Where Tectonics Meet People: Tsunamis
Tsunamis
We have been looking at the ways that rocks move around on Earth and make mountains and some of the ways that this mountain building can threaten humans. Volcanoes and earthquakes are sometimes truly dangerous and damaging. But it is worth remembering that, in the developed world, only a few percent of us die in “accidents,” and car crashes greatly dominate those deaths (so the great majority of us die of other things, such as heart disease, cancer, etc.). With good scientific warnings, good zoning codes, trained medical personnel, hospitals, and ambulances to take care of us, nature kills very few of us (something like 0.03% of deaths in the US in most years). (In the less-developed world, this is, sadly, less true.) For the developed world, things we do to ourselves (smoking, eating, drinking too much, not exercising enough) are far, far more destructive to health and life than anything the planet does to us.
But it is still wise to know about the dangers from the Earth—part of the reason so few of us die from natural disasters is that we are already doing wise things to avoid being killed by nature! Some of those wise things involve preparing for giant waves—tsunamis (and preparing for earthquakes, landslides, volcanoes, floods...). Tsunamis are not directly related to our National Parks in this module, but tsunamis are related to some of the processes that helped make the Great Smokies and the Rockies. Anything that makes earthquakes, volcanoes, or steep slopes in or near the sea might be involved in a tsunami. And tsunamis can be truly horrific. We’ll start discussing a long-ago tsunami from a hot-spot setting and then look at other tsunamis closer to us.
Anomalous deposits are found on the flanks of many of the Hawaiian Islands, including Lanai, Molokai, and Maui, to at least 1,600 feet (500 m) above sea level. These deposits are composed of broken-up, mixed-up, battered corals, other shells, and beach rocks. Corals are undersea creatures and surely don’t grow 1,600 feet above sea level. It is true that some corals grow just below sea level and later are raised above the water by mountain-building processes; however, these Hawaiian deposits occur on islands that are sinking as they slide off the “hill” made by the Hawaiian hot spot, and the deposits are geologically too young to have been raised so far by mountain-building processes. Clearly, something strange happened.

One of the deposits, in particular, is the same age as a nearby, giant underwater landslide, as nearly as the age can be measured. The Hawaiian volcanoes have rather gradual slopes above the water, where the hot, low-silica lavas spread out to make shield volcanoes. But when lava hits the water, the hot flow cools and freezes very quickly, and can make steep piles. When a slope is too steep, it can fail in a great landslide, perhaps when melted rock is moving up in the center of the island and shoving the sides out to make them steeper. Surveys by specially equipped research vessels using side-scanning sonar have shown where several such slides have slipped. Such landslides can be miles thick, tens of miles wide, and over 100 miles long.
If a chunk of rock miles thick and tens of miles wide suddenly starts moving, maybe at hundreds of miles per hour, it will shove a LOT of water out of the way. Where will the water go? The answer is that it will make a huge wave, or tsunami, that will race across the ocean, and up onto any land it encounters. Imagine a wave so huge that it would run far inland and reach heights of 1,600 feet above sea level. Fortunately, the highest deposits in Hawaii are from a tsunami about 110,000 years ago, long before people were living there. Although many such tsunami-generating landslides have occurred, they typically are spaced thousands of years apart or more. But we can’t guarantee that there won’t be another one. See the animation below for a worst-case scenario.
Video: Tsunami visualization (2:05)
The entire mountain swells, which means that the outer slopes get steeper. There are probably a succession of small earthquakes; and then maybe a big earthquake that shakes loose this very steep western flank of the volcano. Almost 1500 cubic kilometers of Earth and rock plummet into the sea. On impact, the land displaces a massive amount of water. The ocean rushes back to fill the giant gap. From this cataclysmic disturbance emerges the tsunami, heading directly for Honolulu. Estimated time to impact, 30 minutes. This is the direction that a big tsunami would come from, the Big Island. The wave first becomes visible as it stands and breaks on a shallow bank 40 km southeast of Honolulu. You'd see the sea rear up in front of you. It would be huge. It would rise up above you the size of a building, the size of a 10-story building. Surging Inland at up to 70 km per hour, the tsunami slams Honolulu. Because the tsunami's energy stretches down the ocean floor, this wave is not clean water, it's filled with sand coral, and rock. Water penetrates 16 km Inland before being sucked back out to sea in a lethal maelstrom of wreckage. It annihilates nearly everyone and everything in its path.
The word tsunami comes from two Japanese words, for harbor and wave, a sort of shorthand for a wave that devastates a harbor. Most tsunamis are generated by undersea earthquakes, but undersea landslides, volcanic eruptions, and even meteorite impacts in the water can generate tsunamis.
Tsunamis move rapidly across the deep ocean, with speeds of 300 to 500 miles per hour (480 to 800 kilometers per hour). In the deep ocean, the “bump” of water that is the wave of a big tsunami may be only a very few feet high but may extend well over 100 miles in the direction it is moving. Waves slow down as they enter shallower water, and the leading edge of a wave hits shallow water before the trailing edge. So, the leading edge slows as it nears the coast, the trailing edge that is still in deep water catches up, and the wave goes from being long and low to being squashed and high. Even so, the tsunami wave is usually not a towering wall of water, but a strong surge, something like the tide coming in but higher (hence the mistaken name “tidal wave”).

An especially nasty feature of a tsunami is that the water often goes out before it comes in. (Waves consist of troughs and crests, and while the crest arrives first in some places, the trough arrives first in other places.) The sudden retreat of water and exposure of the sea floor tempts people to walk out and look around. Then, the ocean returns faster than a person can run. The outcome is very unpleasant.
Video: NOAA Tsunami Animation (1:27) This video is not narrated.
Terrible tsunamis have occurred. The greatest loss of human life from a tsunami was probably the Indian Ocean tsunami of 2004, which was triggered by the second-largest earthquake ever recorded, and killed roughly 230,000 people. Second was the tsunami from the 1755 earthquake near Lisbon, Portugal that killed about 60,000 people, especially in Morocco, Portugal, and Spain. The massive 1883 explosion of the volcano Krakatau in Indonesia essentially destroyed the island, with tsunami waves observed as far away as England. Floods raced miles inland on Java and Sumatra, killing approximately 40,000 people. The volcanic eruption of the Greek island volcano Santorini in the 1600s BCE pushed a tsunami perhaps 300 feet (100 m) or higher across the coast of Crete and may have contributed to the eventual demise of the Minoan civilization there. Many commentators have suggested that this is the source of the myth of Atlantis. The great 1964 Alaska earthquake generated a deadly tsunami that killed 118 people, with deaths as far away as California. In 1958, an earthquake-caused landslide in Lituya Bay, Alaska, caused a tsunami that included a wave 50-100 feet high in the bay, which a father and son safely rode out in a boat. They watched in awe as the wave then ran 1,800 feet up an adjacent coast; five people were killed in the event. Many other destructive tsunamis have occurred.
There isn’t a whole lot that can be done to stop tsunamis, but the loss of life and property damage can be limited. Tsunami warning systems are functioning in many places and are being extended rapidly. When instruments (called seismometers) sense the shaking of the Earth from a large undersea earthquake, volcano, or other disturbance, the signals are analyzed rapidly to see if characteristics suggest that a tsunami is likely, and if so, communications are sent out to various agencies concerned with safety, and sirens or other warnings on beaches are activated to get people away from the coast before the tsunami arrives.
The Indian Ocean tsunami of 2004 seems to have been especially deadly in places where human activities had caused damage to the coral reefs and coastal vegetation that would have blunted the strength of the wave, so maintenance of such natural buffers along these and other coasts can help protect the people living nearby from any future tsunamis. Scientists can figure out where tsunamis are likely, how big and how frequent they are likely to be, and then zoning codes can be enforced so that people build in safe ways on safe land if they want to live in an area.
Virtual Field Trip: Tsunamis
Please join us on a virtual field trip of Tsunamis.
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Tsunami Visualizations
(An extensive collection of animations on this subject)
Mountain Uplift and Erosion
(An extensive collection of animations on this subject)