the hydrosphere. Surface water Ground water. Glacial ice. Impact craters occur rarely on Earth. Removed by weathering and erosion. Unlike other planets in the solar system. Surface Features Earth's surface reveals high con:nents and low ocean basins. Posi:on due to the differing buoyancy of each type of crust. Most land lies within 1 km of sea level. Most ocean floors are close to 5 km depth. Extremes of depth or height are rare. The 2 dominant "levels" reflect con:nental vs. oceanic crust. Hypsometric Curve Fig. 2.12ab W. W. Norton Elemental Composi:on ~ Ninety percent of Earth is comprised of 4 elements. Iron (Fe) ~35% Oxygen (O) ~30% Silicon (Si) ~15% Magnesium (Mg) ~10% The remaining 88 naturally occurring elements? Form ~10% of Earth. Earth Materials Elements combine in a variety of Earth materials. Organic compounds Carbon-containing compounds. Most are residue from once-living creatures. Include wood, peat, lignite, coal, and oil. Geologically rare (decomposes in contact with oxygen). Elements combine in a variety of Earth materials. Minerals Inorganic crystalline solids. Comprise rocks and, hence, most of the Earth. Most rocks on Earth are silicates (based on Si and O). Earth Materials Glasses Non-crystalline mineral-like maeer. Rocks Aggregates of minerals Igneous Cooled from a liquid (melt). Sedimentary Debris cemented from pre-exis:ng rock. Metamorphic Rock altered by pressure & temperature. Earth Materials Metals Solids made of metallic elements. Melts Rocks that have been heated to a liquid. Magma Molten rock beneath the surface. Lava Molten rock at the surface. Vola:les Materials that turn into gas at the surface. H2O, CO2, CH4, and SO2 Vola:les are released from volcanic erup:on. A Layered Earth We live on the thin outer skin of Earth. Early percep:ons about Earth's interior were wrong. Open caverns filled with magma, water, and air. Furnaces and flames. We now know that Earth is comprised of layers. The Crust. The Mantle. The Core. Outer Core. Inner Core. A Layered Earth Several clues indicate a layered structure. Density Plumb bob deflec:on es:mates density. Average density greater than surface density. Density must increase with depth. Shape A rota:ng sphere requires centered mass. Mass away from center would result in a flaeened disk. Solid interior - Surface doesn't undulate from :dal forces. A Layered Earth Earthquake clues - Earthquake energy transmieed as seismic waves that pass through Earth. Seismic waves have been used to probe the interior. Wave velocity changes with density. Velocity changes give depth of layer changes. Changes with depth. Pressure. Temperature. Earth's Interior Layers Earth (and other planets) have layered interiors. Crust Con:nental Oceanic Mantle Upper Lower Core Outer Liquid Inner Solid Fig. 23.02 W. W. Norton The Crust The outermost "skin" of Earth with variable thickness. Thickest under mountain ranges (70 km 40 miles). Thinnest under mid-ocean ridges (3 km 2 miles). The Mohorovicic discon:nuity is the lower boundary. Separates the crust from the upper mantle. Discovered in 1909 by Andrija Mohorovicic. Marked by a change in the velocity of seismic P waves. Two Types of Crust Con:nental crust Underlies the con:nents. Average rock density about 2.7 g /cm3. Average thickness 35-40 km. Grani:c in composi:on. Oceanic crust Underlies the ocean basins. Density about 3.0 g /cm3. Avg. thickness 7-10 km. Basal:c in composi:on. Crustal density controls surface posi:on. Con:nental crust Less dense; "floats higher." Two Types of Crust Oceanic crust More dense: "floats lower." Crustal Composi:on 98.5% of the crust is comprised of just 8 elements. Oxygen is (by far!) the most abundant element in the crust. This reflects the importance of silicate (SiO4-based) minerals. As a large atom, oxygen occupies ~93% of crustal volume. Earth's Mantle Solid rock layer between the crust and the core. 2,885 km thick, the mantle is 82% of Earth's volume. Mantle composi:on is the ultramafic rock perido:te. Below ~100-150 km, the rock is hot enough to flow. It convects: hot mantle rises, cold mantle sinks. Three subdivisions: upper, transi:onal, and lower. The Core An iron-rich sphere with a radius of 3,471 km. 2 components with differing seismic wave behavior. Outer core Liquid iron-nickel-sulfur 2,255 km thick Density 10-12 g /cm3 Inner core Solid iron-nickel alloy Radius of 1,220 km. Density 13 g /cm3 Lithosphere-Asthenosphere Lithosphere The outermost 100-150 km of Earth. Behaves as a non-flowing, rigid material. The material that moves as tectonic plates. Made of 2 components: crust and upper mantle. Asthenosphere Upper mantle below lithosphere. Shallower under oceanic lithosphere. Deeper under con:nental lithosphere. Flows as a sot solid. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 2: Journey to the Center of the Earth ...
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