The New Zealand earthquake broke an iceberg the size of 20 football fields off the country's longest glacier.

The chunk is estimated to have been three-fourths of a mile (1,200 meters) long by 250 feet (75 meters) wide, scientists say.

The iceberg's collapse also kicked up 10-foot (3.5-meter) waves in Tasman Lake.

"We heard a large crack like a high-powered rifle," a U.S. tourist who had been on a glacier tour at the time told the New Zealand Herald.

New Zealand Earthquake Gave Glacier "Last Kick?"

Such an event is rare but not unprecedented, said Martin Truffer, a glaciologist at the University of Alaska's Geophysical Institute in Fairbanks, Alaska.

Pieces of Alaska's Hubbard Glacier also broke off in 1958, following a large earthquake in the state's southeast.

Such events, however, don't occur totally at random. "There has to be an earthquake in the area of a glacier that was ready to cast off a big chunk. ..." Truffer said.

Furthermore, glaciers such as the Tasman—which calve into lakes—tend to be less crevassed, or cracked, at their ends than those that calve into the ocean. This makes such lake-based glaciers more likely to lose big pieces of ice rather than smaller ones.

"A block was probably ready to go, and [the earthquake] just gave it the last kick," Truffer said.

Heavy Rain Eased Ice Breakup

Even before the Christchurch earthquake, tourist guides were already being cautious about not approaching the glacier too closely, due to heavy rain in past weeks, the Herald reported.

The rain had probably raised the lake level, which made it easier for large chunks to break off, Truffer explained. It's likely, he added, that the end of the glacier was floating, not anchored to the lake bed. (See extreme-ice pictures.)

"If you have a floating part of the glacier and you raise lake level, you induce bending [of the ice] that can help the calving process."