Creating a wax works of the Earth's crust to chart global shifting

Bodenschatz

Using a tub of wax, Cornell physicist Eberhard Bodenschatz has shown that wax is an excellent model of ocean floors. From this, he and his former students have produced a predictive mathematical model of tectonic microplates -- one of the very poorly understood features of plate tectonics.

The research is reported in the New Journal of Physics, published jointly by the Institute of Physics (IOP) Publishing and the German Physical Society.

The research gives scientists a clearer understanding of the mechanisms of plate tectonics: how the land masses of the Earth shift and change over time.

Tectonic microplates are dynamic whirlpools of ocean floor found at mid-ocean ridges. Bodenschatz and two former students, Richard Katz, now at Columbia University, and Rolf Ragnarsson, built a scale model of the ocean floor: a tank of wax heated from below and cooled from above. Although scientists have been using wax to simulate the ocean floor since the 1970s, this was the first time that such a model had been used to create genuine patterns in the Earth's crust.

Like ball bearings trapped between two sheets of metal, tectonic microplates are rotating blocks of crust that are born where sections of mid-ocean ridge begin to overlap, then grow larger as they age, and gradually move away from the spreading ridge along with new ocean floor. They can reach sizes of up to 250 miles across, and rotate about 15 degrees every million years (which is fast by geological standards). Only 12 are known to exist, and they are among the least-understood features of plate tectonics.

The experiment began in 1998 as an undergraduate research project in the Cornell Center of Materials Research, deep in the basement of Cornell's physics department, where Bodenschatz set up a large tank filled with wax to mimic spreading ridges on the ocean floor. The wax is heated from beneath, but cooled from above by air-conditioning units so that the surface becomes a rigid crust while the center and base remain molten. A pair of long, straight paddles move slowly away from the center, pulling the crust apart and causing new molten material to rise up and solidify at the surface, just like the creation of new ocean floor at mid-ocean ridges.

Bodenschatz and his research students immediately began to notice features in the wax similar to a variety of geological features seen on Earth. They saw structures growing in the wax that were very similar to transform faults, like the San Andreas fault, rift valleys and also the zig-zag rifts found on the surface of lava lakes in volcanic craters. They also found that when the paddles pull the surface apart at a certain rate, a rare spiral feature of mid-ocean ridges, called microplates, form and evolve, mimicking structures known to exist in the East-Pacific Rise, such as the Easter microplate just off Easter Island.

Katz said: "When I joined the research team at Cornell I became fascinated by the microplates, which they could create in the wax, and thought that we could use the experiment to begin to understand how real microplates on the Earth come about and to accurately describe how they behave mathematically so we can predict their movement."

The researchers made detailed observations of the formation of microplates using a video camera mounted above the tank, looking directly down onto the surface where they were forming. Lamps were mounted in the molten wax and directed upward so that the pictures would show the thickness of the crust because of the difference in brightness.

Using these observations, Bodenschatz, Katz and Ragnarsson developed a mathematical model that closely predicts microplate evolution over geological time. "Microplates have a distinctive pattern on the sea floor and in the wax tank. We can use our model to predict how they will evolve over time and how they will interact with the mid-ocean ridge and their neighboring major plates. It may also help us identify very young microplates in the crust or very ancient ones or even to identify plate tectonics on planets besides Earth," said Katz.

Why do microplates form in the first place? The team speculates that it might be because the mid-ocean ridge that hosts them is a strange chimera: neither transform fault nor spreading ridge, but an unstable form in between. When the crust moves to become more stable, areas of crust overlap and might give birth to rotating microplates because of the forces opposing each other.

Used with the permission of Institute of Physics (IOP) Publishing.