Continual: things that continue for a period of time, but there are interruptions in the process - like arguments or rain: My mother continually tells me to clean my room. / My friend and her husband argue continually.
Continuous: things that never end; things happening without interruption - She cried continuously for hours./ Rivers flow continuously.
"He studied continuously for 8 hours." - He studied for 8 hours without taking a break.