A new view of seafloor spreading, 50 years on

Picture from : https://www.thinglink.com/scene/857449204946042880

I was rushing my way from central area to the King's Building to ensure that I did not missed the seminars. Ocean and its whole content has always amazed me since I was a child, look at that wide blue sea without limit, I can see the sky above it, the boats and ships making their way against the waves but I still do not know what is actually happening inside the ocean. How deep it is, does it even have a basement? what are they look like? what creatures can live in that? Is there even any life there? how deep are they?how is this whole amazing creation first started.

So this talk, given by Prof. Chris McLeod from University of Cardiff, might be able to answer some of my questions.

Here what I have summarized from the talk.

Main Points

2/3 of the earth is covered with oceanic crust but the knowledge on the seafloor is undeniably limited as it is indeed very difficult to reach and get “hands-on” on the sample of the seafloor, unlike the continental surface. Most of seafloor maps are obtained by the satellite.

  

Ocean crust, as known has its own life cycle of being continuously formed and then destroyed in couple of hundred million years period. In mid ocean basin, with mid ocean ridge as the centre, oceanic crust is continuously generated as result of plate diverging due to plate tectonic motion, accreting new materials into the mid ocean ridge.

As seafloor spreading involve movement, it is best to note that the rate of the spreading varies in different places. As example, spreading rate in the Pacific is the fastest and the slowest at the Atlantic Mid Ocean Ridge.

How much do we actually know about Oceanic Crust compare to our knowledge of the Continental Crust?

Seismic studies show that Oceanic Crust is different than Continental crust. The large-scale seismic experiment resulted that the Moho of Oceanic crust are very much shallower than the Moho of that continental. In correspond to Moho Reflection, the seismic structure of oceanic crust is shown to be very regular. Based on geological interpretation from ophiolites and drilling, the ocean crust is actually a layered structure with additional seismic layer 1,2, and 3 with intermediate velocity causes increase in the typical mantle velocity. This is same everywhere, the oceanic crustal structure remains regular regardless of the spreading rate, thickness and the age of the oceanic floor. The characteristic succession above mantle are sediments, pillow lava, sheeted dyke and gabbro.

Pieces of oceanic crust that have been obducted onto the continental and have been widely studied is called Ophiolite. The thing is even with detailed study of ophiolite, it is actually not telling the complete story or knowledge of the seafloor. The question started again when serpentinites are commonly found with ophiolite. 

How does this mantle peridotite rock can be found in the seafloor?

Attempt of summarizing faults information has been made but does not explain the answer to the question. There are peridotites found in the seafloor with massive lava flow directly on top of it with no crust, no stretches and no faulting.

The velocity decreases linearly from peridotite to serpentinite of ~8km/s and ~5km/s respectively. This explains why the seismic layering does not always work in processing geophysical data of the seafloor.

This again leads to more question on how regular oceanic crust structure actually is? How much does it was affected by spreading?

Several attempts to get closer to the sea floor

In doing this, the biggest challenge is the realisation that getting the direct access to the sub surface is very difficult near to almost impossible. One of it is the very expensive submersible that can only sample the surface of the seafloor. Following that is the idea of drilling the Mohole through the oceanic crust to the mantle but they only able to get into 13.5 m of the seafloor. The next attempt in the Pacific Ocean only get through the crust after 8 month which seriously cost millions.

Techniques

In was not until after years of planning that in International Ocean Discovery Program Expedition 360 that scientist decided to drill the Moho at the slower spreading ridge which the nature of the Moho here is significantly shallower due to removal of upper crust by faulting.

Results

One of the key finding is the existence of weak talc which shows that there is fluids role in converting peridotite to talc and serpentine. The importance of serpentinization mechanism is that to see the totally different mechanism of both strain localisation and weakening of the crust. The weak talc allows slipping. A few suggested mechanisms involving fault has been suggested; longer fault? Continuous fault? steep fault which the flattened? Detachment fault model might be favourable in this discussion as the speaker explained that this detachment fault is pulling one side of the plate while the other side move by normal plate diverging mechanism. Slow spreading ridges is significantly form by detachment fault which catches more than half of the separation materials. There are some relations of the targeted place with earthquakes where the targeted areas are actually active earthquake spots.

Conclusion and impact to us

There is more and more knowledge to be discovered by making the best out of the advancement of the technology.

Next post : Brain Evolution in Rodents: What did our ancestor's brain look like?

Date: 11/10/2018

Venue: LT 201

Speaker : Prof. Chris McLeod, Cardiff University