A giant section of a glacier on the Mont Blanc massif in the Alps is in danger of collapsing, experts warned on Wednesday, prompting the precautionary closing of two roads and the evacuation of mountain huts in northwestern Italy.

A block of about nine million cubic feet of the Planpincieux Glacier on the peak of Grandes Jorasses, a mountain on the French-Italian border, has splintered and could give way, according to Daniele Giordan, a geologist working for the Italian National Research Council.

Dr. Giordan, who is part of a team that has been monitoring the glacier since 2013, said by phone on Wednesday that though a fracture in the glacier was common after the warm summer months, the crack detected this time was alarmingly wide.

“This year, we noticed again the presence of a large crevasse,” he said. “The opening of this crevasse can create the collapse of a very large block.”