2-Earth_Composition_Structure.pdf

Earth’s Composition and Structure: A Journey to the Center of the Earth  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 magnetic fie...

Earth’s Composition and Structure: A Journey to the Center of the Earth  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 magnetic field. The Black Canyon of the Gunnison, CO – A layered interior Solid and liquid layers. – A gaseous envelope. i.e. atmosphere The Solar System Human perceptions have changed. – Early history – Planets as moving lights. – 1600s – 1st telescopes saw hazy spheres. – Today – A complex, evolving system. – Structure – History Space probes have photographed and analyzed planets. Scientists have hypothesized likely origins of the solar system.  Earth and the Solar System What would solar system visitors notice? – Magnetic field. – Atmosphere. – Surface features. Continents. Oceans. Polar ice caps. Evidence of humanity? – Structures. » Dams. » Great Wall of China. » Cities. » Roads / canals. – Electric lights.  The Celestial Neighborhood Interstellar space: a ~vacuum with a virtual absence of matter. The amount of matter greatly increases approaching the Sun. The Sun ejects matter outward into space as the solar wind. Solar wind: – Magnetically & electrically charged particles. – Stream outward in all directions. – Consists of… Protons (+ charge). Electrons (– charge). Only a small percentage of the solar wind impinges upon Earth. Five Key Characteristics About Earth’s Structure: 1. Earth has a dipole magnetic field that deflects solar wind and protects earth’s surface from solar radiation 2. Earth has a stratified atmosphere, mainly composed of nitrogen (N2) and oxygen (O2) 3. Earth is made of a variety of minerals, glasses, melts, fluids and volatiles, all left behind during birth of the solar system 4. The Earth has layers: a thin silicate crust, a thick iron- & magnesium silicate mantle, and a thick metallic core 5. Physically, the earth can be divided into a rigid outer lithosphere and a plastic/ductile asthenosphere  Earth’s Magnetic Field Geodynamo - The Earth’s magnetic field is produced by the geodynamo -Flow in the liquid iron outer core creates a magnetic field Magnetic field - region affected by force emanating from a magnet - grows stronger as separating distance decreases - attracts or repels magnetically charged or moving electrically charged objects - compasses work because Earth is a large magnet  Earth’s Magnetic Field Magnetic field - Like a bar magnet, Earth’s magnetic field is a dipole, (has both a N and S pole) - Solar wind contains electromagnetic particles that are deflected by earth’s field. These particles distort the shape of earth’s magnetic field in space - Van Allen belts – two belts in the inner magnetic field where high energy cosmic rays are trapped. Protects us from solar radiation! Northern & Southern Lights Form because of our dipole magnetic field! Aurorae Some ions escape Van Allen belts. – These ions are pulled to the magnetic poles. – The ions create light in the upper atmosphere. Spectacular aurora follow solar flares. – Aurora borealis – Northern lights. – Aurora australis – Southern lights.  Earth’s Atmosphere Distinct layers of gas surround the solid portion of the earth. o Composition is ~uniform regardless of altitude o 78% N2 o 21% O2 o All others ~1% o Ar, CO2, CH4, H2O, Ne, CO, SO2 o Some other Planets have atmospheres too! o None have N2 & O2 as dominant gasses o Earth was oxygen-free until ~2.5 Ga  Earth’s Atmosphere o Pressure decreases with increasing altitude o Reflects # of molecules/volume o Lower pressure = less molecules/volume o Air pressure @ sea level = 14.7 lb/in2 = 1 bar o Pressure is caused by the weight of overlying material o Upper atmosphere has less material above it o Pressure is lower o 99% of atmosphere is below 50 km, the rest is between 50 and 500 km.  Earth’s Atmosphere o Earth’s Atmosphere is divided into distinct layers based on altitude o Exosphere (very thin ~500 km) o Atmosphere merges with space o Thermosphere (>90 km) o Where space shuttles orbit o Mesosphere (50-90 km) o Meteors burn up here o Stratosphere (12-50 km) o Stable air; good for jets o Tropopause (11-12 km) o Troposphere (0-11 km) o Mixing layer o All weather is limited to this layer o “Tropo” = Greek for “turning”  Earth’s Atmosphere o Troposphere o A well-mixed layer dominated by convection of air masses o Convection o Method of heat transfer in a fluid o Think lava lamp! o Cold is more dense = sinks o Hot is less dense = rises o This process results in circular convection cells o Also causes pressure gradients which create wind! o Also applies to the interior of the Earth …this guy likes convection  Earth’s Components o Earth’s surface = ~30% land, ~70% water o unlike any other known planet o Hydrosphere = includes oceans, lakes, seas, rivers, & groundwater o Cryosphere = glaciers, snow, and sea ice o Earth’s surface is not flat; it has topography o Ignoring oceans, Earth’s surface is dominated by two distinct elevations: o Most land is 0-2 km above sea level o Most of the sea floor is 3-5 km below sea level  Earth’s Components o Earth’s elemental composition reflects mostly heavier elements not blown away by solar wind during formation of the solar system o Most abundant elements o Fe, O, Si, Mg o Most common minerals consist 35% of silica (SiO2) mixed in varying proportions with other elements such as Fe, Mg, Al, 30% 10% Ca, K, Na o Felsic = more silica (less Fe/Mg) 10% & less dense o E.g. Granite o Mafic = less silica (more Fe/Mg) 15% & more dense Bulk Earth composition o E.g. Gabbro / Basalt o Range: Felsic / Intermediate / Mafic / Ultramafic  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).  Earth Materials 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). – Glasses – Non-crystalline mineral-like matter. Cool too quickly to form structure – Rocks – Aggregates of minerals. There are many types. Igneous – Cooled from a liquid (melt). Sedimentary – Debris cemented from pre-existing rock. Metamorphic – Rock altered by pressure and 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. Volatiles – Materials that turn into gas at surface temps. – H2O, CO2, CH4, and SO2 – Volatiles are released from volcanic eruption.  A Layered Earth We live on the thin outer skin of Earth. Early perceptions 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. Some basic rules of physics give some clues… From Milton’s “Paradise Lost”  Earth’s Density Earth’s Density gives us clues about its internal structure Density = Mass/Volume Measures how much mass is in a given volume. Expressed in units of mass/volume e.g. g/cm3 Ice floats…why? Estimates of earth’s mass and volume give a whole earth density of ~5.5 g/cm3 Typical rocks at the surface of the Earth have a density of 2.0-2.5 g/cm3 What does this require of the density of material in the Earth’s interior?  Earth’s Density Earth’s shape as a clue to the internal structure of the Earth If density increased gradually and uniformly towards the center, a significant portion of Earth’s mass would be near the outer edges…. Then centrifugal force (not centripetal) would cause the planet to flatten into a disk. This has (obviously) not happened…  Earth’s Layers Earth’s shape as a clue to the layering of the earth If the Earth consisted of a thin solid shell over a thick liquid center, then the surface would rise and fall with tides like the ocean – This does not happen; only the oceans rise and fall. Thus, the Crust does not float over a liquid interior  A Layered Earth Earthquake clues - Earthquake energy transmitted 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. More on this in Chapter 10 and Interlude D!  Earth’s Interior Layers The Earth (and other planets) have layered interiors. – Crust Continental Oceanic – Mantle Upper Lower – Core Outer – Liquid Inner – Solid © 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 Mohorovičić discontinuity or “Moho” 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 Continental crust – Underlies the continents. – Avg. rock density about 2.7 g/cm3. – Avg. thickness 35-40 km. – Felsic composition. Avg. rock type = Granite Oceanic crust – Underlies the ocean basins. – Density about 3.0 g/cm3. – Avg. thickness 7-10 km. – Mafic composition Avg. rock type = Basalt/Gabbro  Two Types of Crust Crustal density controls surface position. – Continental crust Less dense; “floats higher.” – Oceanic crust More dense: “floats lower.”  Crustal Composition 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 (SiO2-based) minerals. – As a large atom, oxygen occupies ~93% of crustal volume. Bulk Earth Composition 35% 10% 30% 10% 15%  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 composition = ultramafic rock called peridotite. Below ~100-150 km, the rock is hot enough to flow. It convects: hot mantle rises, cold mantle sinks. Three subdivisions: upper, transitional, 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 Flow in the outer core generates the magnetic field.  Lithosphere-Asthenosphere The Crust, Mantle, Core boundaries – defined by composition …but sometimes we want to divide the layers of the Earth by their behavior or physical properties Lithosphere – The brittle portion of Earth’s interior. – Behaves as a non-flowing, rigid material. – The material that moves as tectonic plates. – Made of 2 components: crust and upper mantle. Asthenosphere – The ductile portion of Earth’s interior. – Shallower under oceanic lithosphere. – Deeper under continental lithosphere. – Flows as a soft solid.  Boundaries Between Layers The Crust-Mantle boundary = Moho – defined by seismic discontinuity indicating significant change in composition. Lithosphere-Asthenosphere boundary = Brittle-ductile transition – Defined by a significant change in rock physical properties (viscosity) – Also defined as the depth below which earthquakes do not occur. Lithosphere ≠ Crust