Io is the most volcanically active object in the solar system with intense eruptions that cover the entire moon in a centimeter of lava every year.
    
Earth probably went through a similar phase in its youth, back before the planet cooled enough for plate tectonics to start up, according to the study in the journal Nature.
    
Earth formed from the fusion of many small, rocky "planetesimals" about 4.5 billion years ago. These collisions generated lots of heat, as did the subsequent separation of Earth's metallic core and the decay of radioactive elements.
    
Consequently, the ancient Earth harboured far more internal heat than it does today - perhaps five to 10 times more.
    
However, the planet's lithosphere - its rigid outer shell, composed of the crust and upper mantle - was relatively thick and cool in those early days. That shouldn't be the case if plate tectonics were the main means of dissipating interior heat back then, as it is now, researchers said.
    
Instead, the early Earth may have worked more like Io, where heat flows to the surface through volcanic "pipes" in huge quantities.
    
The moon is tugged so hard by Jupiter's powerful gravity that it currently transports about 40 times more internal heat than Earth does despite being just 30 per cent as wide as our planet.
    
"The heat pipe [idea] explains that, by allowing heat through the lithosphere in particular places, the pipes, which allows the rest of the lithosphere to be thick and cold and strong," said study lead author William Moore, of Hampton University in Virginia.
    
The chief alternative to the heat-pipe model developed by Moore and study co-author Alexander Webb of Lousiana State University is a version of plate tectonics, in which Earth's enormous lithospheric plates simply moved faster and transported more heat long ago.
    
But a hotter interior would likely have generated more molten rock, producing thicker, more buoyant plates that would have taken longer to cool down enough to dive back into Earth's mantle, Moore said.
    
Also, rocks that formed about 3.5 billion years ago preserve evidence of periods of intense, continuous volcanism on Earth lasting several hundred million years. That's another knock against the ancient tectonics argument, Moore added.

(Agencies)

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