There is no difference between "andesitic" and "andesite". They are both referring to the same thing.

* Andesite is the name of a type of extrusive igneous rock.

* Andesitic is an adjective that describes something related to or resembling andesite.

Think of it like this:

* Apple is a type of fruit.

* Apple-like is a description of something that resembles an apple.

So, you might say "This rock is andesitic in composition" or "This volcano is known for erupting andesite."

It's like saying "That car is red" or "That car has a red paint job." Both mean the same thing, just phrased differently.