Glaciers are compacted ice. They freeze and melt slightly then refreeze, and when this happens gravity pulls the glacier closer to the center of the earth. Generally, this is downhill. When glaciers move as such, they can pull along abrasive rocks, carving the landscape they are in.
Glaciers move simply because they are very heavy, so that the influence of gravity will continually pull them downhill. They are a plastic formation, meaning that they are able to change shape as they go around obstacles like mountains. As to how they move: a very thin bottom layer of the glacier will melt from the friction with the ground, creating a layer of liquid water that may be only a milimeter thick. This allows them to glide along on to of the water with reduced friction.
Glaciers will advance due to gravity, or retreat as their snow and ice melts (though even as they retreat they will still be pulled forward by gravity). Glaciers usually move very slowly, to that the advance and retrat is unnoticable, but occasionally they surge, moving up to ten meters a day!
Glaciers move because of a variety of factors. One of the largest is the relationship between temperature and pressure. With ice, melting point drops as pressure increases, and the ice at the bottom of a glacier is under extreme pressure. Sometimes with this and latent heat from the Earth’s surface, the ice starts to melt at the bottom and gives the glacier a slick surface to slide/move on, but very, very slowly. Sometimes, glaciers move rather quickly in what is called a surge (ex. 7 miles in three months). Scientists aren’t quite sure why glaciers surge, but it might be due to a delicate balance of many factors riching a tipping point.
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