Using a jackhammer on a rock does not change its structure, just breaks it into smaller pieces. Changing the crystalline structure requires partial or complete melting of the rock (must be hot enough to allow atomic rearrangement
-when Jackhammer is a huge hammer used to breaking up the hard rocks and the hard materials to smaller pieces.
-and the metamorphic rocks are type of rocks which change to metamorphic rock by heating and high pressure so, the chemical and physical properties of the rock will be changed.
-and here in cause of using the Jackhammer there is no change in the chemical or physical properties of the original rock and no high temp(more than 200°C ) or high pressure to change to metamorphic rock . It is just a breaking up of the rock to small parts.
