My greatest dream is to become a geologist.
Because I am very interested in geology specifically, minerals and volcanoes.
I want to find ways to protect the environment from natural disasters.
That's why my greatest dream is to be a geologist.

We can define dream in two ways: It can be in the form of sleep and the other one is hope.

You mentioned that you want to be a geologist. It means this is what you hope. We rephrased some of the words you used to clarify what you mean.