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Plate tectonics. Creation and destruction of lithosphere. Plate tectonics and continent building Accretion through collisions Recycling of material Segregation of melts. The Rock cycle. Evolution of modern plate tectonics.

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creation and destruction of lithosphere
Creation and destruction of lithosphere
  • Plate tectonics and continent building
    • Accretion through collisions
    • Recycling of material
    • Segregation of melts
evolution of modern plate tectonics
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
slide5

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)

slide6

Present-day plate tectonics “begins”

{

Period of heavy bombardment

Period of major accretion

(~ 10-30 my)

alternative views
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
since the archaean
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
age of crustal material
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
slide11

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.

net result
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
slide13

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

slide14

S Atlantic opens

Antarctica moving S

India moving N

Australia separates

and moves N

slide15

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).
tectonic rock cycles

Tectonic Rock Cycles

Chemical evolution

slide17

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
slide18

Involvement of the hydrologic cycle

and biological processes

The Rock Cycle

rock cycle linked to ocean chemistry
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
hydrothermal solutions
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
weathering and erosion processes
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
slide23

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.
slide24

cation-rich Al silicates + H+ -> cation-poor clays + SiO2 + diss. cations

protons come from the hydration of atm. CO2 - produces bicarbonate (HCO3-)

slide25

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.

role of organisms in weathering
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?
slide28

river transport

Weathering is an important part of ocean/atmosphere CO2 cycle

CO2 removal

Fig. 8-17

slide29

river transport

CO2 removal

Fig. 8-17

slide30

river transport

CO2 removal

Fig. 8-17

slide31

river transport

CO2 removal

Fig. 8-17

weathering
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
slide33

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

slide35

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
slide37

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

slide38

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.

slide39

Negative

Feedback

Tectonic forcing

(addition of CO2)

slide41

(?)

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)

the sediment cycle
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
river plumes transport sediments
River plumes transport sediments
  • Mississippi R and the Gulf of Mexico
  • Frazier River
  • World’s big rivers
volcanoes
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
dust plumes
Dust plumes
  • Volcanoes (Mt. Pinatubo)
  • Deserts – Sahara dust signal across Atlantic
slide46
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)
ice as a transport agent
Ice as a transport agent
  • Move rocks in glaciers (e.g., morraines, erratics)
  • Find sediments far from their sources
organisms as transport agents
Organisms as transport agents
  • Kelp
  • Birds
  • Sea Lions (swallow stones for ballast)
  • Unpredictable patterns
early oceans
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
early oceans50
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
controls on the chemical concentration of seawater
Controls on the chemical concentration of seawater
  • Assume rivers are the predominant source of materials to the ocean
early models uni directional formation of the oceans
Early models:Uni-directional formation of the oceans

igneous rocks + “excess” volatiles  seawater + sediments + air

Gives you a salty ocean…. Okay with respect to concentrations of non-volatiles

But it does it too quickly (with respect to age of the ocean) !!!!!! (~100 my or so) (see page 92 of text – this gets you the average residence time of salt in the oceans ~ 100 MY)

concept of residence time
Concept of residence time
  • Time water (or anything else) spends in any one reservoir on average.
    • Units of time [volume/(volume/time)] or volume/flux
  • Larger reservoirs often have longer residence times
    • Residence time in the ocean is long
    • Implications for dumping garbage in the ocean!
  • But, residence times vary depending on fluxes
    • Implications for water quality and planning
slide54

Budgets in a nutshell

Concentration

Pool Size

Maintained

Exports

Inputs

Concentration

Accumulates

Inputs

Exports

Concentration

Declines

Inputs

Exports

recycling
Recycling?
  • Tectonic and rock cycles
  • Inputs via weathering reactions
  • Removal and recycling in the crust
  • Consistent with previous discussions
slide56

But, seawater is not just concentrated river water!

CaCO3 + CO2 + H2O  Ca2+ + 2HCO3-

cation-rich Al silicates + H+ cation-poor clays + SiO2 + diss. cations

Processes lead to formation of highly alkaline (pH 10) soda lake

e.g., Dead Sea or Great Salt Lake

sources of major ions to seawater
Sources of major ions to seawater
  • Rivers
  • Mechanical and chemical weathering
    • Breaking
    • Reactions
      • Dissolution (calcite, halite)
      • Acid-base (carbonic acid + igneous rocks)
      • Products are cations, diss. silica, clays (Al-silicates), bicarbonate
  • Products of weathering are clay minerals
  • A variety of processes are responsible for removal of these elements to maintain steady state concentrations
where do salts come from
Where do salts come from
  • Difference is both in absolute and relative concentrations
  • Composition of salts in water  composition in crustal rocks
  • Composition of salts in water  composition in rivers or salty lakes
  • Principal ions in seawater are Na+ and Cl- while principal ions in rivers are Ca2+ and HCO3-
  • Where do excess volatiles (constituents that are not accounted for by weathering of surface rocks) come from?
we ll talk about that next time but
We’ll talk about that next time but…
  • Upper mantle – contains more of the substances in seawater including water
  • Hydrothermal alteration
  • Excess volatiles include carbon dioxide, chlorine, sulfur, hydrogen, fluorine, nitrogen and water
  • Some constituents are present at concentrations lower than expected (e.g., magnesium and sulfate) – mineral deposits, mid-ocean rifts, biological processes
  • Some ocean solutes are hybrids of weathering and outgassing (e.g., sodium chloride)
why isn t the ocean getting saltier
Why isn’t the ocean getting saltier?
  • Some salty lakes do
  • Chemical equilibrium – proportion and amounts dissolved per unit volume are nearly constant
  • Inputs must equal exports (remember from lecture 4)
  • Steady state ocean
  • Idea of residence times of particular salts
    • Residence time = total amount of element/rate at which element is added or removed
major ions have been constant
Major ions have been constant
  • Inputs = outputs
  • Concentrations don’t change with time
    • dC/dt = 0
  • Box model to calculate residence time (t)
  • t = AT/(dA/dt)
    • Total amount in the ocean AT
    • Removal or input rate (dA/dt)
slide64

Budgets in a nutshell

Concentration

Pool Size

Maintained

Exports

Inputs

Concentration

Accumulates

Inputs

Exports

Concentration

Declines

Inputs

Exports

Reservoirs, fluxes and residence times

residence times fluxes
Residence times & fluxes
  • The reservoir is the ocean
  • Inputs from weathering and outgassing
  • Exports due to sedimentation (including biological particles, adsorption of reactive particles and precipitation of minerals), subduction
  • Residence time depends on chemical activity
  • Distribution of element depends on residence time relative to ocean mixing times (ocean mixing time is on average 1600 years or so)
  • Long residence times ensure complete mixing and is the foundation for principle of constant proportions
slide66

Ocean water

residence time

of about 4100

years based on

precipitation and

evaporation and

the known volume.

conservative and nonconservative constituents
Conservative and nonconservative constituents
  • Conservative constituents occur in constant proportions or change very slowly (long residence times) – distributions affected by physical mixing and diffusion
    • Include major salts
  • Nonconservative constituents are often tied to biological or seasonal cycles or very short geological cycles (short residence times)
    • Include oxygen carbon dioxide, silica, calcium, iron, aluminum, nitrogen & phosphorus
  • Many trace elements have distributions that are nonconservative
reactivity
Reactivity
  • Order of reactivity Si>Ca>Na
  • Biological processes
  • Major ions unreactive so have long residence times
  • Salinity variations caused by evaporation or precipitation
slide72

Salinity map showing areas of high salinity (36 o/oo) in green, medium salinity in blue (35 o/oo),

and low salinity (34 o/oo) in purple. Salinity is rather stable but areas in the North Atlantic,

South Atlantic, South Pacific, Indian Ocean, Arabian Sea, Red Sea, and Mediterranean Sea

tend to be a little high (green). Areas near Antarctica, the Arctic Ocean, Southeast Asia,

and the West Coast of North and Central America tend to be a little low (purple).

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