The kind of rocks that weather most rapidly are sedimentary rocks.
Sedimentary rocks are the rocks with the most fragile structure of the three groups of rocks. They are coming to existence only after the igneous rocks fall apart, which is a process that takes millions of years, and after that after certain conditions the debris left from the igneous rocks are merging and usually making layer type rocks over time and create the sedimentary rocks. They are very soft for rocks and can very easily be distinguished by just scratching them with your nail or with any harder material where they lose their structure and fall apart relatively easily.
