This diagram shows how a Tsunami is formed; the movement of the tectonic plates sometimes cause them to smash together, causing an earthquake, when an earthquake occurs underwater: it creates a large wave above the fault line, and those waves will travel, getting larger and larger the closer they get to the coastal line, and by the time they reach it, they will have grown into a full blown tsunami, possibly higher than 1000-M tall!
Tsunamis have four stages for when they form, the first stage is called "initiation", when an earthquake happens underwater and creates a tsunami. The second stage is called "split", when the wave formed by the earthquake splits and part of it travels towards the ocean, and the other part of the wave go towards the closest land. The third stage is "amplification", when the tsunami hits the shore and enlarges. The last stage is called "run-up", when the tsunami goes over the shore in one of two ways, the most common way is when the shore is flooded as if the tsunami was a very large tide, the second way is when the shore is hit with a huge wall of water.