Plate tectonics

1. Plate tectonics

2. Creation and destruction of lithosphere • Plate tectonics and continent building • Accretion through collisions • Recycling of material • Segregation of melts

3. The Rock cycle

4. Evolution of modern plate tectonics • Presence moderate temperatures – Venus is too hot so lithosphere never cool enough to subduct • Heat removal from mantle through subduction of cool oceanic lithosphere and upwelling of new crust • Drives convection cells • Allows basalt eclogite transition to be shallow • Subduction leads to fractional melting of oceanic crust and segregation to form continental crust • Presence of water • Needed for granite formation • Catalyzes fractional melting in subducting sediments

5. Archaen-Proterozoic transition To modern plate tectonics • 1. Early plates became bigger and thicker • 2. Continued recycling of oceanic crust • formed large amounts of buoyant • continental crust • Continued partial melting/distillation • Separation of Si and other elements from • Mg and Fe • Conversion of mafic material to felsic • material through rock cycle • 3. Decrease in heat production slowed mantle • convection • Drove system to larger convection cells • Allowed larger plates to travel farther • on the Earth’s surface and cool more • Led to subduction rather than collision of • plates • Modern plate tectonics Present-day plate tectonics “begins” period of rapid crustal growth { Period of heavy bombardment Period of major accretion (~ 10-30 my)

6. Present-day plate tectonics “begins” { Period of heavy bombardment Period of major accretion (~ 10-30 my)

7. Alternative views • Does life play a role? (Gaia) • Earth is only planet with life AND plate tectonics • Is there a connection? Cause-effect? • See Lovelock work • Life affects weathering and calcite deposition

8. Since the Archaean • Intensity of plate tectonics has varied over time • Wilson cycles – 500 my cycles • Evidence of supercontinent 600-900 mybp • Pangea formed ~ 300 mybp • Causes not well understood • Periods of rapid sea floor spreading (and vice versa) • Sea level rises because large amounts of shallow basalt form and don’t cool (and subside) much • High CO2 release – released at spreading centers when new crust forms and subducting crust has sediment on it including calcite which releases CO2 when it melts

9. Age of crustal material • Continental crust is older because it doesn’t get subducted • Too buoyant • Becomes “core” for accretion • Collisions (closing of basins) mediate accretion • Losses only from weathering and subduction of sediment • Oldest rocks are 4.3 – 4.4 by old • Oceanic crust is young and constantly recycled (and fractionated) • Oldest oceanic crust is furthest from spreading centers near subduction zones

11. Figure 8.18 Map of a closed Atlantic Ocean showing the rifts that formed when Pangaea was split by a spreading center. The rifts on today's continents are now filled with sediment. Some of them serve as the channelways for large rivers.

12. Net result • Spreading rates at transform faults • Pacific plate moves NW at 8 cm/yr • N American plate moves W at 2 cm/yr • Indian plate moves NE at 12 cm/yr • Pacific Ocean is shrinking and Atlantic is growing • Atlantic opened about 200 MY ago so there should be no rocks older than this in the Atlantic

13. Most recent episode of Seafloor spreading: Pangaea first broke into 2 pieces Sea opens between N and S continents and Between Africa and Antarctica India moves North

14. S Atlantic opens Antarctica moving S India moving N Australia separates and moves N

15. 50 MY in the future: • 1. Africa will move N and close Mediterranean Sea • E Africa will detach (Red Sea rift zone) and move to India • Atlantic Ocean will grow and Pacific will shrink as it is • swallowed into Aleutian trench. • W California will travel NW with the Pacific Plate (LA will • be swallowed into the Aleutian trench in 60 MY).

16. Tectonic Rock Cycles Chemical evolution

17. Creation and destruction of lithosphere • Rock cycle • Weathering destroys continental crust • Materials deposited in sediments • Some subducted and recycled through melts • Some added to continents through collisions • Links to hydrological and biological cycles

18. Involvement of the hydrologic cycle and biological processes The Rock Cycle

19. Rock cycle linked to ocean chemistry • Processes affect ocean chemistry and elemental cycles • Seawater circulates through mid-ocean ridges • Chemical reactions between water and fresh, hot basalt • Hydrothermal fluids have very different composition than seawater (loss of Mg2+ and sulfate, addition of silica and trace metals) • Major role in cycling of some elements in the oceans • Balances riverine inputs (Mg2+ and bicarbonate) • Hydrothermal alteration

20. More on this later with ocean chemistry

21. Hydrothermal solutions • Very acidic – adds protons (H+) to the oceans and helps remove riverine bicarbonate • Titrates bicarbonate back to CO2 • Returns CO2 to the atmosphere

22. Weathering and erosion processes • Weathering of continental crust creates soils • Mechanical weathering • Chemical weathering • Cation-rich Al-silicates + protons (H+)  Cation poor clays + SiO2 + disassociated cations • Different minerals show different stabilities • Weathering is a primary source of major ions to seawater (cations + and anions -) • Major role in controlling ocean composition • Source of protons is hydrated atmospheric CO2 • Rivers transport bicarbonate to the ocean • Atmospheric CO2 sink

23. cation-rich Al-silicates + H+ • cation poor-clays + SiO2 + diss. cations • protons came from acidic excess volatiles • left behind their anions (Cl-, S-2 and HCO3-) • these anions and the cations weathered from rocks led to an increase in the salt content of the early oceans.

24. cation-rich Al silicates + H+ -> cation-poor clays + SiO2 + diss. cations protons come from the hydration of atm. CO2 - produces bicarbonate (HCO3-)

25. Weathering transports bicarbonate to the oceans so it is a CO2 sink H2O + CO2 H2CO3 CO2 removal Bicarbonate transport (consumes H+) Fig. 8-17 Pictorial representation of the carbonate–silicate geochemical cycle.

26. Role of organisms in weathering • Plants accelerate weathering • Mechanical • Chemical • Secrete organic acids • Enhance build-up of CO2 in soils • In the absence of life, pCO2 would have to be much higher so that weathering rates (consumption of CO2) balances CO2 inputs (from vulcanism, metamorphism and diagenesis) • Is this Gaia feedback?

27. Biological involvement in chemical and mechanical weathering

28. river transport Weathering is an important part of ocean/atmosphere CO2 cycle CO2 removal Fig. 8-17

29. river transport CO2 removal Fig. 8-17

30. river transport CO2 removal Fig. 8-17

31. river transport CO2 removal Fig. 8-17

32. Weathering • An acid base reaction • Anions left behind are Cl-, S-2, HCO3- • Weathering produced anions and cations that increased the salt content of the early oceans • At present day weathering rates this could have occurred fairly rapidly (100’s of millions of years) • As the pH rose above ~7.5, carbonate minerals (CaCO3) began to precipitate • Began to buffer the pH of the oceans • Biological or chemical precipitation - stromatolites • Led to large drop in atmospheric CO2 • Initial atm likely had higher total CO2 • Most of this CO2 now sequestered in carbonate rocks

33. the pH rose above approx. 7.5, carbonate minerals (CaCO3) began to ppt • began to buffer the pH of the oceans 3.5 by old stromatolite from the Warrawoona formation in Australia

35. Estimated size of C reservoirs(Billions of metric tons) • Atmosphere • Soil organic matter • Ocean • Marine sediments & sedimentary rocks • Terrestrial plants • Fossil fuel deposits • 578 (as of 1700) to 766 (in 1999) • 1500 to 1600 • 38,000 to 40,000 • 66,000,000 to 100,000,000 • 540 to 610 • 4000

37. CO2 The Carbonate-Silicate Cycle and Long-Term Controls on Atmospheric CO2 CO2 CO2 Weathering of silicate rocks Ions (and silica) carried by rivers to oceans Ca2+ + 2HCO3- (+ SiO2[aq]) Organisms build calcareous (and siliceous) shells + SiO2 CaCO3 + CO2 + H2O (+ SiO2(s)] CO2 Subduction (increased P and T) CaCO3 + SiO2 CaSiO3 + CO2 CaSiO3 + 2CO2 + H2O  Ca2+ + 2HCO3- + SiO2

38. Fig. 8-18 Systems diagram showing the negative feedback loop that results from the climate dependence of silicate–mineral chemical weathering and its effect on atmospheric CO2. This feedback loop is thought to be the major factor regulating atmospheric CO2 concentrations and climate on long time scales.

39. Negative Feedback Tectonic forcing (addition of CO2)

40. Negative feedback on temp. and lowering of CO2 Incr. solar luminosity

41. (?) Present-day plate tectonics “begins” Onset of early “weathering” (perhaps earlier) { { Condensation of water vapor Period of heavy bombardment Accumulation of excess volatiles (Cl [as HCl]; N [as N2]; S [as H2S]; CO2) Period of major accretion (~ 10-30 my)

42. The Sediment Cycle • Mountains rise • Rocks erode (water and wind) • Sediments are deposited • Sediments uplifted or subducted • 15 billion metric tons (16.5 billion tons) of sediments moved by rivers each year! • 100 million metric tons moved by air

43. River plumes transport sediments • Mississippi R and the Gulf of Mexico • Frazier River • World’s big rivers

44. Volcanoes • Come from ash ejected during eruptions, carried by winds and rivers. • Aeolian transport – dust • Dust and climate – trace metals, cooling, nuclear winter, asteroid impacts and extinction events

45. Dust plumes • Volcanoes (Mt. Pinatubo) • Deserts – Sahara dust signal across Atlantic

46. Dust • Dust carried in the atmosphere is < 2 mm • Limit for clean air (US Gov) is 150 mg/m3 (LA is 1250; avg over US cities is 100-125) • 75% of sediments in the N Pacfic, 64% of those to the S Atlantic and 30% of those to the equatorial Atlantic arrive by wind (mostly from deserts – Mohave and Sahara)

47. Ice as a transport agent • Move rocks in glaciers (e.g., morraines, erratics) • Find sediments far from their sources

48. Organisms as transport agents • Kelp • Birds • Sea Lions (swallow stones for ballast) • Unpredictable patterns

49. Early oceans • With onset of these combined reactions cation concentrations reached steady state • Steady state is not chemical equilibrium • Steady state is just input = output; constant concentration • Over last 700 MY concentrations of major ions in seawater have probably not changed by more than a factor of 2 (2x or 0.5x present) • SW composition constrained by distribution of evaporite minerals in geological record • Major changes in SW composition would lead to different evaporite mineral sequence

50. Early oceans • Surface waters were much warmer (~50oC) • Ancient ocean had no dissolved oxygen (no free O2 in atm) • Sulfate content much lower and primarily as H2S, not SO42- • CO2 much higher than today so lower pH • No precipitation yet • Fe was reduced - Fe (II) • So soluble, after oxygen concentrations increased this changed, had Fe(III) which is insoluble