by getting rock smash from oreburgh gym leader
Find a Pokemon that can learn Rock Smash. Like Geodude.
you can use the hm strength you can smash rocks with rock smash
HM06 Rock Smash
Get the HM Rock Smash
Teach the HM Rock Smash to a Pokemon and interact with the rock. Round rocks need the move strength and not all Pokemon can use these moves.
a move in Pokemon to smash rocks so you can get past
you must HM rock smash.
in rocks you smash
You get the HM rock smash and teach it to one of your Pokemon. You get it on island 1.
There is no way to break the boulders that don't have cracks. I've played through the game a few times and I don't recall coming to any place where you need to break one to get to an item or anything.
You use the Rock Smash TM
from the digger