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The Earth's outermost shell is constantly moving, dragging continents apart and pushing them together.

But exactly what caused this colossal movement three billion years ago has been a mystery.

Now, researchers in Australia believe they have the answer: the motion, they claim, started because of gravity.

This animation shows an early buoyant continent slowly spreading toward the still plate (blue). After 45 million years, a short-lived area develops, where the plate goes under. This allows the continent to surge toward the ocean, leading to the detachment of a continental block and triggering modern-day plate tectonics

Scientists at Sydney University suggest that whole continents flattened out under their own weight, and this eventually became a self-sustaining process.

The team has created computer models to explain how layered rocks can appear on a young, hot Earth, even without modern plate tectonics

There are eight major tectonic plates that move above the Earth's mantle at rates up to 150 millimetres every year.

The process involves plates being dragged into the mantle at certain points and moving away from each other at others, in what has been dubbed 'the conveyor belt'.

University of Sydney's cmputer models suggest that these spreading early continents would have imposed an horizontal stress strong enough to force adaject plates to be pushed under their edges

As plates spread out under their own weight, they would begin to melt on the bottom, due to the heat below

This would trigger a subduction - the process that takes place at boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge

When plates smash together, they make mountains, and when they spread apart, molten rock comes to the surface and makes new crust.

The movement depends on the relationship between density of rocks and temperature.

For instance, at mid-oceanic ridges, rocks are hot and their density is low, making them buoyant or more able to float.

As they move away from those ridges they cool down and their density increases until, where they become denser than the underlying hot mantle, they sink and are 'dragged' under.

But three to four billion years ago, the Earth's interior was hotter, volcanic activity was more prominent and tectonic plates did not become cold and dense enough to spontaneously sink.

'So the driving engine for plate tectonics didn't exist,' explained Associate Professor Patrice Rey, from the University of Sydney's School of Geosciences.

'Instead, thick and buoyant early continents erupted in the middle of immobile plates.

When plates smash together, they make mountains such as the Himalayas (pictured), and when they spread apart, molten rock comes makes new crust. But what triggered this process had been a mystery

'Our modelling shows that these early continents could have placed major stress on the surrounding plates.

'Because they were buoyant they spread horizontally, forcing adjacent plates to be pushed under at their edges.'

'This spreading of the early continents could have produced intermittent episodes of plate tectonics until, as the Earth's interior cooled and its crust and plate mantle became heavier.

'Plate tectonics became a self-sustaining process which has never ceased and has shaped the face of our modern planet.'

The Earth's outermost shell is constantly moving, dragging continents apart and pushing them together. Pictured is the fault in the landscape caused by continental drift between North American and Eurasian tectonic plates at Thingvellir National Park near Reykjavik, Iceland