HILO » Last year's gassy, dusty eruption inside Halemaumau Crater did far more swallowing than spitting, the Hawaiian Volcano Observatory said.

Scientists do not know how or why it happened, they said in their weekly “;Volcano Watch”; column.

From March to December the hole at the base of Halemaumau spit out mostly dust, small rocks and bits of fresh lava.

When geologists added up all that stuff, they estimated the volume at 2,540 cubic yards. Half, 1,270 cubic yards, was bits of old rocks, and half was spatters of new lava.

When they looked at the hole in the bottom of Halemaumau, about 300 feet across at its widest and equally deep, observatory scientists calculated that the amount of old rock blasted out should be 290,000 to 360,000 cubic yards.

In really round numbers, the hole is 200 to 300 times bigger than the 1,270 cubic yards of old rock erupted.

So the scientists faced an eruption that swallowed.

“;Clearly, most of the rocks removed to form the vent crater must have collapsed into the underground rather than being tossed onto the ground surface,”; the scientists said. “;What did the rocks collapse into? Was it a cavity, did they fall onto a 'conveyor belt' that carried them away, or what?”;

Cavities underground are “;anathema,”; according to the “;Volcano Watch”; column. Observatory geologist Janet Babb explained that such holes are not included in conceptual models of how volcanoes work, and there is no clear idea of how they would form.

But they have been seen before, indirectly, when eruption sites collapse, forming pit craters. The same thing might have happened at Halemaumau, the scientists said.