A( ) is an enormous mass of moving ice. they form in areas that are so cold that snow stays on the ground year around. because glaciers are so massive, ( ) cause them to flow slowly. fill in the blank

A glacier is an enormous mass of moving ices. They form in areas that are so cold that snow stays on the ground all year round. Because glaciers are so massive, gravity causes them to flow slowly. 
Glaciers are so heavy that the sheer weight of the ice causes them to move slowly down a mountain side. The bottom of the glacier moves more slowly than the top of the glacier. This due to the friction caused by the earth's surface