Sometimes reaching heights of 40 meters (120 ft.) or more, tsunamis are the most dramatic and destructive of waves. Underwater disturbances, such as volcanoes, earthquakes and landslides, are the cause of these monster waves. The larger the disturbance, the larger the tsunami will be.
(NOT
TO SCALE)
In the open ocean, tsunamis
may be hard to spot. Long wavelengths can hide the size of the wave, but
just like other kinds of waves, changes occur when the wave enters shallow
water. The wavelength shortens, and the height increases. The strength of
the disturbance, the distance the wave travels and the shape of the
coastline combined determine the tsunami's height, and ultimately, its
destructiveness.
EARTHQUAKES ----------LANDSLIDES------------VOLCANOES
All three can cause a tsunami!
Information provided by: http://www.onr.navy.mil