A tsunami is a sequence of powerful ocean waves generated by the significant, rapid displacement of a large volume of water. These waves are distinct from typical ocean swells, as their immense energy originates from sudden disturbances beneath or near the ocean surface. Understanding the various natural phenomena that can lead to these impactful events provides insight into their formation.
Underwater Earthquakes
Underwater earthquakes represent the most frequent and impactful cause of tsunamis. These seismic events commonly occur in subduction zones, which are areas where one tectonic plate slides beneath another. As these plates interact, immense stress accumulates over extended periods, sometimes decades or centuries, until the locked sections suddenly slip.
When this sudden release of energy occurs, it causes a significant vertical displacement of the seafloor, either uplifting or dropping it. This abrupt movement directly transfers energy to the overlying water column, pushing a massive volume of water upwards or downwards from its equilibrium position. This initial disturbance creates the nascent tsunami wave.
Not all underwater earthquakes generate tsunamis; those that do typically have a magnitude greater than 7.0 and occur at a shallow depth, generally less than 100 kilometers below the Earth’s surface. Earthquakes deeper than this are less likely to cause the necessary vertical seafloor displacement. The size of the affected seafloor area and the amount of vertical movement also influence the resulting tsunami’s scale.
The energy from this seafloor displacement propagates horizontally through the water column, forming long-wavelength waves that can travel across entire ocean basins. While these waves may be barely noticeable as small swells in the deep ocean, they can reach speeds of up to 800 kilometers per hour. As they approach shallower coastal waters, their speed decreases, but their height dramatically increases, leading to destructive coastal impacts.
Submarine Landslides and Volcanic Activity
Beyond earthquakes, submarine landslides and volcanic activity can also trigger tsunamis through rapid water displacement. Submarine landslides occur when large masses of sediment, rock, or debris slide rapidly down underwater slopes. This sudden movement can push a significant volume of water, generating tsunami waves.
These landslide-generated tsunamis can be particularly dangerous near their source, potentially arriving within minutes with little to no warning. While they may be larger than earthquake-generated tsunamis close to their origin, they tend to lose energy more quickly and typically do not affect distant coastlines.
Volcanic activity contributes to tsunami formation in several ways, primarily through explosive underwater eruptions or the collapse of volcanic structures. Underwater eruptions can directly displace water, while the collapse of a volcanic caldera or a large coastal flank into the ocean can create a massive splash, generating waves. Such events, like landslides, involve the rapid displacement of water by a large mass of material, converting gravitational potential energy into wave energy.
Extraterrestrial Impact
While exceptionally rare in recorded human history, an extraterrestrial impact, such as a large meteor or asteroid striking an ocean, could generate a global-scale tsunami. The immense kinetic energy of such an object would instantly displace an enormous volume of water upon impact. This displacement would create a colossal wave that would propagate outwards across the ocean. The scale of the resulting tsunami would depend on the size and speed of the impacting object, as well as the depth of the ocean at the impact site.
Submarine Landslides and Volcanic Activity
Beyond earthquakes, rapid mass movements such as submarine landslides and volcanic activity can also trigger tsunamis. Submarine landslides involve large volumes of rock, sediment, or debris sliding down underwater slopes. As these masses move, they displace significant amounts of water, generating waves that can evolve into tsunamis.
Landslide-generated tsunamis often impact nearby coastlines within minutes, providing little warning. While they can be quite large near their source, such as the 1958 Lituya Bay event in Alaska which created a wave that ran up over 500 meters, their energy typically dissipates quickly, limiting their reach to distant shores. Many of these landslides are themselves triggered by earthquakes, even those not strong enough to directly cause a tsunami.
Volcanic activity also contributes to tsunami formation through various mechanisms. Explosive underwater eruptions can directly displace water, while pyroclastic flows, which are fast-moving currents of hot gas and volcanic debris, can generate tsunamis if they enter the sea. The 1883 eruption of Krakatoa, for instance, produced devastating tsunamis largely due to pyroclastic flows entering the ocean, resulting in waves up to 40 meters high.
The collapse of a volcanic caldera or a large coastal flank into the ocean can also displace vast quantities of water. While caldera collapse was once a favored theory for tsunamis from events like the Minoan eruption of Santorini, recent research suggests that pyroclastic flows and submarine slumping were the primary tsunami-generating mechanisms in that instance. However, large flank collapses, though infrequent, have the potential to create significant local tsunamis.