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At school, most of us learnt that the Earth's tectonic plates 'float' like large rocky rafts.

But teachers were vague about what the pieces of Earth's crust actually float on because no one really knew.Now scientists have solved the mystery, however.

Geologists have used tonnes of dynamite, creating their own seismic waves, to discover that the plates move across a thick layer of jelly-like rock.

Geologists have used tonnes of dynamite to create their own seismic waves and discover that tectonic plates move across a thick layer of jelly-like rock. Here, instruments are used to measure the waves

Together with experts from the US and Japan, a team of geologists from Victoria University of Wellington, in New Zealand, developed a new method to get the most detailed images yet of the base of the tectonic plate beneath Wellington.

In the past, geologists have studied earthquakes and the speed of the seismic waves produced, in a bid to unravel the mystery. 

'Rather than relying on earthquake waves that come from below we create our own "earthquakes" with dynamite shots,' study leader Tim Stern told Cosmos Magazine.

The team detonated dynamite underground across the southern part of the North Island and listened for echoes to build an image of the bottom of the Pacific Plate, which is 62 miles (100km) beneath the Earth's surface. 

They used 877 Coke can-sized seismometers set up over 52 miles (85km) to take their measurements. 

Experts already knew that plates are made of a thick layer of hard rock, called the lithosphere, above a softer layer called the asthenosphere (shown in this diagram), but the boundary between them was a mystery

In the past, geologists have studied earthquakes and the speed of the seismic waves produced, in a bid to unravel the mystery. This stock image shows seismic waves created by a small earthquake

Experts already knew that plates are made of a thick layer of hard rock, called the lithosphere, above a softer layer called the asthenosphere, but the boundary was a mystery.

The recordings were clearer than before and showed that Earth's tectonic plates are gliding on a distinct layer of 'soft' rock, six miles (10km) thick and weak enough to allow the plates to shift many centimetres per year.

'The idea that Earth's surface consists of a mosaic of moving plates is a well-established scientific paradigm, but it had never been clear about what actually moves the plates around,' said Professor Stern of the university's geography department.

'To work this out requires an understanding of what happens at the bottom of a tectonic plate. It's been difficult to obtain the necessary detailed images at such great depths using the usual method of recording natural earthquake waves.

'But by generating our own seismic waves using higher frequency dynamite shots we were able to see how they became modified as they passed through different layers in the earth.

'This, along with some new techniques in seismic reflection processing, allowed us to obtain the most detailed image yet of an oceanic tectonic plate.'

He says that the thinner layer beneath the plate appears to contain pockets of molten rock that make it easier for the plates to slide across them.

'This means that the plates can be pushed and pulled around without strong resistance at the base.

'A weak slippery base also explains why tectonic plates can sometimes abruptly change the direction in which they're slipping. It's a bit like a ski sliding on snow.'

The rock may be soft and jelly-like because of a higher concentration of water or magma than in rock in the lithosphere, which is 0.1 per cent magma.

The experts say that if the rock was made of just 2 per cent magma, it may explain its unusual consistency.

'On a million-year time scale this would appear weak and jelly-like,' Professor Stern said.

He said that understanding the boundary between the base of cold, rigid tectonic plates and the underlying hot mantle underneath is central to our knowledge of plate tectonics, as well as the very formation and evolution of our planet.

HOW EARTH GOT ITS CRUST: TINY GRAINS LED TO PLATE FORMATION

Researchers say weak zones caused the Earth's mantle to crack, causing tectonic plates to form

They are responsible for the formation of continents, and are still active - as earthquakes and volcanic eruptions show. 

However, researchers have never been able to work out why the Earth developed tectonic plates, but other planets did not.

On April, Yale said it had solved the mystery - and say the key is tiny minerals within rocks.

The research suggests how and when Earth came to develop one of its most distinct features — rigid tectonic plates — and why Venus, Earth's twin-like neighbor, never has.

'We think it all comes down to the behavior of tiny grains of minerals within rocks,' said Yale geophysicist David Bercovici, lead author of research published online April 6 in the journal Nature.

The researchers argue that in Earth's early days, mantle convection caused weak zones in the lithosphere - the outermost shell of the planet.

These zones persisted, developed into plate boundaries, and ultimately connected to form a global network of tectonic plates. The weak zones, they argue, resulted from the tendency of mineral grains in rocks to shrink as the rock deforms.