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