GEOS 251 — Physical Geology

1. GEOS 251 — Physical Geology 21 January 2014 • Announcements • No labs this week; they begin next week • People who want to add the class • Office Hours: Wed 3:00-4:00pm, Thurs 2:00-3:00pm • Today • Lecture #2: The Dynamic Earth • Handout • Lecture summary

2. Society of Earth Sciences Students http://www.geo.arizona.edu/SESS/ A club for all UA students interested in Earth Science, regardless of major. MEETINGS: Every Friday at Noon Gould-Simpson Room 201 Free pizza! • Activities: • Field Trips • Guest Speakers • Social Events • Education Outreach

3. Compliments • You were great last lecture • Good interaction • Thank you for filling out the questionnaire

4. Keys to doing well in this course • Attend the lectures • Reading the slides on line is a poor substitute for hearing the “voice over” and associated class discussion • My experience shows: mean scores of students who attend lectures is at least one grade level above the mean of those who do not regularly attend lectures • Be present for all of the exams, and turn in all lab and field trip assignments • When you miss an activity, you earn a zero • This is far below the lowest failing grade for an assignment and thus is hard to compensate for later in the course • Students who attend lectures and turn in all assignments rarely earn less than a C in this class • Advice • Study and review the lecture summaries and do the reading • If you are not interested in attending lectures and completing the work • Please drop the class soon to make room for those who would like to take it

5. Last Time • Introductions and syllabus • Geology–study of the earth • Natural science, importance of time • Scientific method • Hypothesis (e.g., continental drift); criticism and repeated questioning • Theory (e.g., plate tectonics) • Fact (e.g., radius of Earth) • Nucleosynthesis–Origin of the elements • Origin of Solar System–Nebular hypothesis • Cataclysmic impact–Earth-Moon system • Radius of Earth 6370 km • Relationship to Earth’s composition and divisions

6. Common elements in Earth (why? exceptions?)

7. Where do elements originate?Nucleosynthesis in stars • Hydrogen and helium the starting materials • Series of nuclear reactions related to stellar evolution • Hydrogen burning (4 1H to 4He), followed by • Helium burning 3 4He to 12C, the adding more 4He >> • Advanced burning (e.g., 16O, 20Ne, 24Mg, 28Si) • Equilibrium process (Fe peak, most stable element) • Neutron-capture reactions (heavier elements) in supernova • Basic to understanding abundance of elements

8. Meteorites Evidence for the nebular building blocks of Earth iron meteorite(planetesimal core) chondritic meteorite(silicate and volatile-rich fraction)

9. Accretion; formation of Moon All in ≤ 50 million years (≤ 1% of Earth history)

10. Some key questions • What are the major divisions of the modern Earth? (observation and description) • How did they get to be that way? (process) • When did they get to be that way? (time) • Which are active today? Are any “dead”? Upcoming... • What drives activity? How? • How “fast” is active? And why?

11. Radius 6370 km

12. Differentiation Process Homogeneous early Earth Melting, differentiation—iron sank to center, light material floated to surface; promoted escape of volatiles Cooled; mostly solidifieddense core, light crust, residual mantle in between (largest volume)

13. Differentiation • Rocky mass of accreted planetesimals converted into layered Earth (r=6370 km) • Core (solid iron inner; liquid iron outer) • Mantle • Crust • Continental, oceanic • Fluid envelope—Hydrosphere, atmosphere • Early in Earth’s history, planet partially melted • Heat sources: accretion / impact, core formation, radioactive decay

14. Early Earth Time Line nebula-accretion-moon-crust Nearby supernova contributes distinctive chemical isotopes (e.g.,26Al, pure 16O)

15. How Old is Earth? • Estimated at 4.56 billion years (Ga), but • Oldest earth minerals are zircons at ~4.4 to 4.2 Ga, from Western Australia • Oldest earth rock from Greenland at ~3.8 Ga Why did the earth’s odometer start so late?

16. What might we infer about the outer part of the Earth? • From the existence of ocean basins and continents? • High versus low: Differences in properties or forces • From knowing that the outer “solid” parts of the Earth are active? • Active: parts move; cannot behave as solidly as we may think

17. Lecture 2: The Dynamic Earth • Last time: Earth as a planet • Dynamic processes • Physical properties, energy sources, and responses • Time scales and geologic time • Plate tectonics • Evidence, mechanism, and importance • Next time: Minerals

18. Density separation • Early Earth: molten, started differentiation early, at about 4.5+ Ga. Oldest minerals ~4.2 Ga • Densest compounds sink, forming a three-fold division of the earth (based on composition): • Highest density core: Fe and Ni; solid inner (intense pressure), liquid outer • *solids usually occupy less space, except water • Intermediate density mantle - Fe, Mg; hot, plastic, deforms like putty except for the upper part--the bulk of the Earth • Low-density crust- Si, O, Al, Na; cool, rigid. The outer 5-65 km, thin.

19. • Relative strengths and densities • - Stacked in order of density • - Cooler = stronger (for same material) • - Why is thicker crust higher?

20. Outer Earth: Chemical vs. Strength Divisions • Chemical division: Crust vs. mantle • Strength division: Lithosphere vs. asthenosphere • Lithosphere (= crust + upper mantle) • Cooler, strongerlithospheric plates • Asthenosphere (= lower mantle) • Hotter, weakplastic

21. Given the division between fluid and rigid behavior how might the Earth’s crust accommodate movement?

22. Driving forces: What are the energy sources for the dynamic Earth? • Internal • Sources: • Residual from Earth’s origin • Continual radioactive decay • Gravitational energy • Results: • Move lithosphere, build mountains, make volcanoes • External • Source: Solar energy • Results: • Energizes atmosphere, oceans (climate, weather) Relates to topics later in the semester

23. Heat Transfer • Manner in which heat is redistributed; drives plate tectonics • Radiation: Heat from a fire (across the room), or from the Sun • Conduction: Heat transferred along contacts • Convection: Heat transferred by overturning of cold and hot air/liquid masses; convection of mantle occurs in response to redistributing energy from the sun and from the Earth’s interior

24. Internal heat drives convection making for an active planet • Where does (did) the heat come from? • —Accretion / impact, core formation, radioactive decay

25. Geochemical Cycles Movement and transformation of materials driven by internal and solar energy; they have many different time scales

26. How fast? How long? How fast do these processes take place? Are they all the same? How long have they been operating? What are the consequences?

28. Geologic Processes and Time • Many different time scales — Some fast, some slow— From <1 second to 100’s of millions of years • Uniformitarianism vs. catastrophism— Continuous vs. episodic— “The present is the key to the past” (James Hutton) • Geologic time and time scale(return to in a few weeks)

29. Geologic Time Scale • How high on you? Review question from handout for first lecture

30. Question If your height is proportional to the age of the Earth, where would the following key events fall? • Accretion? • Bottom of feet • Oxygenation of the atmosphere (approximately the Archean-Proterozoic boundary)? • Upper leg • The Cambrian "explosion" (evolution of complex fauna at the beginning of the Phanerozoic)? • Neck • The Mesozoic Era (age of dinosaurs)? • Eye brows • The Ice Ages (Pleistocene to Recent)? • Scalp • Recorded human history (ca. last 5000 years)? • Thickness of a hair

31. Times Scales and Geologic Time • Earth processes have greatly different rates depending on energy and materials • Convection in the atmosphere: ~0.01 year • Convection in the mantle: ~100 million years • Convection in the ocean: ~1000 years • How does does the flow of thermal energy (heat flow) correlate with these?

32. Tectonic heat flow:10-2 watts per m2 Atmospheric heat flow:103 watts per m2

34. The Moon and Mars - Early activity, now inactive- Why?

35. Loss of heat

36. The active solid Earth The hypothesis of continental drift The theory of plate tectonics

37. Reminder of Scientific Method • Science: Observations, questions, experiments • Hypothesis (continental drift) • Tentative explanation based on data collected through observation and/or experiment • Exposed to criticism, repeated testing • Theory (plate tectonics) • Survived repeated challenges, has strong support; mechanism postulated and evidence found • Theories someday may be considered fact, but are never immune to question nor are they finally proven

38. Continental Drift and Plate Tectonics • Early maps suggested match across Atlantic--“continental drift” (A. Wegener, 1915) — hypothesis • Observations made; rates of movement and forces responsible (mechanism) were postulated, but these turned out to be incorrect • Plate tectonics (1960’s) — theory • New observations made; mechanism postulated, and supporting evidence was found

39. Continental Drift Historic development of ideas based on geologic observations — but lacked a mechanism that worked Observational evidence: • Jigsaw-puzzle fit of continents • Trends of rock types and structures • Terrestrial fossils (e.g., Mesosaurus) • Glacial deposits and other ancient climate indicators (e.g., late Paleozoic glacial deposits currently near equator)

40. Mesosaurus—Permian reptile • Fossils of non-swimming reptile present in South America and Africa

41. The key to developing plate tectonics:Sea-floor spreading • Mid-ocean ridges long recognized, but magnetic structure only known since WWII (anti-submarine warfare) • Symmetric distribution of magnetic stripes and age of ocean crust was the key clue • Demonstrated youth of ocean basins and continual formation of crust • Mechanisms related to mantle convection

42. Topography and symmetry of the ocean floor At Mid-Atlantic Ridge SW of Iceland

43. Plate Tectonics • The unifying theory of global geology • Plates: physical definition • Plate boundaries (3 kinds) • Constructive (divergent) • Destructive (convergent) • Transform (strike slip) • Fits many observations • Geologic (e.g., continental drift, paleoclimate) • Geophysical (e.g., earthquakes, heat flow) • Petrologic (e.g., origins of magmas and basins)

45. Plate boundaries: Three types • Divergent(mid-ocean ridges - new crust forms) • Convergent(trenches and continental collision zones - crust is destroyed) • Transform faults (crust is conserved)

46. Divergent boundaries Divergence—the crustal birthplace Plates move apart as magma upwells from asthenosphere, forming new crust e.g., mid-ocean ridges (“sea-floor spreading”): East Pacific Rise, Mid-Atlantic Ridge (above sea level at Iceland), Red Sea

47. Convergent boundaries Convergence—the crustal graveyard Continent-oceanic collisions Denser crust “subducts” beneath less dense crust; at depth, the subducted crust is melted and surfaces along volcanic chains e.g., Andes, Japan, Pacific Northwest Continent-continent collisions No subduction: big mountains are built (example?)

48. Mustang Valley, Nepal

49. What type of boundary ?

50. Divergent and transform boundaries