You can picture riven as a log that has been split with a wedge rather than cut with a saw. The crack follows the wood's own grain, so it opens along a line that was already inside the wood, and the two faces are rough. Hold that picture and the harder use follows: a group can crack the same way, along lines that were already inside it.
可以想成：riven 是一塊被楔子劈開的木頭，不是被鋸子鋸開的。裂痕順著木頭自己的紋理走，所以那條線本來就在裡面，只是被打開了，斷面也是粗的。把這個畫面記住，另一個用法就很好接：一群人也可以這樣裂開，沿著本來就存在的那幾條線。
From the word’s history
The packet's source gives riven as the past participle of the verb rive, from Middle English riven, and points to Old Norse rífa, to tear or scratch. Tearing has been the picture from the start.
來源記到 riven 是動詞 rive 的過去分詞，來自中古英文的 riven，並跟古北歐語的 rífa（撕、抓）有關。所以「撕開」這個畫面從一開始就在這個字裡面，不是後來才加上去的。
Wiktionary, "riven"
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