Most people consider the landscape to be unchanging, but Earth is a dynamic body, and its surface is continually altering---slowly on the human time scale, but relatively rapidly when compared to the great age of Earth.

【翻译】大部分人认为地形外貌是一成不变的，然而地球是一个动态的物体，其表面随着人类发展的进程在持续缓慢地变化着。当然，与地球冰河时代的地貌变化相比，这个变化相对快了很多。

为了理解方便，保留必要的成分：

its surface is altering relatively rapidly when it is compared to the great age of Earth.

when compared省略了it is，it应该指代its surface，to 之后似乎省略了that of。