Principle of Inclusion

Notice in the image above, there are rock fragments located in a bed of granite—the rocks are "included" in the existing rock. Imagine an existing rock, Rock A. Then imagine that a body of magma moves upward through the crust and dislodges and engulfs large fragments of Rock A. The magma then cools and hardens to form Rock B. Do you see how Rock B contains fragments of Rock A?

This is a simple but important principle: the fragments of Rock A in Rock B are older than Rock B. Summed up, the principle of inclusion says that a fragment of a rock incorporated or included in another is older than the host rock. Fragments may be incorporated by a moving magma body or they may be fragments weathered off another rock and subsequently made into a new sedimentary rock. It makes sense that the fragments that make up a sedimentary rock are older than the sedimentary rock itself.

From the previous unit on rocks and minerals, can you remember a sedimentary rock that contains large pebble-size fragments from other rocks? What does the principle of inclusion tell you about the fragments in that rock?

Conglomerate and breccias are two examples of sedimentary rocks that contain pebble-size fragments weathered off of previous rocks. The principle of inclusion tells you that the fragments are older than the rock.