In places where it gets cold and snowy in winter, many meters of snow can fall. In some the following winter, adding a new layer to what was already there. Over hundreds to thousands of years, this process creates big sheets of ice called glaciers. Glaciers are found in the Arctic and Antarctic. They’re also common high in the mountains, even those close to the equator. Oddly enough, glacial ice is considered metamorphic rock!
The ice in glaciers starts off as individual snowflakes, which are themselves tiny, six-sided crystals of ice. As snowflakes form in a cloud, water molecules grab on to those sides (called facets). This is how facets grow the delicate branches that give snowflakes their distinctive look. When snowflakes collect on the ground, they trap air between them. This is what makes freshly fallen snow so fluffy. But as more snow falls, the bottom layers get squashed together.
As layer upon layer is added, the weight of the ice becomes heavy. It squeezes out the pockets of air. When the pressure becomes great enough, it presses the snowflakes together, causing them to transform into larger ice crystals. This process is called metamorphosis. The new crystals that form are larger and denser than snowflakes. Some can grow to the size of a baseball. The transformation turns the snow into a solid field of ice.