Fox Glacier under highest peaks in New Zealand - Mt. Tasman 3,497m and Mt Cook 3,754m

When do we call a chunk of ice a glacier? Usually, the ice mass has to be at least 100m x 100m in size and needs to show some signs of a present or past movement.
Generally, glaciers are divided into two main groups – Ice Sheets and Valley Glaciers, each with several sub-types.
Ice Sheets or Continental Glaciers are the largest masses of ice on Earth spreading over 50,000 square kilometres with the depth of ice sometimes more than 4,200m. They are only found in Antarctica and Greenland. Ice Shelves are floating extensions of Ice Sheets and surround most of Antarctica. Ice masses smaller than 50,000km2, with a dome-shaped accumulation of ice and snow completely covering mountainous areas, are called Ice Caps and are found in high elevations of polar and sub-polar regions. Similar but smaller than Ice Caps are Ice Fields which can cover low-relief mountain plateaus providing a source of ice for Mountain or Valley Glaciers. Relatively smaller than its originating ice source are Outlet Glaciers, these are rapidly moving frozen streams flowing out of larger bodies of ice.

The ice flow of a glacier is divided into two types, internal deformation and basal sliding. Internal deformation is movement of parts of the glacier relative to itself. Throughout the descent of the glacier, the ice at the sides and bottom side of the glacier are subject to more friction from the valley floor and walls, and therefore forced to move at a slower rate than the ice in the centre of the glacier. As a result the ice deforms within itself. Basal sliding occurs as water, either rain or melt passes underneath the glacier and acts as a lubricant between the ice and the bedrock. This is the main source of terminal velocity of temperate glaciers.

The bottom layers of the glacier flow deform plastically under the pressure allowing the glacier as a whole to slide down the valley like a huge toffee. It is the upper layer of around 30metres where the glacier is more brittle, therefore the ice here cracks and breaks into crevasses and seracs in order to bend over uneven bedrock.