Ocean Ridges and Transforms Flashcards
Ocean Ridges and Transforms
- Oceanic lithosphere
- Hydrothermal circulation at ridges
- Axial magma chambers
- Transform faults and ridge segmentation
Heat flow, q
- Heat flow density, mW/m^2
- Energy per unit time (watts) flowing through a unit area
- Fourier’s Law
Fourier’s Law
Heat flow, q = -k (dT/dz)
- Where dT = temp change, dz = thickness, k = thermal conductivity
Heat flow at Earth’s surface
- dT/dz = 20-30 degrees K/km, k = 2-3W/m/degree K
- q = 40-90 mW/m^2
- Continents = 55mW/m^2
- Oceans = 80-90mW/m^2
q at continents
55mW/m^2
- approximately 1/2 is crustal radioactivity
q at oceans
80-90mW/m^2
- approximately 75 percent of Earth’s heat flow
Heat Flow Probe
- Temperature gradient measured over known distances (drill hole up to a few km, sediment probe 3m length)
- Conductivity measured in lab, or in situ using decay of a heat pulse from the probe
What are the 2 plate models?
- GDH1
- PSM
Half-space
- Boundary layer cooling model
- Material cools and contracts as it moves away from ridge
- Surface layer cools from top down
- Lithospheric thickness can be calculated
Lithosphere, HS model
- Defined as region w/ temp below certain value
- eg. base of lithosphere = 1100C or 1300C
- Thickness increases away from ridge
- Can calculate thickness using model
HS Lithospheric thickness calculation
- L = 11 x sq.root t
- q = 1/sq.root t
- Where L is thickness in km, t is age in Ma, q is heat flow
- Thickness increases with age
HS cooling model, seafloor depth d
- From age or distance from ridge
- As material cools, density increases
- Isostasy leads to calculation
- d = 2.5 plus 0.35 x sq.root t
- Implies typical ridge depth is 2.5km
What are the exceptions to the HS model for depth of ridge?
- Typical depth is 2.5km
- Iceland = 0km
- Pacific-Antarctic Discordance zone = 3km
HS boundary layer cooling model comparison with observations
- Heat flow is too low for ages greater than 120Ma
- Depths are too large for ages greater than approximately 70Ma
- Model lithosphere continues to cool w/out limit but a constant rate of cooling must be reached (about 70Ma)
Plate model
- Assumes fixed lithospheric thickness L of 95km
- Assumes fixed temperature at base of lithosphere and vertical boundary below ridge (1450C, Stein model)
- Far from ridge the plate is far from high T influence and equilibrium is reached, constant heat loss
GDH model
- Relationships for depth and heat flow
- Different eons for different age ranges
- T< or > 20, T< or> 55
Problem with plate models
- Seismic evidence suggests lithosphere is thinner under ridge
- Therefore thickness, L, cannot be constant
Both models (HS and GDH) vs. Heat flow
- Plate models fit depth observations better than heat flow
- Both models over-predict heat flow in young lithosphere
- Large data scatter near ridge
- Therefore hydrothermal circulation has an influence
Hydrothermal flow at ridges
Seawater near ridges:
- Penetrates and cools new ocean crust through cracks
- Heated and driven out at hydrothermal vents
- Carries away heat by convection rather than conduction
Black smokers
- Leach things out of rocks and then precipitate metal sulphides at vent and change minerals in basalt to be more hydrous
- Organisms use chemosynthesis to survive in this environment w/ no light
Seismic data from Juan de Fuca ridge
- Regimes for hydrothermal flow
- W/ distance from ridge, open to sediment sealed circulation, changes in heat flow, fluids, seismic velocity
- Effect of basement highs, forced fluid flow
Transition from open to sediment-sealed hydrothermal circulation
- Heat flow and basement temperature increase away from ridge
- Hydrothermal circulation nearer ridge cools younger rock more than expected
- Increasing seds covering and filling cracks away from ridge decrease hydrothermal circulation (hemipelagics settling out from column, turbidites and mixing/continental influence further from ridge)
Near ridge envr
- Seds: Very thin hemipelagics
- Heat flow much lower than expected
- Basement Temp 10C
- Pore fluids like seawater
- Seismic layer 2A velocity 3.0-3.5km/s
20km from exposed basement envr
- Seds: Turbidites, provide a hydrologic barrier
- Heat flow approaches expected value
- Basement Temp 40-50C
- Pore fluids depleted in Mg, enriched in Ca, elevated chlorinity
- Seismic layer 2A velocity >5km/s
Implications of seds, heat flow, and basement temps for near ridge
- Near-ridge hydrothermal circulation
- Open fissures, cracks
- Carries heat away
Implications of pore fluids and seismic velocities near ridge vs. 20km away from exposed basement
- Alteration due to hydration reaction in crust
- Porosity, permeability decreases due to cracks and fissures infilling further from ridge
Seismic layer 2A
- Pillow basalt layer of ophiolite
- Velocity increases further from ridge
- b/c rock more altered and precipitated minerals have potentially filled cracks
- Therefore less pores and less impedance for seismic waves
Basement highs (high heat flow)
- Forced fluid flow
- Rugged basement topography, 300-500m relief
- Ridges mostly sediment covered, some exposed
- Thinner sed cover = increased heat flow
- Fluid’s leak through sed seal
- Massive discharge through outcrop and form precipitates
Expression of ridge depending on speed
- Fast spreading ridge, E. Pacific rise, Smooth bathymetry
- Slow spreading ridge, Mid-Atlantic ridge, Rugged bathymetry
Slow spreading ridge
- Rugged bathymetry
- More broken up looking
- More discernable transforms
- Median valley w/ discontinuous axial high
- Coalescence of small volcanoes, axial volcanic ridge
Fast spreading ridge
- Smooth bathymetry
- Relatively linear
- Less discernible transform faults and valleys
- Axial high continuous, buoyant hot shallow rock
AMC
Axial Magma Chamber
AMC: Fast spreading ridges
- High magma supply
- Melt lens: 10’s - 100’s m thick, 1-2km wide
- Crystal mush zone, partially solid, surrounded by transition zone to solid rock
- Differentiation of lavas
- Variations along-axis, pockets of melt, tapped in eruptions
AMC: Slow-spreading ridges
- Low magma supply
- No long-term melt lens
- Dike-like mush zones, crystallize to ocean crust
- Eruptions related to injection of new magma from mantle
- Undifferentiated lavas, injected magma mixes with crystal mush, no melt separation
- Tilted fault blocks, rift valley, volcanoes in back valley
AMC: East Pacific Rise, approx. 14 degrees S
- Seismic reflection along ridge
- AMC is continuous for 10’s of km, width 250-4500m
How does percent melt affect basalt seismic velocity
- 0 percent melt (eg 1000C): P-vel = 6.2km/s, S-vel=3.4km/s
- 100 percent melt (pure basalt melt): P-vel=3.0-3.4km/s, S-vel=0
- Partial melt: P-vel decreases w/ increasing melt, S-vel to 0 once crystals lose inter-connections (variable percentages of melt for S-vel to become 0)
Roof and base of AMC
- Observations: P-vel = 6.0km/s, S-vel = 3.2km/s
- Interpretation: mostly solid, approx. 2 percent melt, gabbros (cooling and crystallization w/in AMC)
Within AMC
- At 1625, P-vel = 4 km/s, S-vel = 2.3km/s, mush 40-60 percent melt
- At 2488, P-vel = 3.4km/s, S-vel = 0km/s, Pure melt 90-95 percent melt
Above AMC roof
- At 2488 (More melt area)
- In a approx. 200m thick region above roof P-vel, S-vel are 0.3-0.5km/s lower than solid basalt
- Interpretation: Reduced velocities from hydrothermal fracturing (fracture porosity approx. 7 percent)
Magma chamber vs. above roof
- Magma chamber has high velocity floor (150-200m) and roof (50-60m)
- Above roof 150-200m thick low-velocity zone likely results from hydrothermal fracturing
Melt lenses
- only 2-4km in length
- Steady-state at fast ridges over 100’s yrs
Hydrothermal plumes
- Associated w/ melt lenses in crust
- Fresh supply of magma from mantle
Observations of Melt vs. Hydrothermal plumes
- Melt lenses steady-state at fast ridges over 100’s years
- Hydrothermal plumes have fresh supply of magma from mantle
- It would take approx. 50 yrs to solidify a 50m thick melt if not replenished
