Seismic Tomography and the Mid-Atlantic Ridge

Mantle Transition Zone and the Mid-Atlantic Ridge (Matthew Agius et al.)

WARNING: Seriously geeky geology ahead. And serious simplification will occur.

Geologists have attributed most tectonic plate movement to gravity pulling down the colder, denser slabs in subduction zones. The northwesterly movement of the Pacific Plate, for example, is attributed to cold, dense ocean bottom being pulled under the North American Plate at southern Alaska and the Aleutian Islands. The subduction of the northwesterly Pacific Plate pulls the rest of the plate along. If you press geologists on issues like friction, total mass of the entire Pacific Plate and the tensile strength of the rock involved, you get a lot of throat-clearing, hemming and hawing and admissions that the mechanisms involved are “not well understood.”

Yet the plates are demonstrably moving. The Hawai’ian plume has left geologic footprints across the Pacific Ocean floor. The Big Island in Hawai’i is just the latest footprint. You can trace those footprints all the way to offshore of Kamchatka, in Siberia, where they disappear in the subduction zone that created the many Kamchatkan volcanoes. And the Atlantic Ocean is measurably widening.

And, speaking of the Atlantic Ocean, the point at which it is widening is the Mid-Atlantic Ridge, that long, submerged volcanic range extending from the far South Atlantic to north of Iceland. There are no subducting plates on either side of the Atlantic; in the north Atlantic, the Mid-Atlantic Ridge is the boundary between the North American Plate and the the Eurasian Plate. What’s pushing or pulling the two plates apart?

Enter seismic tomography. Tomography generally is using some kind of wave and a great deal of computer power to image the inside of an otherwise opaque object. Ultrasound, for example, is used to check for cancerous tumors in a woman’s breast. X-rays are used in CT (computed tomography) scans. Magnetic resonance imaging – MRI – is used to peer into soft tissues of the human body to diagnose problems.

In the case of seismic tomography, the energy waves used are the waves generated by earthquakes. With a large array of seismometers, and an active seismic zone, geologists can peer inside the earth in a fuzzy, slightly indistinct way. There are challenges: earthquakes generate two types of seismic waves, p-waves and s-waves, and each has strengths and weaknesses for seismic tomography. But, generally, warmer rocks transmit the waves more slowly than colder rocks, and reflections and refractions occur as the waves cross boundaries in the interior of the planet. It’s more complicated form of ultrasound, but the principle is the same.

A team of seismologists led by Matthew Agius, deployed an array of 39 seismometers across the bottom of the Atlantic Ocean, near the equator, straddling the Mid-Atlantic Ridge. The Mid-Atlantic Ridge is seismically active, with lots of little quakes. So there was lots of data. And after a very impressive amount of computer processing. measuring the elapsed time from the hypocenter of the earthquake to the arrival of the seismic waves, they were able to create a picture of the area up to 700 kilometers underneath that part of the Ridge.1

What they found was surprising. Under the Mid-Atlantic Ridge, the transition zone between the Upper Mantle and the Lower Mantle, called, cleverly enough, the Mantle Transition Zone, was compressed. It’s usually a band extending from 410 to 660 kilometers down. It’s thinner at the Mid-Atlantic Ridge than elsewhere. And there are mantle plumes rising through that thinned area, transmitting additional heat, a lot of additional heat, from the Lower Mantle up into the Upper Mantle. And that may very well be what is driving the spreading center at the Mid-Atlantic Ridge.

The effect is not trivial. It’s created the vast Atlantic Ocean which, in geologic terms, wasn’t there until recently. A mere 145 million years. About an inch a year, if you pencil it out. Over WC’s lifetime, the Atlantic has widened by about as far as WC is tall.

So why is this happening at the Mid-Atlantic Ridge? Why is the Mantle Transition Zone thinned in a narrow band along nearly the entire length of the Atlantic Ocean? Why has the effect persisted for 145 million years? Well, ahem . . er. . . the geologists are still working on that. Presently, the reasons are “not well understood.” You can bet that similar studies are planned for elsewhere on the Mid-Atlantic Ridge and along the other mid-ocean ridges on the planet. That’s how science works: each question you answer raises a dozen more questions.

1 The hypocenter is the three dimensional point where the earthquake occurred, or originated. The epicenter is the point on the surface immediately above the hypocenter.

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