A Dublin-based scientist hopes to solve the riddle of why continents drift across the Earth's surface. More data and faster computers will help him achieve his goal
WHY IS the Mediterranean bigger than it used to be and why is Tibet the shape and size it is? It all comes down to plate tectonics, where huge slabs of the Earth's crust drift about and collide.
A scientist working in Dublin hopes to answer one of the big questions about plate tectonics, whether we are floating on a sea of molten soup or sitting on rigid blocks, grinding against one another.
"In the 1960s, the plate tectonics revolution occurred," says Prof Sergei Lebedev of the Dublin Institute for Advanced Studies. Plate tectonics tells us that all land masses and oceans sit on massive moving structures called plates and forces underneath these plates in the Earth's mantle are responsible for their movement.
Our main experience of this movement is "earthquakes and volcanoes, which happen at the plate boundaries", says Prof Lebedev. There are two types of plates, those beneath oceans and those that continents lie on, with one major difference being thickness. "Continental boundaries are different from oceanic ones because continental crust is thick and crust is weaker than the mantle," he says. The movement of the oceanic plates is relatively well understood, with one plate thought to slide under another "descending deep into the Earth".
Depending on the circumstances involved, there are sometimes spectacular and sometimes catastrophic consequences. The creation of the Galapagos Islands or Surtsey in Iceland, "born" on November 15th, 1963, are beautiful examples of the former. The horrendous St Stephen's Day tsunami in South-East Asia in 2004 is a stark reminder of the latter.
What isn't well understood, however, is "how continents deform", either by colliding or moving apart, according to Prof Lebedev.
The boundaries of contact are much more widespread than for oceanic plates, causing a "broad deformation zone", and creating an array of obvious topographic features. The results can be seen in the massive Tibetan plateau formed by India hitting the Asian plate. It is also visible in the Mediterranean, which has been "stretched into a shallow sea over 30 million years" by the African plate moving away from the Eurasian plate.
Two competing theories exist to explain the movement of these continental plates. The first suggests that they sit on rigid formations under the surface that are constantly sliding against one another. The other proposes that all plates float on a "viscous fluid" of crystalline material. Depending on the currents within this fluid, there would be different effects on the overlying plates.
The main way to resolve this debate, says Prof Lebedev, is by looking at movement of the Earth's mantle at depth, since all we see at the surface is "a very thin layer of brittle rock".
Until recently, these theories have been just that, plausible theories. But thanks to a Science Foundation Ireland grant to Prof Lebedev, it will be possible to test these theories. Two factors are going to help, more seismic data and much faster computers, he adds.
His team is working with geologists in the US, France and Switzerland, collecting data from arrays of seismic stations all over the world including in Tibet, across the US and the Aegean Sea. Each station records movements of the Earth's surface so that "signals from distant earthquakes can be measured . . . and as the wave propagates across the array we can use the arrival time to infer the seismic structure of the region".
The next step will involve computers processing the seismic data to "model" the possible structure of the mantle. Prof Lebedev says this will give his team a measure of the "level of anisotropy" 25km to 50km under the Earth's surface. Anistropy refers to the extent of alignment of the mineral crystals within the mantle. If there is high anistropy, the free-floating theory wins; if there are lower levels, continents may be sitting on rigid blocks.