Why do tsunamis grow into large waves as they approach land?

When the waves come closer to the land they rub against the sea floor, and friction causes the waves to slow down and build up from behind creating huge piles of water to crash on the land A tsunami spreads out from an earthquake's epicenter and speeds across the ocean. In the open ocean, the height of the wave is low. As a tsunami approaches shallow water, the wave grows into a mountain of water.