On June 17, 2017, a bizarre event happened
in Greenland. Nuugaatsiaq, a remote fishing village, was
devastated by one of the tallest tsunamis in recorded history, a huge, one hundred meter
monster of a wave. The wave, about as tall the Statue of Liberty,
washed away eleven houses and killed four people. It was so big that its impact registered
as a 4.1 magnitude earthquake on nearby seismometers. Now, that confused geologists a bit -- because
tsunamis are usually caused by earthquakes out at sea. But when they looked into it further, they
found no actual earthquakes had been recorded in the right time frame to cause the wave. Meaning this Greenland tsunami wasn't a conventional
tsunami. Instead, scientists determined that this event
was something called a megatsunami. So what makes a tsunami mega? It tends to be bigger than an ordinary tsunami,
but that's not how you tell the difference. Instead, what distinguishes them from ordinary
waves is not their size, but their formation. What you need for a megatsunami is a huge
amount of material plunging into a body of water, like an ocean or a lake. It's a little like the splash caused when
someone does a cannonball into a swimming pool. But you need more than just your buddies jumping
into the ocean. You need like a -- you need a lot a buddies. Or like an asteroid -- or, as was the case
for Greenland, a landslide. The Greenland event happened when a huge landslide
hit a fjord one kilometer up, displacing a massive area of rock. All that earth tumbled down into the fjord. The resulting wave dissipated quickly, but
it was still enough to raise the water level on shorelines about 30 kilometers
away. But this isn't the first megatsunami we know
of. In recent history, we know of very few besides
the Greenland event. On July 9, 1958, a magnitude 7.8 earthquake struck Alaska's Lituya Bay. The earthquake caused a rockslide that dumped
82 million metric tonnes of material into the narrow bay. The resulting megatsunami was 524 meters high. That's just recorded history, though. These things are more than big enough to leave
their mark for thousands of years. And we've been able to document a few that
may have happened long before anyone was around to see it. One of the largest megatsunamis that we know
of happened sometime around 73,000 years ago, off the coast
of West Africa. A huge chunk of the eastern flank of the Fogo volcano fell off at once, striking the surface
of the sea. This produced a wave roughly 170 meters tall, almost twice the size of the Greenland wave. But even bigger waves might have struck
the Hawai'ian archipelago, more than 100,000 years ago. The clue lies in limestone-bearing gravel on the coastal slopes of the island Lanai. Now, limestone normally forms underwater,
but these rocks are found 326 meters above sea level. In a 1984 study, researchers proposed that
these rocks were deposited not by simple changes in sea level but by an enormous megatsunami,
caused by landslides on the island's steep slopes. To produce this, the wave that struck the
island of Lanai would have had to be at least 300 meters high, again twice as large
as the Fogo wave. Witnessing that wave would have been like
watching a wall of water almost as tall as the Eiffel Tower coming towards you -- out
of nowhere. Further back from that, it becomes a little
more difficult to directly attribute events to megatsunamis, but we do have a few likely
candidates. For example, we know the impact that ended the age of dinosaurs about 66 million years
ago splashed down off the coast of the present day Yucatan peninsula -- so it's very likely
to have caused a megatsunami. That impact sent shockwaves throughout the
entire global ocean system, starting with the nearby Gulf of Mexico. In a 2018 presentation, researchers suggested
that the asteroid would have caused a wave 1500 meters high. It's hard to imagine how much water that is,
but if you took Mount Kilimanjaro and put it right in the path of that wave, the water
would have gotten a quarter of the way up to the summit. Yeah, most of the dinosaurs worldwide were
done for anyway -- but the ones in the path of that wave did not last a day. If there's an upside to megatsunamis, it's
that they don't last as long as their seismically-created counterparts. Megatsunamis tend to have a stronger local
impact. So their impact is limited compared to regular
tsunamis, which can still be destructive after traveling thousands of kilometers. But in their local areas, they can be incredibly
destructive. Now, here's the bad news: megatsunamis are
really rare, but scientists think that climate change might make them more common in our
near future. Regions like Greenland and Alaska have permanently
frozen soil known as permafrost. Permafrost never thaws -- not even in the
summer. Effectively, these places are built on a layer
of solid icy ground. But as global temperatures warm, that permafrost
does melt. This makes the ground suddenly unstable, which
could trigger more landslides. And when landslides happen in coastal areas,
well, they could cause more megatsunamis. In theory, early warning systems could help
protect the four million or so people who live in the Arctic. Which they're going to need, if more megatsunamis
threaten their communities. Thanks for watching this episode of SciShow. Today's the last day of April, and that means
it's the last day you can order the Space Pin of the Month -- which is this cute little
Vostok 1 satellite. It can be yours, but only if you order it right now. Tomorrow we will have a new pin, and there
won't be any more of this one -- but there will be other very good ones. Check it out at the link in the description!
