Where and Why are Metamorphic Rocks Formed?

2.1 Introduction

Each of the two images scrolling at the top of this page is an example of a set of rocks with something in common. All the rocks in each series have approximately the same chemical composition. The rocks in one set have the composition of mudrocks, shales. The rocks in the other set have the composition of carbonate rocks, siliceous dolomites. Can you tell which is which? How can you tell?

Even though the rocks in each set have approximately the same composition, they do not look like one another. Because both groups have sedimentary rock compositions, it is reasonable to conclude that both once were sedimentary rocks, but have been changed by metamorphic (meta- = change + morph = form) processes. Why did this happen? Where on earth, geologically, did it happen? This chapter considers these questions.