Less dense substances move upwards to form a crust.
密度小的物质向上浮动并形成一层皮。
《牛津词典》
2
The wetter snow associated with warmer temperatures in recent years causes a frozen crust to form over the plants, which the reindeer can't break through.
最近几年，与较高温度相关较湿的雪地导致了在这些植物上面形成一层冷冻外壳，而驯鹿却无法打破这层外壳。
3
One moment the crust took the form of a shattered windshield, then it coalesced into a jigsaw puzzle, then a ragged map of the world.