Why Are Sedimentary Rocks More Useful Than Metamorphic Or Igneous Rocks In Establishing The Relative Ages Of Rock?

Why are sedimentary rocks more useful than metamorphic or igneous rocks in establishing the relative ages of rock?

 

Because sedimentary rocks are made of sediment. That is the goop at the bottom of the seas or lakes or whatever build up over time as sediment. They uplift into the air, or become a dry sea bed and become rocks. As the sediment "built up" it stored a record of what made up the layers of sediment at the time it existed as goo. That is where the rock scientists get their information. While igneous rocks are produced by great heat such as a volcano or meteor strike. Basically, they are rocks that started out molten. They do not record time very well unless connected to a known geological incident. And metamorphic rocks are rocks that changed form or composition through some natural or unnatural process. Their ages are hard to establish unless connected to some known geological incident.