In this passage, Joseph Smith edited three of the four instances of is to are. For each case of grammatical emendation, he seems to have interpreted the rocks as the referent for the relative pronoun which, thus the need for the plural are. But for the fourth instance of is, Joseph left the is in the singular, undoubtedly because its subject noun phrase was the singular “the more part of it”. Interestingly, the 1908 RLDS edition restored the original singular is in the third case, perhaps because the editors felt that the referent for the relative pronoun which was the singular the earth rather than the plural the rocks. One could also interpret the occurrence of it in the fourth case as referring to the earth, although more likely the it actually refers to the rocks since the next verse uses the plural pronoun they in a yea-clause to restate the previous statement that refers to the rocks being broken up (“yea they shall be rent in twain”). Usage elsewhere in the Book of Mormon shows that the singular pronoun it can refer to a collective plural (such as the rocks of the earth). For another example of this kind of usage, see under Helaman 13:18–20 for the use of the pronoun it to refer to treasures. Here in Helaman 14:21–22, the critical text will restore all the original instances of is that Joseph edited to are.
Summary: Restore or maintain, as the case may be, each instance in Helaman 14:21 of original is; usage elsewhere in the text shows that the singular pronoun it as well as the singular is can be used to refer to collective plurals, such as the rocks of the earth.