They are a collection of volcanic islands in the Pacific Ocean near the equator,about 600 miles west of south America. 在 west of south America 中,west of 为“复合介词”,结构是:adv. + prep.,由于 west 是副词,所以不能、也不需要加 from。 about 600 miles 修饰介词短语,说明 volcanic islands 与 south America 的具体距离,而且是“在南美以西”的距离。
去掉 about 600 miles 仍然是正确的句子,只是不如原句的内容具体细致:
They are a collection of volcanic islands in the Pacific Ocean near the equator,west of south America.