I'm a bit confused about how divergent boundaries between tectonic plates work, or just why plates move. I've read that these ridges that are in the place of divergent boundaries are created when magma arises from the mantle convection, probably powered by a mantle plume. This creates a hotspot, but in every illustration I have seen, they use these plumes or just the mantle convection force to explain why plates move apart each other:

enter image description here

Here, that convection of the mantle is used to explain the formation of a ridge and why the plates move apart.

However, this is easy to understand with a cross section of the Earth like in that picture. But I can't wrap my head around the idea of this phenomenon creating the linear ridges we see extending across thousands of kilometers, like the East Pacific Rise. Because, that "ridge" we see in the above picture is just an elevation seen in a cross section of the Earth, that could very well be just a volcano or a single oceanic mountain, not a linear ridge of continental dimensions.

I mean this:

enter image description here

That can't be produced by a single mantle plume, right? Because a mantle plume is just in one spot. Is the convection force of the mantle rising across that entire line that is kilometers wide, creating a linear mantle plume? If that last one is the case, I would be even more confused about the shape of the convection cells in the mantle. Or are there multiple individual mantle plumes conveniently aligned across and beneath that ridge?

So, I don't understand how the ridge is formed. Because the first picture helps me only to understand how a single elevation spot is created, like an underwater mountain, or a volcano, or even a volcanic island, but not an entire ridge.

Now, I do know that the chain of Hawaiian islands up to Siberia was created due to the tectonic plate passing above a plume, or just a hotspot, and that created the entire "ridge". But are all oceanic ridges formed in that same way? Due to a single plume creating a hotspot and the plate moving above it drawing the ridge as it passes? Or is this a different process? Because I also know this is happening in the middle of a plate, not a divergent boundary

This is a picture about the Hawaiian island formation I read:

enter image description here

I hope someone can help me understand this a little better! Thanks!


Hot spots and ridges are two different things.

Hot spots stay in one place while the earth crust above them drifts and thus create a line of dormant volcanoes, they are a point features so to say.

Ridges are places where the mantle convection wells up and spreads in two directions, gently pulling the earth crust with them and creating new crust where the magma wells up. Ridges may be described as linear features.

So, without convection and thus without ridges, which form new earth crust, the hot spots wouldn't create a chain of volcanoes.

  • $\begingroup$ Thanks for your answer. I understood then, that a hot spot is a different thing, what I mentioned about Hawaii. But about ridges, I still don't understand how the mantle convection forms a linear ridge so extense. Doesn't mantle convection generate point like plumes that arise in the middle of a convection cell? I understand how that's able to elevate the terrain in a particular point of the oceanic crust, forming maybe a mountain. But how does the elevation extend across the ridge linear shape? My question is more about the shape of the ridge rather than the elevation itself. $\endgroup$ – Dimakhaerus Apr 23 '19 at 13:14
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    $\begingroup$ There is no middle of the convection cell. We're talking about a linear, even global phenomenom like the Hadley Cell instead of a single, thunderstorm-like convection. And it makes sense, since there are long lines of subduction faults, where material is passed from crust to mantle, there also have to be long lines of ridges where a similar amount of material wells up from the mantle and forms new crust. Don't cling to much to 2D-drawings, they don't show the whole theory but only a crossection. $\endgroup$ – Erik Apr 23 '19 at 13:20
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    $\begingroup$ Thanks again! This is indeed a very good answer. So the portion of mantle that ascends in the convection process has indeed a linear like shape beneath the ridge. I had doubts about it as I always saw it in those cross section pictures and it looked like a point. The comparison to the Hadley Cell also helped me to understand! $\endgroup$ – Dimakhaerus Apr 23 '19 at 13:48
  • $\begingroup$ Hot spots control how spreading centers form, they are not unrelated. $\endgroup$ – John Apr 24 '19 at 15:18

hot spots help/control how spreading centers form but they are not what keeps them going.

If you push on a thin amorphous material perpendicular to the surface they tend to crack/split at three cracks roughly ~120 degrees to each other. If you push at several points close enough to each other these cracks will tend to link up as long lines with 120 degree angle shifts spread throughout it. Rising mantle plumes create doming which splits in these 3 120 degree angles. These are called triple junctions. Plate boundaries can be mapped by connecting the dots between known triple points, usually with two active and one dead fracture, continental crust boundaries in particular are dominated by these angles. Basically hotspots create uneven distribution of stress and weaknesses in the crust for convection currents to exploit, affecting where the crust breaks. as the crust breaks and seperates the mantle can more easily push up creating a kind of feedback that turns single points into a line of spreading. The smaller and more isolated the plume the more likely it will just stay just a hot spot.

Note this is different than how hot spot induced island chains like Hawaii form. Which is caused by a single isolated hot spots punching a series of holes in crusts that is dragged over it.

enter image description here

It helps if you remember mantle plumes and convection cells are not completely seperate things. Mantle plumes vary dramatically in size and distribution, if you have several mantle plumes together that is a large convection cell.


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