1. Chapter 6
The Earth
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2. Size & Shape of the Earth
Earth is a huge, rocky/iron sphere spinning in space
Moving around Sun at ~100 miles every few seconds (67,000 mph!)
Blanket of air & magnetic field protect surface from radiation & colliding debris

3. Earth’s Equatorial Bulge
Like all (official) planets, large enough for gravity to shape into a sphere

4. Earth’s Equatorial Bulge
Like all (official) planets, large enough for gravity to shape into a sphere
How much do mountains weigh??

5. Gravity makes planets spheres!
If not as massive, gravity won’t be able to pull matter into a sphere!
Look at Asteroids!

6. Earth’s Equatorial Bulge
Rotation makes equator bulge out into oblate spheroid
You weigh more at North Pole than at the Equator!!

7. Composition of Earth Most common elements of surface rocks:
oxygen (45.5% by mass),
silicon (27.2%),
aluminum (8.3%),
iron (6.2%),
calcium (4.66%), and
magnesium (2.76%)
Silicon & oxygen = silicates
Sand is silicate mineral quartz (nearly pure silicon dioxide)

8. Density of Earth
Density measures how much material (mass) is packed into a given space (volume)
Units: grams per cubic centimeter (g/cm3)
Density of water = 1 g/cm3
Density of ordinary surface rocks = ~3 g/cm3
Density of Iron = 8 g/cm3
Average density for Earth = 5.5 g/cm3
Earth’s interior (core) probably iron/nickel ~ g/cm3

9. Density of Earth
Hypothesis: Earth’s interior (core) probably iron/nickel ~ g/cm3
Critical Test:
How do we know if we have never been there?
Use seismic waves!
Inge Lehmann (1936)

10. Probing Earth’s Interior
Earthquakes generate seismic waves
Waves move through Earth’s interior with speeds depending on properties of material.
Speeds determined by timing arrival of waves
Seismic “picture” generated of Earth’s interior along path of wave

11. The only way we have to probe Earth’s interior!
A Sonogram of the Earth!
The only way we have to probe Earth’s interior!

12. S-waves & P-waves
P waves S waves
P waves compress material & travel easily through liquid or solid
S waves move material perpendicular to wave travel & only move through solids

13. Interior Structure
Observations show P waves but no S waves at detecting stations on the opposite side of the Earth from the origin of an Earthquake
Þ the Earth has a liquid core!

14. Interior Structure of the Earth
Solid, low-density thin crust made mainly of silicates
Hot, thick, not-quite-liquid mantle with silicates
Liquid, outer core mixture of iron, nickel & sulfur (?)
Solid, inner core iron & nickel & (?)

15. Layers of Earth = Differentiation!
Densest materials at center; least dense at surface
Naturally occurs in mix of heavy & light materials…
If liquid …
For long enough time…
In a strong gravitational field
Earth must have been almost entirely liquid in the past

16. Differentiation

17. Thought Question Do rocks s-t-r-e-t-c-h?
No—rock is rigid and cannot deform without breaking.
Yes—but only if it is molten rock.
Yes—rock under strain may slowly deform.

18. Thought Question Do rocks s-t-r-e-t-c-h?
No—rock is rigid and cannot deform without breaking.
Yes—but only if it is molten rock.
Yes—rock under strain may slowly deform.

19. Strength of Rock
Rock stretches when pulled slowly but breaks when pulled rapidly.
Gravity of large world pulls slowly on rocky content, shaping world into a sphere.

20. Temperature Inside the Earth
Estimated temp of Earth’s core = 6500 K
At least two causes:
Heat from impact of small bodies in Earth’s formation by gravitation
Radioactive decay of elements that occur naturally in mix of materials that made up the Earth

21. Cooling of Terrestrial Bodies
Thermal energy trapped inside Earth’s interior takes a LONG time to move to surface & escape

22. Radiometric Dating Radioactive decay used to determine Earth’s age
Radioactive atoms decay into daughter atoms
More daughter atoms relative to original radioactive atoms => older rock

23. Motion in the Earth’s Interior
Heat generated by radioactive decay in Earth creates movement of rock called convection
Hotter material is less dense than surroundings
Rises while cooler material sinks

24. Heat Drives Geological Activity
Convection: hot rock rises, cool rock falls.
One convection cycle takes ~100 million years on Earth.

25. Sources of Internal Heat
Accretion releases gravitational potential energy
Differentiation
Radioactivity

26. Heating of Interior over Time
Accretion and differentiation when planets were young
Radioactive decay is most important heat source today

27. Cooling of Interior
Convection
Conduction
Radiation

28. Cooling of Interior
Convection transports heat as hot material rises and cool material falls
Conduction transfers heat from hot material to cool material
Radiation sends energy into space as IR light (heat!)

29. Thought Question What cools off faster?
A grande-sized cup of Starbucks coffee
A teaspoon of cappuccino in the same cup

30. Thought Question What cools off faster?
A grande-size cup of Starbucks coffee
A teaspoon of cappuccino in the same cup

31. Surface Area to Volume Ratio
Heat content depends on volume.
Loss of heat through radiation depends on surface area.
Time to cool depends on surface area divided by volume
Larger objects have a smaller ratio & cool more slowly.

32. Thought Question What cools off faster? A big terrestrial planet
A tiny terrestrial planet

33. Thought Question What cools off faster? A big terrestrial planet |
A tiny terrestrial planet
Brrrr!

34. Role of Size Smaller worlds cool off faster & harden earlier
Moon & Mercury now geologically “dead”… is Mars?

35. Changing Face of the Earth

36. Plate Tectonics
Early researchers noted that South America & Africa appeared to fit together
Also shared similar fossils
Wegener proposed “continental drift” (1912) that all continents were single supercontinent.
Earth’s surface continually building up & breaking down over millions of years

37. Continental Drift

38. The Earth’s Magnetic Field
Magnetic forces are communicated across distances by a magnetic field
Magnetic fields depicted by magnetic lines of force
Lines represent direction a compass would point
Density of lines indicate strength of field
Polarity (N-S)

39. Magnetic Fields Visualized
Magnetic fields are generated by large-scale currents OR
Atomic “currents”
Electron “orbits”
Electron “spin”
Nucleus “spin”

40. Origin of the Earth’s Magnetic Field
Generated by currents flowing in molten iron core

41. Magnetic Effects in the Upper Atmosphere
Earth’s magnetic field screens planet from charged particles emitted from Sun
Earth’s magnetic field deflects charged particles into spiral trajectories towards poles

42. Aurora
Charged solar particles stream into Earth’s atmosphere & generate electrical currents in upper atmosphere
Particles collide & excite gaseous nitrogen & oxygen.
Atoms & molecules de-excite, emitted photons we see - creating aurora

47. Earth’s Atmosphere THIN “veil” of gases
Unique relative to other planets: Oxygen, H20, temperature profile
Comparing with Venus & Mars helps us learn (Jupiter, Saturn, Uranus, Neptune too!)

48. Origin of the Earth’s Atmosphere
Several theories:
Asteroid impacts or volcanoes release gas originally trapped when Earth formed (“Outgassing”)
Brought to Earth by impact with comets

49. The Early Atmosphere
Much more methane (CH4) & ammonia (NH3)

50. The Early Atmosphere
Solar UV intense enough to break Hydrogen from CH4, NH3 , & H2O
Left carbon, nitrogen, & oxygen behind while H escaped into space
Ancient plants further increased levels of atmospheric oxygen through photosynthesis

51. Earth’s Atmosphere Today
78% Nitrogen, 21% Oxygen, traces of CO2, H2O, Argon
Extends hundreds of kilometers, but very “tenuous” at high altitudes
Half of atmosphere’s mass within first 4 kilometers

52. Structure of the Earth’s Atmosphere
US Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar

53. The Greenhouse Effect

54. The Greenhouse Effect on Earth
FIGURE 6-4 The Greenhouse Effect
(a) Sunlight and heat from Earth’s interior
warm Earth’s surface, which in
turn radiates energy, mostly as infrared
radiation. Much of this radiation is
absorbed by atmospheric carbon dioxide
and water, heating the air, which
in turn raises Earth’s temperature even
further. In equilibrium, Earth radiates
as much energy as it receives.

55. The Greenhouse Effect
Visible light reaches Earth’s surface, absorbed, & reradiates as heat (IR)
IR trapped by atmosphere (especially CO2, H20, CH4)
Reduces rate of heat loss
Makes surface hotter than it would be otherwise

56. Climate Change
Tracers of the Earth’s temperature and atmospheric carbon dioxide content for the past 800,000 years show a strong correlation.
Many scientists are concerned that humans are adding so much carbon dioxide to the atmosphere that we might trap so much heat that Earth’s temperature will climb—a process called global warming.

57. How is human activity changing our planet?

58. Most of the CO2 increase has happened in last 50 years!
CO2 Concentration
Most of the CO2 increase has happened in last 50 years!

59. Keeling Curve

60. CO2 Concentration
Global temperatures have tracked CO2 concentration for the last 500,000 years.
Antarctic air bubbles indicate the current CO2 concentration is at its highest level in at least 500,000 years.

61. Dangers of Human Activity
Human-made CFCs in atmosphere destroy ozone, reducing protection from UV radiation.
Human activity driving many species to extinction.
Human use of fossil fuels produces greenhouse gases that can cause global warming.

62. Global Climate Change
Earth’s average temperature has increased by 0.5°C in the past 50 years.
Concentration of CO2 is rising rapidly.
Unchecked rise in greenhouse gases is leading to global climate change.

63. Modeling of Climate Change
Build climate models based on current/past data
Models suggest recent temperature increase is consistent with human production of greenhouse gases.

64. Air and Ocean Circulation
In the absence of any force an object will move in a curved path over a rotating object
This apparent curved motion is referred to as the Coriolis effect

65. The Coriolis Effect Responsible for:
The spiral pattern of large storms as well as their direction of rotation
The trade winds that move from east to west in two bands, one north and one south of the equator
The direction of the jet streams, narrow bands of rapid, high-altitude winds
The deflection of ocean currents creating flows such as the Gulf Stream

66. Rapid Spin Equals High Force
The faster the spin, the more dramatic the effect
The atmospheric band structure and storms of the rapidly rotating Jupiter, Saturn, and Neptune are due to the Coriolis Force.

67. Age of the Earth
Radioactive potassium has a half-life of 1.28 billion years and decays into argon, which is a gas that is trapped in the rock unless it melts
Assume rock has no argon when originally formed
Measuring the ratio of argon atoms to potassium atoms gives the age of the rock
This method gives a minimum age of the Earth as 4 billion years
Other considerations put the age at 4.5 billion years

68. Convection Convection in the Earth’s interior
The crust and mantle are solid rock, although when heated, rock may develop convective motions
These convective motions are slow, but are the cause of: earthquakes, volcanoes, the Earth’s magnetic field, and perhaps the atmosphere itself

69. Rifting
Rifting
Hot, molten material rises from deep in the Earth’s interior in great, slow plumes that work their way to the surface
Near the surface, these plumes spread and drag the surface layers from below
GPS measurements show the continents moving up to 10 cm/year relative to each other

70. Subduction Subduction
Where cool material sinks, it may drag crustal pieces together buckling them upward into mountains
If one piece of crust slips under the other, the process is called subduction

71. The Magnetosphere
Region of the Earth’s environment where the Earth’s magnetic field affects particle motion is called the magnetosphere
Within the magnetosphere charged particles are trapped in two doughnut shaped rings that encircle the Earth and are called the Van Allen radiation belts

72. The Ozone Layer
Oxygen in the atmosphere provides a shield against solar UV radiation
O2 provides some shielding, but O3, or ozone, provides most of it
Most ozone is located in the ozone layer at an altitude of 25 km
Shielding is provided by the absorption of UV photons by oxygen molecules (both O2 and O3) and their resultant dissociation
Single O atoms combine with O and O2 to replenish the lost O2 and O3
It is doubtful that life could exist on the Earth’s surface without the ozone layer