Are only sedimentary rocks used for relative age determination? Why?
The relative age is determined by index fossils and the Darwinian theory of evolution. The simpler a fossil is the older the fossils is determined to be. If a complex fossil is found in a strata beneath strata containing an index fossil thought to be older based on Darwinian Evolution the lower strata is classified as younger.
Only sedimentary strata contain fossils, so only sedimentary strata can be used to determine relative age determination.