1. Chapter 2 Internal Energy and Plate Tectonics Lecture PowerPoint
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2. Origin of the Sun and Planets
Solar system began as rotating spherical cloud of gas, ice, dust and debris
Gravitational attraction brought particles together into bigger and bigger particles
Cloud contracted, sped up and flattened into disk
Formation of Sun
Greatest accumulation of matter (H and He) at center of disk
Temperature at center increased to 1 million degrees centigrade
Nuclear fusion of hydrogen (H) and helium (He) began, producing solar radiation

3. Origin of the Sun and Planets
Figure 2.1

4. Origin of the Sun and Planets
Formation of planets
Rings of concentrated matter formed within disk
Particles within rings continued to collide to form planets
Inner planets (Mercury, Venus, Earth and Mars) lost much gas and liquid to solar radiation, becoming rocky (terrestrial)
Outer planets retained gas and liquid, as gas planets
Impact Origin of the Moon
Early impact of Mars-sized body with Earth
Impact generated massive cloud of dust (from Earth’s crust and mantle) and gas which condensed to form Moon
Lightweight gases and liquids lost to space
Lesser abundance of iron (from Earth’s core) in Moon

5. Earth History Earth began as aggregating mass of particles and gases
Aggregation took 30 to 100 million years
Occurred nearly 4.6 billion years ago
Processes of planet formation created huge amounts of heat
Impact energy
Decay of radioactive
elements
Gravitational energy
Differentiation into
layers
Figure 2.2

6. Earth History Differentiation into layers
As temperature rose above 1,000 centigrade, iron melted
Liquid iron is denser than remaining rock, so sank toward center of Earth to form inner and outer core
Release of gravitational energy produced additional heat
Remaining rock melted, allowing low-density material to rise
Low-density material formed crust, oceans and atmosphere
4.4 billion years ago: large oceans, small continents
3.5 billion years ago: life (photosynthetic bacteria)
2.5 billion years ago: large continent
1.5 billion years ago: plate tectonics

7. Side Note: Mother Earth
Analogy with Mother Earth, 46-year-old woman:
(1 Mother Earth year = 100 million geologic years)
First seven years unaccounted for
42 years old: life appeared on continents
45 years old: flowering plants
8 months ago: dinosaurs died out
Last week: human ancestors evolved
Yesterday: humans evolved
Last hour: discovered agriculture, settled down
1 minute ago: Industrial Revolution

8. The Layered Earth Differentiated into layers of increasing density
Center of Earth: Iron-rich core 7,000 km in diameter
Inner core is solid and 2,450 km in diameter
Outer core is liquid and has viscous convection currents, responsible for Earth’s magnetic field
Surrounding core is Earth’s mantle, 2,900 km thick
Stony in composition (like chondritic meteorites)
83% of Earth’s volume, 67% of Earth’s mass
Low-density elements have been ‘sweated’ out of the mantle to form the crust, atmosphere and oceans

9. The Layered Earth
Figure 2.3

10. The Layered Earth Layers can be described in terms of
Different density (different chemical and mineral compositions)
Crust overlies mantle
Or different strength
Lithosphere overlies asthenosphere
Lithosphere is rigid (solid rock)
Asthenosphere is fluidlike (plastic rock)
Mesosphere is solid mantle below asthenosphere
Figure 2.4

11. Side Note: Volcanoes and the Origin of the Ocean, Atmosphere and Life
Volcanic gases:
Hydrogen (H), oxygen (O), carbon (C), sulfur (S), chlorine (Cl), nitrogen (N)
Combine to make:
Water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), carbon monoxide (CO), nitrogen (N2), hydrogen (H2), hydrochloric acid (HCl), methane (CH4), and others
Dominant volcanic gas is water vapor – more than 90%

12. Side Note: Volcanoes and the Origin of the Ocean, Atmospheres and Life
Volcanic rocks:
Oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), potassium (K)
4.5 billion years of volcanism has brought light weight elements to the surface to make up
Continents
Oceans
Atmosphere
CHON (carbon, hydrogen, oxygen, nitrogen) elements of life

13. The Layered Earth Behavior of Materials
Gas, solid and liquid are obvious terms, but should be considered with respect to time
Over longer time periods, solids may behave as liquid
Glacier: solid ice, yet flows (ultrahigh viscosity liquid) over years
Elastic deformation is recoverable – object returns to original shape
Ductile deformation is permanent – stress applied over long time or at high temperatures
Brittle deformation is permanent – stress applied very quickly to shatter or break object

14. The Layered Earth Permanent stress occurs when yield stress is reached
Rocks at Earth’s surface (low temperature, low pressure): brittle
Rocks in asthenosphere: “soft plastic”
Rocks in deeper mantle: “stiff plastic”
Figure 2.5

15. The Layered Earth Asthenosphere is plastic About 250 km thick
Comes to surface at mid-ocean ridges
Lies more than 100 km below surface elsewhere
Allows Earth to be oblate spheroid (flattened during rotation; like Solar System during formation)
Allows continents to ‘float’ atop the mantle, by principles of isostasy

16. Isostasy
Isostasy: Less dense materials float on top of more dense materials (i.e. iceberg floating in ocean); buoyancy principle
Earth is a series of density-stratified layers
Core – densities up to 16 gm/cm3
Mantle – densities from 5.7 to 3.3 gm/cm3
Continents (crust) – densities
around 2.7 gm/cm3
Oceans – densities around
1.03 gm/cm3
Atmosphere – least dense
Figure 2.4

17. Isostasy Examples of isostasy:
Impoundment of water in Lake Mead behind Hoover Dam caused area to sink 175 mm over 15 years
Scandinavia is currently rising (about 200 m so far)
Had been depressed under weight of ice sheets during last Ice Age (since 10,000 years ago)
Ground ruptures and earthquakes are present
Viking ship buried in the harbor mud of Stockholm was lifted above sea level
Another 200 m of uplift is likely

18. Internal Sources of Energy
Impact energy
Tremendous numbers of smaller bodies hit the Earth early after its formation, converting energy of motion to heat
Gravitational energy
As Earth pulled to smaller and denser mass, gravitational energy was released as heat
Heat from both of these early sources is still flowing to the surface today, as heat conducts very slowly through rock

19. Internal Sources of Energy
Radioactive isotopes
Unstable radioactive atoms (isotopes) decay and release heat
Early Earth had much larger amount of short-lived radioactive elements and therefore much greater heat production than now
Radioactive decay process:
Measured by half-life: time for half the atoms of a radioactive element (parent) to disintegrate to decay (daughter) product
Half-lives against time is negative exponential curve

20. Internal Sources of Energy
Figure 2.8
Figure 2.7

21. Internal Sources of Energy
Sum of internal energy from impacts, gravity and radioactive elements (plus tidal friction energy) is very large
Internal temperatures have been declining since early Earth maximum, but still significant enough to cause plate tectonics, earthquakes and volcanic eruptions

22. Internal Sources of Energy
Age of Earth
Oldest Solar System materials are 4.57 billion years old
Measured using radioactive elements in Moon rocks and meteorites
Oldest Earth rocks (found in northwest Canada) are billion years old
Oldest Earth materials (zircon grains from Australian sandstone) are 4.37 billion years old

23. Internal Sources of Energy
Age of Earth
Earth must be younger than 4.57 billion years old materials that formed the planet
Seems that Earth has existed as coherent mass since about 4.54 billion years ago
Probably took 30 million years (0.03 billion years) for Earth to form
Collision that formed the Moon seems to have occurred between and billion years ago
Earth must be older than 4.4 billion years old zircons
Conclusion: Earth has existed about 4.5 billion years

24. In Greater Depth: Radioactive Isotopes
Elements defined by number of positively charged protons
Isotopes are different forms of the same element with different numbers of neutrons
Radioactive isotopes are unstable and release energy through their decay process to more stable isotopes
Knowing the half-life of radioactive isotopes allows us to use their quantity as a clock to date rocks

25. In Greater Depth: Radioactive Isotopes
Nuclear fission:
Parent atom sheds particles to become smaller daughter atom
Alpha particle: two protons and two neutrons (helium atom)
Beta particle: electron
Gamma radiation: lowers energy level of nucleus
Figure 2.9

26. In Greater Depth: Radioactivity Disasters
Chernobyl disaster of 1986, in Ukraine
Explosion released 185 million curies of radioactivity, affecting much of Europe
50 deaths in area, with many more later deaths from cancer
Possibly caused by misinterpretation of small earthquake
Can such a thing occur in nature? Depends on relative amounts of U-238 and U-235:
Most uranium ore is U-238, about 0.7% is U-235
Uranium ore used in reactors is enriched to 2-4% U-235
Because U-235 decays more rapidly than U-238, at some point in the past all uranium ore would have had about 2-4% U-235
Sites in West Africa were natural nuclear reactors about 2.1 billion years ago, at about 400 degrees centigrade temperatures

27. Plate Tectonics Tectonic cycle:
Melted asthenosphere flows upward as magma
Cools to form new ocean floor (lithosphere)
New oceanic lithosphere (slab) diverges from zone of formation atop asthenosphere (seafloor spreading)
When slab of oceanic lithosphere collides with another slab, older, colder, denser slab subducts under younger, hotter, less dense slab
Subducted slab is reabsorbed into the mantle
Cycle takes on order of 250 million years

28. Plate Tectonics
Tectonic cycle:
Figure 2.11

29. Plate Tectonics Lithosphere of Earth is broken into plates
Plate Tectonics: Study of movement and interaction of plates
Zones of plate-edge interactions are responsible for most earthquakes, volcanoes and mountains
Divergence zones
Plates pull apart during seafloor spreading
Transform faults
Plates slide past one another
Convergence zones
Plates collide with one another

30. Plate Tectonics
Lithosphere of Earth is broken into plates separated by: divergence zones, transform faults, convergence zones
Figure 2.12

31. Development of the Plate Tectonics Concept
1620: Francis Bacon noted parallelism of Atlantic coastlines of Africa and South America
Late 1800s: Eduard Suess suggests ancient supercontinent Gondwanaland (South America, Africa, Antarctica, Australia, India and New Zealand)
1915: Alfred Wegener’s book supports theory of continental drift – all the continents had once been supercontinent Pangaea, and had since drifted apart
Theory of continental drift was rejected because mechanism for movement of continents could not, at the time, be visualized

32. Development of the Plate Tectonics Concept
20th century: study of ocean floors provided wealth of new data and breakthroughs in understanding
Lithosphere moves laterally
Continents are set within oceanic crust and ride along plates
Theory of plate tectonics was developed and widely accepted

33. In Greater Depth: Earth’s Magnetic Field
Earth’s magnetic field acts like giant bar magnet, with north end near the North Pole and south end near the South Pole
Magnetic pole axis is now inclined 11o from vertical (tilt has varied with time) so that magnetic poles do not coincide with geographic poles (but are always near each other)
Inclination of magnetic lines can also be used to determine at what latitude the rock formed
Magnetic field is caused by dynamo in liquid outer core:
Movements of iron-rich fluid create electric currents that generate magnetic field
Figure 2.13

34. In Greater Depth: Earth’s Magnetic Field
Problematic details of Earth’s magnetic field await resolution:
Strength of magnetic field waxes and wanes
Magnetic pole moves about geographic pole irregularly, crossing 5o to 10o of latitude each century
Magnetic polarity reverses
Every several thousand to tens of millions of years
Orientation of magnetic field switches from north (normal) polarity to south (reverse) polarity
Reverses take few thousand years to complete, with complex field present during switch
Changes in magnetic field are preserved in most volcanic and some sedimentary rocks

35. Magnetization of Volcanic Rocks
Magnetic patterns of ocean floor first observed in mid 20th century – very important to theory of plate tectonics
Why does the ocean floor have a magnetic pattern?
When lava cools to below 550oC (Curie point), atoms in iron-bearing minerals line up in direction (polarity) of Earth’s magnetic field
Polarity of Earth’s magnetic field can be either to north or to south and depends on time in Earth’s history

36. Magnetization of Volcanic Rocks
Successive lava flows stack up one on top of another, each lava flow recording Earth’s polarity at time it formed
Each lava flow can also be dated using radioactive elements in rock to give its age
Figure 2.14

37. Magnetization of Volcanic Rocks
Magnetic patterns of ocean floor
What does magnetic polarity of lava flows tell us?
Plotting the polarity of different lava flows against their ages gives us a record of the Earth’s polarity at different times in the past
Timing of polarity reversals (north to south; south to north) seems random
Reversals probably caused by changes in the flow of iron-rich liquid in the Earth’s outer core
Figure 2.15

38. Magnetization Patterns on the Seafloors
Atlantic Ocean floor is striped by parallel bands of magnetized rock with alternating polarities
Stripes are parallel to mid-ocean ridges, and pattern of stripes is symmetrical across mid-ocean ridges (pattern on one side of ridge has mirror opposite on other side)
Pattern of alternating polarity stripes is same as pattern of length of time between successive reversals of Earth’s magnetic field
Figure 2.16

39. Magnetization Patterns on the Seafloors
Magma is injected into the ocean ridges to cool and form new rock imprinted with the Earth’s magnetic field
Seafloor is then pulled away from ocean ridge like two large conveyor belts going in opposite directions – seafloor spreading
Figure 2.17

40. Other Evidence of Plate Tectonics
Earthquake epicenters outline plate boundaries
Map of earthquake epicenters around the world shows not a random pattern, but lines of earthquake activity that define edges of tectonic plates
Figure 2.18

41. Other Evidence of Plate Tectonics
Deep earthquakes
Most earthquakes occur at depths less than 25 km
Next to deep-ocean trenches, earthquakes occur along inclined planes to depths up to 700 km
These earthquakes are occurring in subducting plates
Figure 2.19

42. Other Evidence of Plate Tectonics
Ages from ocean basins
Oldest rocks on ocean floor are ~200 million years old (<5% Earth’s age)
Ocean basins are young features – continually being formed (at mid ocean ridges) and destroyed (at subduction zones)
Hot spots in the mantle (plumes) cause volcanoes on overlying plate, which form in a line as plate moves over hot spot, getting older in direction of plate movement
Sediment on the seafloor is very thin at mid ocean ridges (where seafloor is very young) and thicker near trenches (where seafloor is oldest)
Figure 2.21

43. Other Evidence of Plate Tectonics
Oceanic mountain ranges and deep trenches
Ocean bottom is mostly about 3.7 km deep, with two areas of exception:
Continuous mountain ranges extend more than 65,000 km along ocean floors
Volcanic mountains that form at spreading centers, where plates pull apart and magma rises to fill gaps
Narrow trenches extend to depths of more than 11 km
Tops of subducting plates turn downward to enter mantle

44. Other Evidence of Plate Tectonics
Systematic increases in seafloor depth
Ocean floor depths increase systematically with seafloor age, moving away from mid-ocean ridges
As oceanic crust gets older, it cools and becomes denser, therefore sinking a little lower into mantle
Weight of sediments on plate also cause it to sink a little into mantle
Figure 2.22

45. Other Evidence of Plate Tectonics
The Fit of the Continents
If continents on either side of the Atlantic Ocean used to be adjacent, their outlines should match up
Outlines of continents at the 1,800 m water depth line match up very well
1,800 m water depth line marks boundary between lower-density continental rocks and higher-density oceanic rocks
Continental masses cover 40% of Earth’s surface, ocean basins cover other 60%

46. Other Evidence of Plate Tectonics
Changing Positions of the Continents
220 million years ago, supercontinent Pangaea covered 40% of Earth (60% was Panthalassa, massive ocean)
Figure 2.23

47. Other Evidence of Plate Tectonics
Changing Positions of the Continents
180 million years ago: Pangaea had broken up into Laurasia and Gondwanaland
Figure 2.24a

48. Other Evidence of Plate Tectonics
Changing Positions of the Continents
135 million years ago: north Atlantic Ocean was opening; India was moving toward Asia
Figure 2.24b

49. Other Evidence of Plate Tectonics
Changing Positions of the Continents
65 million years ago: south Atlantic Ocean was opening; Africa and Europe had collided
Figure 2.24c

50. Other Evidence of Plate Tectonics
Changing Positions of the Continents
Present: India has collided with Asia; Eurasia and North America are separate; Australia and Antarctica are far apart
Figure 2.24d

51. The Grand Unifying Theory
Tectonic cycle:
Rising hot rock in mantle melts to liquid magma
Buildup of magma causes overlying lithosphere to uplift and fracture; fractured lithosphere is then pulled outward and downward by gravity, aided by convection in mantle
Asthenosphere melts and rises to fill fractures, creating new oceanic lithosphere
New oceanic lithosphere becomes colder and denser as it gets older and farther from the ridge where it formed
Eventually oceanic lithosphere collides with another plate; whichever is colder and denser will be forced underneath and pulled back down into the mantle
Lateral spreading may be aided by convection cells of mantle heat

52. The Grand Unifying Theory
Figure 2.25

53. The Grand Unifying Theory
Tectonic cycle:
When two plates collide, denser (colder, older) plate goes beneath less-dense (warmer, younger) plate in subduction
Oceanic plate beneath oceanic plate:
Volcanic island arc next to trench (Aleutian Islands of Alaska)
Oceanic plate beneath continental plate:
Volcanic arc on continent edge next to trench (Cascade Range)
Plate tectonics requires time perspective of millions and billions of years
Plate movement may be 1 cm/year  75 cm in human lifetime
Uniformitarianism: small events add up to big results

54. How We Understand the Earth
Must think in terms of geologic time rather than human time – thousands, millions and billions of years
In 1788, Hutton introduced concept of geologic time:
“No vestige of a beginning, no prospect of an end.”
Everyday changes over millions of years add up to major results
Uniformitarianism: natural laws are uniform through time and space; present is the key to the past
Contrast to previously believed catastrophism
Currently modified actualism: rates of Earth processes can vary

55. End of Chapter 2