状语从句省略形式的主语判断

问题描述

老师您好，请教这句：

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.  

省略的状语从句 when compared to ... 其被省略的主语是什么？

 这好像不符合“状语从句省略，其逻辑主为主句主语”规则，因为此处 compared to 对比的是 the great age of Earth 是一个时间，而主句主语 its surface 并不是时间。

老师答疑

刘老师：

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。