Answer: Made up of
Step-by-step explanation:
Composed can mean several things such as being able to remain calm or being made up of something. In this case it is the latter. When they say that something is composed of something else, it means that the first thing is made up of the second thing.
In this case therefore, the oceanic crust being composed of pieces that cover the ocean floor means that the oceanic crust is made up of those pieces.