He noticed that a river of liquid rock was coming towards him.
这里的that在口语和非正式文体中可以省去。 因为这个that引导的是一个宾语从句 ,他作谓语动词noticed 的宾语。 根据动词notice的用法,除了带宾语从句外,也可以写成主谓宾+宾补结构,即: He noticed a river of liquid rock coming towards him. (他注意到一股岩浆正朝他涌来。)
that a river of liquid rock was coming towards him (一股岩浆正向他涌来)是谓语动词noticed宾语从句。英语语法规定that引导的宾语从句在口语和非正式文体中可以省去。