The deccan plateau which was with the african continent got separated and started drifting towards asia.As it joined or collided with the asian continent the lates starting projecting upwards,this lead to the formation of himalayas.
According to continental drift theory, the earth was as angara land and gondwana land n they are seperated by tethys sea.Before some million years ago due to plate tectonics the water of this sea had raised up n cooled down, then it formed as present today's himalayas(as per my knowledge)