Hawaii's Kilauea Volcano could be about to get a lot more violent.

The summit sat quiet behind a cloud of ash and rain on Wednesday.

But geologists report that it's started hurling chunks of rock the size of microwaves.

The US Geological Survey said that these so-called "ballistic blocks" could signal the start of bigger, explosive eruptions as steam builds up beneath the surface.

Residents were also rattled by a 4.2 magnitude earthquake which shook buildings and sent cracks along a major highway.

Geologists say the real danger is if Kilauea's lava lake falls under the water table.

That could create enough steam to trigger a major eruption - sending ash plumes as high as 20,000 feet and hurling boulders the size of small cars up to half a mile away.

Meanwhile, Hawaii's governor says the state is getting ready for a bigger disaster, organising a joint task force with the military and the national guard to carry out mass evacuations, if needed.