Hobbies And Interests
The Shifting Earth
Tectonic plates lie above the Earth̵7;s mantle. These plates cover the surface of the planet, including tall mountains and deep oceans. Tectonic plates move constantly, though mostly shifting back and forth just a few inches per year. Sometimes, forces below the surface build up, and the plates shift more violently. Earthquakes along fault lines, volcanic eruptions and large-scale landslides are all possibilities as the forces release energy. When plates move enough that one slides under another, the result is called a subduction earthquake.
The Defining Event for a Tsunami
When subduction earthquakes occur on the ocean floor, the violent upward shift of one tectonic plate also moves tons of seawater upward. Should the subduction quake be strong enough and sudden, that column of water moves outward as a tsunami. Subduction earthquakes are a leading cause of tsunamis, but they are not the only one. Large landslides that occur underwater or near a shoreline are also capable of triggering a tsunami if the movement displaces a large volume of seawater. Calving glaciers and underwater volcanoes close to the sea̵7;s surface may also produce a tsunami. Not every one of these events immediately results in a tsunami and, should one occur, it may not race across the ocean at great speeds, nor may it be large enough to impact a coastline. The ocean often absorbs much of the energy released by these events. Ocean depth and nearby landmasses may also absorb the energy or influence the creation of a tsunami.
Tsunamis at Sea
A tsunami travels outward from its source at speeds close to those of a jetliner. At 805 kilometers per hour (500 miles per hour) or more, a tsunami that occurs off the Asian coast can reach the Pacific Coast of the United States in about 24 hours. The volume of ocean water moving in a tsunami reaches all the way to the ocean floor. As a result, the tsunami wave on top of the ocean is often of such insignificant height that those aboard a seagoing vessel might not realize a tsunami passed beneath them. In the open sea, a tsunami wave height may only reach less than a meter, or about 1 or 2 feet.
Shoreline Effect
Ocean depths diminish rapidly as a tsunami reaches the shore. The friction of water against the shallow sea bottom forces the wave to slow down, but it cannot release the energy still stored inside the wave. The combination of a slowing wave and a constant energy source makes the wave move in the only direction it can -- straight up. As the back end of the wave catches up to the portion slowed by shallow water, the wave grows to heights of 30 meters (100 feet) or more. The wave̵7;s trough reaches the shoreline first, and waters actually recede form the beach and bays empty of water. This temporary drawback only lasts about five minutes before the top of the wave strikes land with the full force of the energy it has stored within. Tsunamis often consist of more than one wave. Depending on the distance between them, the waves may pile up to form a massive tsunami wave or produce a series of devastating tsunami waves, each adding to the destruction of the coast.