2. Creation of new oceanic crust at spreading centres

Question 1

Historical overveiw of layered structure theories

Answer 1

Cold war: Sonobouys invented to hunt soviet subs
60s/70s: Sonobouys used to collect seismic refraction profiles (2 ships, 1 with explosives the other sonobouys)
seismic velocity varies with depth and suggests how rigid the rock is
1972: Penrose model of oceanic crust - lavas, dykes, gabbro
Swift et al. (2008):
Velocity through the crust mainly depends on the abundance of fractures and lithostatic pressure, which strongly affect rock rigidity.
Vp=((K+4/3u)/p)^1/2
Seismic velocity is no so useful for discriminating the different lithologies within the crust, it is useful for measuring the total thickness of the crust given the large v contrast at the Moho

Question 2

Explain Mantle Bouguer anomalies

Answer 2

Lin et al. (1990): corrected free-air gravity anomalies for the topography if the seabed and the moho density contrasts (assuming a constant thickness crust). The resulting maps of ‘‘Mantle Bouguer anomalies’’ show a series of lows on slow-spreading ridges. These partly reflect the hotter (and hence less dense) mantle under the ridge axis, so they made a further correction to remove the effect of varied mantle temperature. The resulting ‘‘residual mantle Bouguer anomalies’’ show a series of bands of low and high values, with the low values in the centres of spreading segments. As these areas commonly contain abundant volcanic features, they interpreted them as areas where crust is thicker due to greater supply of melt from the mantle.

Question 3

Explain segmentation of MORs

Answer 3

1985: interpreted axial rift valley segments ascaused by a fluid-dynamical instability in the Earth’s upper mantle beneath the ridge axis, where low-density mantle asthenosphere underlies a higher density mantle diapirs, creating this segmentation. The spacing of spreading segments reflects the density contrast and thicknesses of the two layers

1987: v-shaped off axis trends linked to the movement of the ridge axis over the mantle

Question 4

How does geochemistry provide evidence for the extent of mantle melting

Answer 4

Klein and Langmuir (1987): individual segments are fed magma of a common composition which then fractionates in magma chambers to varied NaO and MgO.
Na is released by early melting minerals but is then diluted, therefore there is an inverse correlation with crustal thickness and Na8.0 (NaO corrected to a common 8% MgO)
Na8.0 also correlates with depth of the ridge axis. This is an effect of isostasy and the varied thickness of the crust.

Question 5

What is the ‘onion model’?

Answer 5

Put forward by Cann (1974)
Model of magma chambers at fast spreading ridges to potentially explain tick homogeneous gabbros found in some ophiolites, the sheet dyke layer and the tilting of lavas into the axis observed in Iceland
Lavas dip towards axis because of axial subsidence
Crystallization around a large magma chamber at fast ridges
Turned out to be wrong (magma chambers are likely much smaller)

Question 6

Why is the ‘‘onion model’’ thought to be wrong?

Answer 6

• seismic refraction experiments have failed to find a large liquid-filled magma chamber
• Dunn et al. (2000) carried out a tomographic inversion of data from ocean bottom seismometers (OBSs)(Seismic refraction), which suggest a lower crust at the axis of mainly solid but lowered velocity, hence probably containing a large proportion of belt between crystals. The velocity anomaly increases upwards, suggesting the accumulation of the magma in a more melt-rich layer at a higher level.
• Seismic reflection experiments have revealed the axial magma chamber to produce a reversed phase reflection (the seismic impedence (=velocity x density) of the magma is smaller than the rock above because it is less rigid) Singh et al. (1998) suggested the ‘‘magma lens’’ or ‘‘magma sill’’ is only a few 10s of m thick and mostly comprises a crystal mush, not pure melt

Question 7

What is seismic refraction?

Answer 7

Seismic refraction involves working out structures from how fast the waves travel through them and are bent (refracted).

Question 8

Describe seismic reflection methods

Answer 8

Seismic reflection methods involve setting off mini-explosions at the surface, usually with compressed air, and recording the echoes arriving from beneath the seabed. These methods are used when looking for oil and gas.

Question 9

Why is there an absence of axial magma chambers at slow spreading centres in seismic data?

Answer 9

The seabed is more rugged at slow-spreading ridges making it more difficult to image features in the crust (the rough topography breaks up the seismic wave). With even when using more sophisticates seismic experiments, it appears there are fewer AMCs at slow rates

Question 10

What led to the rethinking of the layered structure?

Answer 10

Dredges and other sampling has revealed an abundance of ultramafic rocks at the seabed at slow and ultra-slow spreading ridges. Cannat et al. (1995) much of slow-spreading ridge crust contains serpentinite

Question 11

Explain the interpretation of knobs observed by multibeam data around the MAR

Answer 11

Smith & Cann (1993): individual eruptions over dykes linked to individual ephemeral magma lenses.
Smith & Cann (1999): modified to consider the possibility of rootless cones being fed by lava tubes from eruptio sites along neovolcanic ridges

Question 12

TOBI images (deeply towed sidescane sonar)

Answer 12

Question 13

Explain Head et al. (1996) Hawaiian ‘‘wall of fire’’ analogy to explain knobs (small cones)

Answer 13

• Hawaiian eruptions tend to start by first erupting from a long dyke as a wall of fire (think iceland fountains)
• As the pressure driving the eruptions wanes and cooling stops eruption along most of the dyke, eruptions focus at single points producing cinder cones.
• Applying this to the MAR the cones (knobs) are the focused eruptions and the hummocky terrain represents the earlier wall of fire phase, which under the high pressure of the ocean produced pillow lavas

Question 14

Volcanic geomorphology key terms

Answer 14

axial volcanic ridge
hummocks
Smooth lava patch
crater
faults/fissures

Question 15

4 types of lava flow

Answer 15

pillow flow
lobate flow
ropy sheet flow
jumbled sheet flow

Pillow lavas are more abundant at slow spreading ridges
sheet flows more abundant at fast spreadung ridges
1998

Question 16

Describe the findings of Gregg & Fiske (1995) experiments with wax

Answer 16

results suggest that pillow lavas are emplaced with slow effusion rates, fast cooling and on low gradients, whereas sheet flows are emplaces with the opposite conditions. this is consistant with cooler magma occuring at slow-spreading ridges and being extruded in smaller quantities

As surface morphology varies with flow rate and gradient, potentially a single lava flow can transform from one type to another. This has been observed on land at Kilauea. This complicates mapping individual lava flow at MORs

Question 17

How are lavas dated + give examples

Answer 17

Uranium disequilibrium series used to date the timescale of residence in the magma chamber and date lavas emplaces on the seabed

East Pacific Rise: U-series dates are slightly younger than expected from distance from ridge axis - either erupted off-axis or flowed from the axis

Question 18

How are lava flows detected on the galapagos spreadign centre?

Answer 18

The spreading centre lies under the eqiutorial region of high planktonic productivity, hence it recieves higher than normal amounts of pelagic sediment.
sediment thickness profiles shpw areas of uniform sediment and steps up over faults, these are likely to be the lava flows (speculated to affect ocean temps and circulation)