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Andrew Alden

Iceland Tectonics

By April 20, 2010

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As the Eyjafjallajökull eruption continues, the news outlets are catching up with the background information that the larger neighbor volcano Katla is a concern. Eyja has erupted three times before in Icelandic history, and the last two times Katla has followed it within months. We should remember that this argument is not scientific, but anecdotal.

The news outlets are also starting to fill us in on basic plate tectonics, including Iceland's unique position directly upon a spreading ridge. One basic fact of plate tectonics is that volcanoes and magma and eruptions are not forced out of the Earth. Lava melts when the pressure upon it is relaxed. Volcanism emerges where the crust is being stretched apart, allowing lava to form and rise, by buoyancy, to the surface.

The motive force of plate tectonics is the sinking of cold crust. That causes two kinds of stretching, or extension. The first kind of extension is what we see at the opposite end of the plate, in Iceland, the midocean ridges, and a few other places like the African Rift and the Salton Sea. The analogy is two bedsheets stuck together by static cling—as you pull them apart, the sparky place in the middle is where the midocean ridges and eruptions (and Iceland) are. (But remember that in the real Earth the bedsheets, the plates, are actually created at the ridges.) This kind of extension happens at the opposite end of the plates.

The second kind of extension happens at the same places where the cold plates (which geologists call slabs) sink. The motion of a sinking (subducting) slab is not like water going over a fall, although there is a degree of forward motion. There's also a degree of straight-downward motion of the sloping slab that stirs the underlying soft rock of the mantle. On the downward-facing side of the slab, the mantle is compressed, and on the upward face it is extended. So if you picture the west side of South America, the Pacific mantle is being pushed westward and the South American mantle is being sucked westward, along with the continent on top of it. That extension allows the volcanoes of the Andes to form, fed by the water and rocks of the slab beneath it. (Edit: A commenter corrects me about the Andes; the volcanoes of the Aleutian Islands are a better example.) That's why the volcanic "Ring of Fire" surrounds the Pacific. This kind of extension is directly associated with subduction, which accounts for nearly all of Earth's volcanoes on land. But not Iceland.

More background:
Plate tectonic basics
Plate tectonics of the past 1 million years
Volcanism in a nutshell


April 20, 2010 at 9:01 pm
(1) Hephaistos says:

I think you may have slightly oversimplified the situation; it’s not universally agreed that magmatic arcs above subduction zones must always be extensional. Magma in subduction zones is created primarily due to the introduction of water to the mantle wedge from the downgoing slab. Slab-driven convection of the mantle wedge is believed to keep it hot, increasing melt fractions, but extension is not fundamentally necessary.

Also, let’s be a little more careful about how we use the term “extensional.” You label the Andes as extensional, and indeed, there is extensional normal faulting at the surface, but I would argure that it is more appropriately classified as a convergent mountain belt. Remember that the Andean forearc is approaching stable South America at a rate of over a centimeter a year! The minor extension in the Andean plateau is just due to its gravitational instability: it’s being squeezed up from below and spreading out like sunscreen out of an upright tube. At mantle depths, I guarantee you it’s wholly compressional.

In the ductile regime, a column of magma can generally rise anywhere that it is more buoyant (on average) than the adjacent column of rock, regardless of regional stress state*.
Extension does make it a lot easier to initiate throughgoing vertical cracks or conduits in the brittle crust, but magma can still rise through a compressional region of brittle crust without difficulty so long as the intermediate principle stress does not greatly exceed lithostatic pressure (that would be sill-y).

(*Admittedly, if you want to pick a nit, it will always be locally extensional above an upward-propagating intrusion due to the hydraulic forces exerted by the buoyant magma body, but this says nothing about the regional stress balance.)

April 21, 2010 at 12:18 am
(2) Geology Guide says:

Thank you for keeping me honest. I picked a poor example by picking a familiar one. Fortunately for my point, exceptional Iceland is definitely not shoving itself apart but stretching apart. The force causing Eyja’s fire fountains is the expansion of gas, which is a local, vertically directed force in a regional setting of horizontal extension.

April 21, 2010 at 11:39 am
(3) Hephaistos says:

Iceland certainly is a good example of an area that is being stretched apart.

(Though I have another nit to pick which I would have let slide if I weren’t busy preparing a lecture on stress and strain: The expansion of gas bubbles does local, vertically directed work on the magma, lifting and accelerating it upwards into fire fountains. The stress (force/area) exerted by the expanding bubbles is actually equal in all directions. And most of the force exerted by a vertically stretched (as they tend to be) bubble is horizontal, because the vertical bubble walls, which recieve a horizontal push, are greater in area (force = stress times area). It’s just that equal and opposite forces keep the frothy magma from expanding in every direction but up.)

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