Do mantle convection cells move?

[Ostsol]

Hello, PhysicsForums!

I've been discussing what little I know of plate tectonics with someone and the other party has suggested that convection cells within the mantle (assuming convection really occurs at all) should eventually move such that they no longer push more matter up through oceanic-ridges. Given this assumption, said ridges should eventually become inactive and perhaps new ridges should form in other places. So, have there been any studies into whether convection cells themselves eventually migrate?

Thanks.

 

[Andre]

You are referring to Mantle Plumes

The Canary islands are an example with complicated shifting of the hot spot, generating a lot of hypotheses here.

 

[billiards]

Well there are two modes of convection which operate in the Earth's mantle (and yes the mantle convects and that is well established): 1. Mantle plumes, 2. Broad scale convection typical of a fluid heated from within. There is actually some debate (although it is becoming increasingly muted) as to the existence of mantle plumes (the evidence seems to suggest that there are), but there is no debate as to the existence of convection in the mantle, in fact convection is how the Earth transports over 99% of its heat near to the surface, and this is widely accepted.

Now to tackle another apparent misconception. Mid ocean-ridges are where mantle material is upwelled, but contrary to your understanding, mid-ocean ridges are pulled apart and material flows up to fill the gap. They are completely passive features. It is not that forced upwellings push the plates apart. Rather, to the contrary, as structural geologists have observed, mid-ocean ridges are being pulled apart rather than being pushed apart, and this is why there are extensional faults in Iceland for example. Mid-ocean ridges form wherever they happen to be, so yes, they can move, but not because the convection cells move.

As to the nature of convection cells in the mantle. I think that nobody really knows for sure what they look like, but they almost certainly aren't the neat cells pictured in certain schematic cartoons. There are certainly large downwelling areas related to the subduction of ocean lithosphere at the subduction zones. These subduction zones tend to occur where continental plate meets oceanic plate (with some exceptions) and we know that continents can move, and indeed that they can come together to form super continents (e.g. Pangea). The coming together of super continents requires that everything in between the super continents must be subducted to make room for the continents to come together. So we can envisage a massive downwelling beneath the super continents. But then when the continents have come together there is nothing left to subduct, apart from a bit of dense continental root, perhaps. And so the downwelling drys up. Eventually there must be upwelling beneath the continents which forces the continents to break apart. So it seems that mantle convection must move (or perhaps completely change their character) in order to recreate the supercontinent cycle. So I guess the short answer to the question would be: yes.

 

[Ostsol]

Thanks for the prompt responses!

 

[Endervhar]

Looking at a map of tectonic plates, the African plate seems to present a problem. I believe it was Warren Carey who claimed that plate tectonics must be wrong because there was a spreading ridge on each side of Africa, with no subduction; yet Africa was not being squashed. An answer to this could be that the spreading ridges on either side are moving away from each other.

 

[billiards]

Looking at a map of tectonic plates, the African plate seems to present a problem. I believe it was Warren Carey who claimed that plate tectonics must be wrong because there was a spreading ridge on each side of Africa, with no subduction; yet Africa was not being squashed. An answer to this could be that the spreading ridges on either side are moving away from each other.

That's an old dilemma.

The answer is that spreading ridges are completely passive features. They are not actively pushing material outwards, rather, material is being pulled away and material at spreading ridges passively rises to fill the gap.

 

[EricNoot]

To really see evidence of the mantle convective hotspot, look at the Emperor Seamounts of the Pacific. Here the Pacific seems to have moved over a stationary hotspot and the plate seems to have changed direction some time ago. http://www.ngdc.noaa.gov/mgg/image/2minrelief.html

 

[Endervhar]

Great images there EricNoot, thanks.

Next question: The direction of plate movement over the hotspot is very clear, but there seem to be lines that suggest movement in more than that one direction. How can that be?

 

[EricNoot]

From my notes, the Emp. seamounts started about 75 Mya as the Pacific plate seems to have moved SSE-wards, then 43 Mya, it took a turn more towards the ESE and becoming the Hawaiian ridge. There is also a set of mounts with the Tuamotu group and the "Line Islands" called the Christmas Ridge, plus those of Ratak chain, running from the Marshall Islands to Tuvalu that parallel the Emperor seamounts. Their magma source may have cooled since the Pacific's turn, but because they are all parallel, I don't imagine that three magma pockets would move in the same direction and the same rate, but one plate floating on top would.

 

[Endervhar]

Possibly the other features are not island chains, or directly resulting from the movement of the plate.