The thick layer of ice and snow that has formed a permanent crust over Alaska and Antarctica is called an ice sheet.

Here's why:

* Ice sheet: A vast expanse of ice covering a landmass, typically more than 50,000 square kilometers (19,000 square miles). They are formed by the accumulation and compression of snow over thousands of years.

* Glacier: A smaller body of ice that flows downhill, often confined to valleys.

Both Alaska and Antarctica have massive ice sheets that are among the largest in the world.