Officials believe that ice plugged up a pipeline and likely caused a rupture that sent 46,000 gallons of crude oil and water gushing onto snow-covered tundra on Alaska's North Slope late last month.

The spill is one of the worst by volume since the March 2006 spill of 200,000 gallons of crude at Prudhoe Bay, the biggest spill ever on the North Slope, according to Department of Environmental Conservation figures.

BP spokesman Steve Rinehart said Wednesday that an ice buildup is likely to blame in the Nov. 29 spill, leading to an increase in pressure that caused the 18-inch diameter pipe to rupture.

Oil and water sprayed out of a 2-foot lengthwise rupture along the bottom of the pipe. Up to three-quarters of an acre of tundra was affected. Most of the oil and water congealed in a large pile under the pipe.

"There is a lot of material on the ground," said Tom DeRuyter, the on-scene spill coordinator for the Alaska Department of Conservation.

The pipeline normally carried 75 percent water and 25 percent oil, as well as gas, to a processing center at the Lisburne oil field. It is not known what the percentages were when the line ruptured, Rinehart said.

Responders were using a variety of methods to clean up the spill. Methods include applying steam to loosen the congealed material and vacuum it up. Equipment also was brought in to scoop up the oil and frozen water and transport it to an area where it will be melted, separated and measured.

"That mechanical cleanup has proven to be pretty effective," Rinehart said.

The ruptured pipeline, which is about 5 feet above the ground, is not affecting production from the Prudhoe Bay oil field, North America's largest oil

Rinehart said the definitive reason for the most recent spill won't be known until an investigation is completed.

BP is currently on probation for the 2006 spill after pleading guilty to a misdemeanor conviction and paying $20 million in fines and restitution. That spill was blamed on corrosion in a pipeline.

Rinehart said several weeks before the rupture the line was shut down because of restricted flowAnother larger pipeline adjacent to the pipe was handling the extra volume.

Rinehart said the paired pipelines were each equipped with individual temperature sensors near where the lines enter the processing center. He said he did not know if the sensors indicated there was a problem. A BP employee discovered the rupture in the line during a routine early morning inspection.

The line was last inspected in 2008 and found to be serviceable, he said.