PAHOA, HAWAII - "Ballistic blocks" the size of microwave ovens shot from Hawaii's Kilauea volcano on Wednesday (May 16) in what may be the start of explosive eruptions that could spew huge ash plumes and hurl smaller rocks for kilometres, the US Geological Survey said.
Such eruptions, last seen nearly a century ago, have been a looming threat since Kilauea, one of the world's most active volcanoes, erupted nearly two weeks ago.
The explosive, steam-driven eruptions could drive a 6,100-metre ash plume out of the crater, hurl boulders the size of small cars up to 800 metres and scatter smaller rocks over 19km, the USGS has warned.
This type of eruption has the potential to carpet the Big Island in much thicker ashfalls than those up to now and possibly spread ash and volcanic smog across the Hawaiian islands and farther afield if it enters the stratosphere.
"This morning, dense ballistic blocks up to 60cm across were found in the parking lot a few hundred metres from Halemaumau (Kilauea's crater)," the USGS said in a statement.
"These reflect the most energetic explosions yet observed and could reflect the onset of steam-driven explosive activity."
The USGS cautioned that "additional such explosions are expected and could be more powerful".
A 4.2-magnitude earthquake at the volcano at 8.36am prompted authorities to issue an alert reassuring rattled Big Island residents that there was no risk of a tsunami from the volcanic activity.
Smog from Hawaii's Kilauea volcano drifted north up the island chain after communities south of its summit were warned of up to a 0.6cm of ashfall as the nearly two-week eruption intensified.