1. Great Ideas in Science: Lecture 9 – Earth as a Planet
Professor Robert Hazen
PROV 301
Great Idea: Earth, one of the planets that orbits the Sun, formed 4.5 billion years ago from a great cloud of dust and gas.

2. Today: Key Ideas About Earth
Earth, one of the planets that orbits the Sun, formed 4.5 billion years ago from a great cloud of dust and gas.
2. The surface of our planet changes constantly; no feature is permanent.
3. Earth is still changing, due to the slow convection of soft, hot rocks deep within the planet.
4. Earth materials move in cycles; a change in one cycle affects others.

3. Clues to the Origin of the Solar System
Solar System = Objects gravitationally bound to the Sun

4. Clue #1: Planetary Orbits
Features of solar system
All planets orbit in the same direction
All planets orbit in the same plane
Most planets rotate in the direction of orbit

5. Clue #2: Distribution of Mass
Almost all mass is in the Sun (99.9%)
Two types of planets
Small, rocky inner planets [“Terrestrial”]
Gas giant outer planets [“Jovian”]
Other objects
Moons, asteroids, comets

6. A Hierarchy of Scientific Ideas
Fact (a confirmed observation)
Hypothesis (an educated guess)
Law (a predictive mathematical description of nature)
Theory (a well established explanation of nature)

7. The Nebular Hypothesis

8. Rocky (Inner) Planets Mercury, Venus, Earth, Mars
These planets are rocky, dense, and relatively small.

9. Gas Giant (Outer) Planets
Jupiter, Saturn, Uranus, Neptune:
These planets are rich in gas, low density, and much larger than the terrestrial

10. Pluto and the Plutoids Pluto has only 0.3% of Earth’s mass
Pluto has three moons
Could be a captured comet or asteroid?
Is it a planet?

11. Asteroids, Comets, and Meteors
“Small” rocky bodies
They orbit the Sun
Many in asteroid belt
Comets
They’re like dirty snowballs
They have highly elongated orbits
Stardust and Deep Impact missions
Meteors and Meteorites
Original solar system material
Watch for meteor showers!

12. The Formation of Earth
The Great Bombardment

13. Earth’s Layers (think onions)
Heat was produced from collisions and by radioactive elements.
Dense material sank to the center, while lighter material rose to the surface
The result: A layered structure
Core
Mantle
Crust

14. Volcanoes and Earthquakes: Evidence of Earth’s Inner Forces
Volcanoes: Magma breaks through surface

15. Volcanoes and Earthquakes: Evidence of Earth’s Inner Forces
Rocks breaks along a fault.
Energy is transmitted as a wave through solid rock.
Tsunamis are ocean waves after big earthquakes.

16. The Case of the Disappearing Mountains
Old
Young

17. James Hutton at Jedburgh Scotland
Deep Time
James Hutton at Jedburgh Scotland
ca.1790

18. Deep Time
Sediments
Burial
Uplift

19. Plate Tectonics
Great Idea: Earth is constantly changing, due to the slow convection of soft, hot rocks deep within the planet.

20. Plate tectonics – 6 lines of evidence
Physiology: Similar shapes of continents

21. Plate tectonics – 6 lines of evidence
Physiology: Shape of continents
Seismology: Distribution of earthquakes

22. Plate tectonics – 6 lines of evidence
Physiology: Shape of continents
Seismology: Distribution of earthquakes
Geology: Distribution of rocks and fossils

23. Plate tectonics – 6 lines of evidence
Physiology: Shape of continents
Seismology: Distribution of earthquakes
Geology: Distribution of rocks and fossils
Oceanography:
Sea floor topography
Bruce Heezen and Marie Tharp

24. Plate tectonics – 6 lines of evidence
Physiology: Shape of continents
Seismology: Distribution of earthquakes
Geology: Distribution of rocks and fossils
Oceanography: Sea floor topography
Volcanology: Ages
of Atlantic volcanoes
<5 95 75 55
125

25. Plate tectonics – 6 lines of evidence
Physiology: Shape of continents
Seismology: Distribution of earthquakes
Geology: Distribution of rocks and fossils
Oceanography: Sea floor topography
Volcanology: Ages
of Atlantic volcanoes
<5 95 75 55
125

26. Plate tectonics – 6 lines of evidence
Physiology: Shape of continents
Seismology: Distribution of earthquakes
Geology: Distribution of rocks and fossils
Oceanography: Sea floor topography
Volcanology: Ages of Atlantic volcanoes
Paleomagnetism of sea floor rocks

27. New Support for Plate Tectonics
Satellite measurements of the distance between continents
North America and Europe are separating at ~5 cm per year

28. Earth’s Tectonic Plates

29. Three Main Boundary Types
Plate Boundaries
Three Main Boundary Types
Divergent
Convergent
Transform

30. Divergent Boundaries

31. Convergent Plate Boundaries
Three Types:
1. Ocean-ocean

32. Convergent Plate Boundaries
Three Types:
1. Ocean-ocean
2. Continent-continent

33. Convergent Plate Boundaries
Three Types:
1. Ocean-ocean
2. Continent-continent
3. Ocean-continent

34. Transform Plate Boundary

35. Another Look at Volcanoes and Earthquakes
Volcanoes Occur:
1. At Divergent Plate Boundaries
2. Close to Convergent Plate Boundaries
3. Above Hotspots

36. Another Look at Volcanoes and Earthquakes
Earthquakes Occur at:
Transform Plate Boundaries
Convergent boundaries

38. Earth’s Geochemical Cycles
Earth materials move in cycles;
a change in one cycle affects the others.
1. Atoms recycle; they’re used over and over.
Reservoirs (Aluminum can)
Hydrologic cycle
Atmospheric cycle
2. Energy flows through the system.

39. Geochemical Cycles Account for all the repositories of that substance.
Document processes by which the substance moves from repository to another.
Gold

40. The Carbon Cycle

41. Atmospheric Cycle Air mass: Uniform temperature and moisture
Weather: State of the atmosphere
Climate: Long-term average of weather

42. The General Circulation of the Atmosphere
Circulation powered by Sun:
Air is heated and then rises at the Equator.
Air cools and sinks at the poles.
The prevailing winds are caused by Earth’s rotation.

43. Five variables define the state of atmosphere, i.e. the weather:
Temperature
Air pressure
Humidity
Cloudiness
Prevailing winds

44. Climate
Climate, unlike weather, refers to gradual change over long periods
Influences on climate include:
Large bodies of water
Ocean currents
Mountain ranges
Position of tectonic plates
Solar output
Greenhouse gases

45. The Water (Hydrologic) Cycle
The total amount of Earth’s water is constant; the same atoms cycle from one reservoir to another. The total volume is ~1.4 billion km3
Oceans 97.3%
Lakes and Rivers 0.01%
Groundwater 0.6%
Ice Caps & Glaciers 2.1%
Atmosphere 0.001%
All life %

46. How does water move among reservoirs?

47. How does water move among reservoirs?

48. Ocean Currents
Redistribute heat across planet

49. Cycle tied to Earth’s tilt and orbit
Ice Ages
As the ice caps and glaciers grow, the sea level drops.
Cycle tied to Earth’s tilt and orbit

50. Two facts about water use
A human requires about 2 liters of water per day to survive
The average American uses about 6,000 liters of water per day

51. The Water Table
The water table will drop when discharge exceeds recharge (like a bank)
In the US we “mine” about 100,000,000 gallons of water every day (more than the recharge)
Artificial recharge helps
Urbanization and pollution hurt