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Is this correct?
Due to the weathering process the rocks are decaying and are broken apart.
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Almost. The only thing I would change is the end bit. You have a choice of:

1 "...are decaying and are BEING broken apart. "
2 "...are decaying and are breakING apart. "
3 "...are decaying and are HAVE BEEN broken apart. "

The mix of present and past tense participles sounds a bit strange there, that's all. The "weathering" bit is absolutely fine.

Rommie
Comments  
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3 "...are decaying and are HAVE BEEN broken apart. "

could I use "...are decaying and are HAVE BEEN breaking apart. "?
actually is "...are decaying and HAVE BEEN broken apart. ".. right??

could I use "...are decaying and HAVE BEEN breaking apart. "?
Sure, but they mean different things.

have been broken apart => the breaking apart process is finished.
have been breaking apart => the breaking apart process is still going on.
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