What's the study of how tiny particles of iron in rock aligned at the same time if their formation?

Answer is: magnetism. 
In rock formation, the tiny particles of iron in rock aligned at the same time with Earth's magnetic field.
Ferromagnetism is mechanism in which certain materials (for examle iron) form permanent magnets or are attracted to magnets.

Answer the use of flood plains and irrigation;

Ibelieve it's the pacific islands