When I first introduced plate tectonics, it sounded like a relatively boring force. It's kind of cool that the Earth's surface is made up of gigantic slabs, which move around and collide. But these slabs move at the same rate that fingernails grow. Even with the huge mass of each plate, you wouldn't expect things moving at 5 cm a year to do much. However, as demonstrated by earthquakes and some types of volcanoes, the slow-moving plates can have pretty devastating effects. Along with these, there's a third type of natural disaster often associated with plate tectonics. And this one comes by sea.
Tsunamis, also called tidal waves (although this is a misnomer) are some of the most impressive natural disasters, second only to volcanoes. They are a gigantic wave of water that crashes onto land. The 2004 tsunami in the Indian Ocean was over 50 ft tall (15 m) in some areas. This, like most tsunamis, was caused by an earthquake. A tsunami is basically a series of enormous ripples, when a large amount of water is displaced. In the deep ocean, tsunami waves are very small, barely enough to rock a boat. It is when they get to land that they become devastating. The column of water drags along the ocean bottom, while the top part continues to move quickly. This speed difference causes the tsunami to reach devastating heights. The animation below shows how this works.
Like I mentioned above, tsunamis are caused by displacement. Underwater earthquakes are the primary cause of these. However, large landslides into the ocean can also cause tsunamis. This occurred in Lituya Bay Alaska, in 1958 causing one of the largest tsunamis ever recorded. Geologists and oceanographers also predict that the next mega-tsunami will occur if volcanic activity causes a landslide in the Canary Islands. A good portion of the island would collapse into the ocean, sending a tsunami wave towards the east coast of the United States. Between that, and the Yellowstone caldera, there is not a lot of the United States that will be secure from natural disasters. Fortunately, these gigantic events are rare, and can be monitored using seismographs and other devices, so there will should be plenty of warning before a giant tsunami, or some other mega-disaster occurs.