Wax works: Wax proves a perfect model of the Earth’s crust

Physicists in the US have proven that wax is a perfect model of the ocean floors. Using a tub of wax, geophysicists at Cornell and Columbia have produced a predictive model of tectonic microplates – one of the most important and poorly understood features of plate tectonics – for the first time. This research is reported today in the New Journal of Physics (www.njp.org) published jointly by the Institute of Physics and the German Physical Society (Deutsche Physikalische Gesellschaft).

This breakthrough gives scientists a clearer understanding of the mechanisms of plate tectonics: how the landmasses of the Earth shift and change over time, how earthquakes are generated, volcanoes erupt, and precious metals are concentrated in rich seams. Tectonic microplates could also help identify whether this process, which many scientists argue was a key factor in triggering the evolution of life on Earth, occurs on other bodies in the Solar System.

Richard Katz, now at Columbia University, and Eberhard Bodenschatz from Cornell University where the research was carried out, have produced the first mathematical model which successfully describes how ’tectonic microplates’ – dynamic whirlpools of ocean floor found at mid-ocean ridges – evolve and move over time. Writing in the New Journal of Physics, they announce their model which successfully predicts microplate behaviour as observed in a scale model of the ocean floor: a tank of wax heated from below. Scientists have been using wax to simulate the ocean floor since the 1970s. This research links these ingenious wax models with genuine patterns in the Earth’s crust for the first time.

Like ball-bearings trapped between two sheets of metal, tectonic microplates are rotating blocks of crust which 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 400km across, and rotate about 15 degrees every million years (fast by geological standards). Only 12 are known to exist, and they are one of the least well-understood features of plate tectonics.

The experiment began in 1998, deep in the basements of Cornell’s physics department. A large tank filled with wax had been set up by Professor Eberhard Bodenschatz 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 centre and base remain molten. A pair of long straight paddles move slowly away from the centre 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 on the Earth.

Bodenschatz and his team of 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 which 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 in the Pacific.

Richard Katz from Columbia University 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 model 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”.

They 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 upwards so that the pictures the camera took showed the thickness of the crust because of the difference in contrast.

Using these observations, Katz and his supervisor Eberhard Bodenschatz set out to write a mathematical expression based on existing assumptions about microplate behaviour. They found that their model predicts microplate evolution perfectly, and so can now predict how they’ll behave.

Katz said: “Microplates have a distinctive pattern on the sea-floor and in the wax tank. We can now use this model to predict how they’ll evolve over time, how plates near them will move and shift as they grow older and how microplates will affect the surrounding crust and the mid-ocean ridges which give birth to them. It should also help us identify very young microplates in the crust or very ancient ones. It might even help us identify plate tectonics on other bodies in the Solar System.”

In their paper, Katz and Bodenschatz give an insight into why microplates form in the first place. It turns out that it might be because the crust around 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.