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Chapter 3: the composition and structure of the earth

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The document summarizes key aspects of Earth's structure and composition. It describes Earth's distinct atmospheric layers and how pressure decreases with altitude. It also outlines Earth's internal layering, including the crust, mantle, and core. The crust is divided into continental and oceanic types based on differences in density and thickness. The mantle conveys heat via convection currents. The liquid outer core generates Earth's magnetic field through the geodynamo process.

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CHAPTER 3: The Structure & Composition of The Earth
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 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 of silica (SiO2) mixed in varying proportions with other elements such as Fe, Mg, Al, Ca, K, Na O Felsic = more silica (less Fe/Mg) & less dense O E.g. Granite O Mafic = less silica (more Fe/Mg) & more dense O E.g. Gabbro / Basalt O Range: Felsic / Intermediate / Mafic / Ultramafic Bulk Earth composition 35% 30% 15% 10% 10%
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
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
–Crust •Continental •Oceanic –Mantle •Upper •Lower –Core •Outer – Liquid •Inner – Solid Earth’s Interior Layers
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.
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. •Flow in the outer core generates the magnetic field. –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 •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 ductile solid. –Contains a small percentage of melt (< 2%)
Boundaries Between Layers •The Crust-Mantle boundary = Moho –defined by seismic discontinuity indicating significant change in composition. •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
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!

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Chapter 3: the composition and structure of the earth

  • 1. CHAPTER 3: The Structure & Composition of The Earth
  • 2. 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
  • 3. 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.
  • 4. 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”
  • 5. 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
  • 6. 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 of silica (SiO2) mixed in varying proportions with other elements such as Fe, Mg, Al, Ca, K, Na O Felsic = more silica (less Fe/Mg) & less dense O E.g. Granite O Mafic = less silica (more Fe/Mg) & more dense O E.g. Gabbro / Basalt O Range: Felsic / Intermediate / Mafic / Ultramafic Bulk Earth composition 35% 30% 15% 10% 10%
  • 7. 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).
  • 8. 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.
  • 9. 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
  • 10. 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
  • 12. 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.
  • 13. 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
  • 14. Two Types of Crust •Crustal density controls surface position. –Continental crust •Less dense; “floats higher.” –Oceanic crust •More dense: “floats lower.”
  • 15. 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.
  • 16. 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.
  • 17. The Core •An iron-rich sphere with a radius of 3,471 km. •2 components with differing seismic wave behavior. •Flow in the outer core generates the magnetic field. –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
  • 18. 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 ductile solid. –Contains a small percentage of melt (< 2%)
  • 19. Boundaries Between Layers •The Crust-Mantle boundary = Moho –defined by seismic discontinuity indicating significant change in composition. •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
  • 20. 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
  • 21. 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!