Final answer:
The movement of ice within a glacier is best defined as flowing downhill under the influence of gravity, facilitated by partial melting and water at its base providing lubrication.
Step-by-step explanation:
The movement of ice within a glacier is best defined as flowing downhill under the influence of gravity. This movement occurs because a glacier is a dynamic system, which forms when winter snowfall exceeds summer melting. Glaciers in places like Glacier National Park are under continuous change, responding to temperature and precipitation. Scientists from the US Geological Survey (USGS) define a glacier as a body of snow and ice large enough to move under its own weight, and this movement is often indicated by the presence of crevasses. Glaciers retreat when melting exceeds the accumulation of snow.
Under the immense pressure of its own weight, a glacier can partially melt, producing a layer of water at its base that serves as lubrication, facilitating its downhill motion. This process, called basal sliding, along with internal deformation of ice, explains how glaciers are capable of moving. It is noteworthy that Option 1, referring to accumulation at the glacier's base, and Option 3, mentioning sublimation, do not adequately define the movement of glacier ice. Option 4 is also incorrect as glaciers are not stationary at all times. Therefore, Option 2 is the correct answer in this context.