Seminar Sum-Up: Earth and Planetary Science Seminar 11/10/18



When I first started at university studying science I thought there would never be any uncertainty in what the right answer was however, after three and a half years this is certainly not the case. Chris MacLeod’s seminar ‘New View of Seafloor Spreading – 50 years on’ perfectly demonstrates this. Certainly, in first year and on websites the uniform ‘layer cake’ structure to the oceanic crust is taught. A conceptual model has been created due to evidence from the MOHO and Ophiolites. The MOHO or Mohorovičić discontinuity is the boundary between the Earth’s mantle and crust and is much shallower under the oceans. An Ophiolite is a section of the oceanic crust that has been uplifted. Since only 10% of oceanic lithosphere has been studied in any detail comparing ophiolite structure to seismic imaging of oceanic crust reveals oceanic crust has a layered rock series very similar to the Ophiolite, creating this uniform ‘layer cake’ model thought to be true (Fig 1). Ocean crust is created at spreading ridges that can be fast or slow and Fig 2 shows the traditional model of symmetrical spreading ridges at Mid Ocean Ridges. 

Figure 1: Typical layer cake model for oceanic crust and ophiolites (http://earth.s.kanazawa-u.ac.jp/ishiwata/ophiol_e.htm).
Figure 2: Traditional symmetrical sea floor spreading model(https://jeremyvosplatetectonicstimeline.weebly.com/sea-floor-spreading.html).


However, Chris MacLeod has focused his research on slow spreading ridges and how these models are not always the case

Techniques such as dredging have been carried out on the Mid Atlantic Ridge and serpentinite mantle rocks were found where basaltic crust should have been on the seafloor. This does not conform to the layer cake model, therefore further investigation began. A Side Scan Sonar was used across the Mid Atlantic ridge, this machine sends acoustic pulses out and a proportion of the reflected sound as an echo is received back, the echo provides information on the sediment. Dark images are produced for soft sediment and light for fresh volcanics. Another technique carried out was drilling – using a sea bed rock drill in the Atlantis Bank area (Fig 3). 
During this research they found some peridotite on the flanks of Atlantis Bank. With further exploration they found curved striations – lines – on flat topped areas of Atlantis Bank a clear sign of detachment faulting. Which is a mechanism that could explain the serpentine mantle rocks on the seafloor. Detachment faults are low angle faults that can promote lots of extension - enough to bring mantle rocks to the surface.
Figure 3: Atlantis Bank location (https://joidesresolution.org/atlantis-bank-a-mountain-under-the-sea/).

 Therefore Chris and other researchers could finally come to the conclusion during slow spreading ocean detachment faults can be recognised as a clear mode of sea floor spreading (Fig 4).
Figure 4: Slow sea floor spreading facilitated by detachment faulting, more asymmetrical and heterogenous than the traditional model (Fig 2)(https://www.researchgate.net/figure/The-Chapman-model-8-of-lithospheric-accretion-and-oceanic-core-complex-formation_fig5_254261082).


Today Chris’ research focuses on whether detachment faults in the Mid Atlantic Ridge have a different mechanism to other mid ocean ridges. Looking at whether the Mid Atalntic Ridge has a cold mechanism and other hot, so really his research was just beginning again.

In conclusion, it has taken many decades but Chris and others have proved there are different mechanisms of sea floor spreading than first thought and the model layer cake can only be applied to fast spreading ridges. Slow spreading ridges with a serpentinised floor can be explained by detachment faulting. 


This seminar was particularly difficult to understand due to the complex processes and often hard to visualise features however Chris has explained clearly sometimes the answer is not as simple as we think, and it is worth putting years of work into finding the right one. Chris has also set up a website of teaching resources centred around mid-ocean ridges and sea floor spreading which I think is a great initiative to encourage school teachers and kids to be more informed about these processes and spark an interest in Earth science from a young age.

-       Meg

Seafloorspreading.com

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