I think the type of addiction marijuana causes is mainly a psychological one how ever some very small elements of the addiction in extreme cases are physical

I got this from wiki

"The medical community now makes a careful theoretical distinction between physical dependence (characterized by symptoms of withdrawal) and psychological dependence (or simply addiction). Addiction is now narrowly defined as "uncontrolled, compulsive use"; if there is no harm being suffered by, or damage done to, the patient or another party, then clinically it may be considered compulsive, but to the definition of some it is not categorized as "addiction". In practice, the two kinds of addiction are not always easy to distinguish. Addictions often have both physical and psychological components."