From Darkest Hour: The True Story of Lark Force at Rabaul – Australia’s Worst Military Disaster of World War II, by Bruce Gamble (Zenith, 2006), Kindle Loc. 391-417:
The most recent of these caldera-forming eruptions occurred sometime around AD 600, though its exact date is a mystery. The eruption was cataclysmic—one of the most powerful since the time of Christ—and utterly devastated hundreds of square miles of New Britain and the surrounding islands. It likely began with a period of vigorous seismic activity which generated large quantities of magma beneath the existing ring fractures. Numerous tremors shook the island over a period of days or even weeks as pressurized gases weakened one of the old fault lines. The earthquakes grew in frequency and intensity until the conditions underground finally reached a critical state. At some point, the magma chamber not only boiled over, it blew apart.
The noise must have been stupefying. The ground literally ripped apart around the weakened ring fracture, from which a great ring of fire twenty miles in circumference burst forth. Pent-up gases exploded from below, hurling a thick column of rock, dust, and ash into the sky. The tiniest particles, boosted by heat and convection, soared an estimated one hundred thousand feet into the upper atmosphere. Larger rocks and glowing blobs of magma arced back to the surface, where they splattered against the ground or struck the sea with the sound of thunder.
The greatest devastation resulted from the terrible cloud itself. Most of the material hurtling skyward eventually lost momentum, then gravity took over and the outer portions of the dark, roiling column collapsed. Superheated to more than 1,800 degrees Fahrenheit, the material accelerated as it fell, and when it hit the ground it burst outward at more than one hundred miles an hour. Known as “pyroclastic flow,” the incandescent cloud spread rapidly over the ancient volcanoes and raced downhill to the sea, boiling the water spontaneously as it blasted across the surface. Outlying islands were wiped clean in seconds. By the time the energy finally dissipated, the fiery cloud had killed every living thing on land and marine life near the ocean’s surface for thirty miles in every direction.
Other destructive effects reached even farther. The prevailing winds carried heavy accumulations of ash fifty miles southwest of the volcano. Huts collapsed, crops were ruined, and the surviving islanders groped through blinding, polluted air. They too would be wiped out, doomed to eventual starvation unless they could quickly find a source of unaffected food.
Sometime after the eruption subsided, the unsupported roof over the empty magma chamber caved in. An oblong area approximately seven miles long and five miles wide collapsed suddenly, sliding downward for hundreds of feet. Additionally, the sea breached a portion of the southeastern rim and flooded most of the huge depression.
After the dust finally settled and the sea calmed, a large portion of the island resembled a bizarre moonscape. The pyroclastic flow had deposited grayish veneers of ash and pumice on the steep slopes of the old volcanoes, and low-lying areas around the caldera were buried under a hundred feet or more of the stuff. Based on vulcanologists’ estimations, the eruption had disgorged ten cubic kilometers of magma and debris from the earth. (By comparison, the eruption of Mount Vesuvius in AD 79 displaced only three to four cubic kilometers, and the explosion of Mount St. Helens in 1980 displaced less than one cubic kilometer of material.)
Compare Krakatoa and Long Island (Papua New Guinea), which produced similarly massive eruptions.