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EOS410PltTec > Ocean Ridges and Transforms > Flashcards

Ocean Ridges and Transforms Flashcards

1
Q

Ocean Ridges and Transforms

A
  • Oceanic lithosphere
  • Hydrothermal circulation at ridges
  • Axial magma chambers
  • Transform faults and ridge segmentation
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2
Q

Heat flow, q

A
  • Heat flow density, mW/m^2
  • Energy per unit time (watts) flowing through a unit area
  • Fourier’s Law
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3
Q

Fourier’s Law

A

Heat flow, q = -k (dT/dz)

- Where dT = temp change, dz = thickness, k = thermal conductivity

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4
Q

Heat flow at Earth’s surface

A
  • 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
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5
Q

q at continents

A

55mW/m^2

- approximately 1/2 is crustal radioactivity

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6
Q

q at oceans

A

80-90mW/m^2

- approximately 75 percent of Earth’s heat flow

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7
Q

Heat Flow Probe

A
  • 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
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8
Q

What are the 2 plate models?

A
  • GDH1

- PSM

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9
Q

Half-space

A
  • 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
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10
Q

Lithosphere, HS model

A
  • 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
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11
Q

HS Lithospheric thickness calculation

A
  • 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
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12
Q

HS cooling model, seafloor depth d

A
  • 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
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13
Q

What are the exceptions to the HS model for depth of ridge?

A
  • Typical depth is 2.5km
  • Iceland = 0km
  • Pacific-Antarctic Discordance zone = 3km
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14
Q

HS boundary layer cooling model comparison with observations

A
  • 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)
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15
Q

Plate model

A
  • 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
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16
Q

GDH model

A
  • Relationships for depth and heat flow
  • Different eons for different age ranges
  • T< or > 20, T< or> 55
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17
Q

Problem with plate models

A
  • Seismic evidence suggests lithosphere is thinner under ridge
  • Therefore thickness, L, cannot be constant
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18
Q

Both models (HS and GDH) vs. Heat flow

A
  • 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
19
Q

Hydrothermal flow at ridges

A

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
20
Q

Black smokers

A
  • 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
21
Q

Seismic data from Juan de Fuca ridge

A
  • 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
22
Q

Transition from open to sediment-sealed hydrothermal circulation

A
  • 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)
23
Q

Near ridge envr

A
  • 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
24
Q

20km from exposed basement envr

A
  • 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
25
Q

Implications of seds, heat flow, and basement temps for near ridge

A
  • Near-ridge hydrothermal circulation
  • Open fissures, cracks
  • Carries heat away
26
Q

Implications of pore fluids and seismic velocities near ridge vs. 20km away from exposed basement

A
  • Alteration due to hydration reaction in crust

- Porosity, permeability decreases due to cracks and fissures infilling further from ridge

27
Q

Seismic layer 2A

A
  • 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
28
Q

Basement highs (high heat flow)

A
  • 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
29
Q

Expression of ridge depending on speed

A
  • Fast spreading ridge, E. Pacific rise, Smooth bathymetry

- Slow spreading ridge, Mid-Atlantic ridge, Rugged bathymetry

30
Q

Slow spreading ridge

A
  • Rugged bathymetry
  • More broken up looking
  • More discernable transforms
  • Median valley w/ discontinuous axial high
  • Coalescence of small volcanoes, axial volcanic ridge
31
Q

Fast spreading ridge

A
  • Smooth bathymetry
  • Relatively linear
  • Less discernible transform faults and valleys
  • Axial high continuous, buoyant hot shallow rock
32
Q

AMC

A

Axial Magma Chamber

33
Q

AMC: Fast spreading ridges

A
  • 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
34
Q

AMC: Slow-spreading ridges

A
  • 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
35
Q

AMC: East Pacific Rise, approx. 14 degrees S

A
  • Seismic reflection along ridge

- AMC is continuous for 10’s of km, width 250-4500m

36
Q

How does percent melt affect basalt seismic velocity

A
  • 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)
37
Q

Roof and base of AMC

A
  • 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)

38
Q

Within AMC

A
  • 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
39
Q

Above AMC roof

A
  • 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)
40
Q

Magma chamber vs. above roof

A
  • 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
41
Q

Melt lenses

A
  • only 2-4km in length

- Steady-state at fast ridges over 100’s yrs

42
Q

Hydrothermal plumes

A
  • Associated w/ melt lenses in crust

- Fresh supply of magma from mantle

43
Q

Observations of Melt vs. Hydrothermal plumes

A
  • 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

EOS410PltTec (12 decks)

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