# Hawaii Coastline Collapses: Why no Tsunami? **URL:** \ **Category:** Factual Questions\ **Created:** [December 2, 2005, 9:39pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817 "2005-12-02T21:39:11Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Ponder\_Stibbons](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponder_stibbons/32/343_2.png) [@Ponder\_Stibbons](https://boards.straightdope.com/u/Ponder_Stibbons)\ **Post date:** [December 2, 2005, 9:39pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/1 "2005-12-02T21:39:11Z")
We all know that the main cause of tsunamis is earthquakes. But anyone who has watched tsunami specials on the Discovery channel knows that the really scary prospect is tsunamis created by landslides, although (fortunately) such landslides are very rare. This last Monday, a stretch of volcanic coastline in Hawaii [collapsed into the sea](http://www.msnbc.msn.com/id/10298728/). Some 44 acres of land disappeared beneath the waves. Yet, no tsunami. Anyone know the details on this? ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [December 2, 2005, 9:46pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/2 "2005-12-02T21:46:35Z")
I’m gonna say that 44 acres is a mere pebble in the ocean. Also, doesn’t the lanslide need to take place below water and create a more significant disturbance than just 44 acres falling _into_ the sea? Trouble usually arises when the force creates an uplift…creating the wave of energy thru the water until the energy releases at a shore line, raising the seas. ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [December 2, 2005, 9:47pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/3 "2005-12-02T21:47:57Z")
I think I know this one. Tsunami’s typically are caused by underwater shifts in tectonic plates or gigantic underwater landslides. If you think about it a huge about of the crust has to move up and down in the ocean to generate a wave that travels thousands of miles. I wouldn’t think a landslide into the ocean would cause anything like a tsunami. What’s strange to me is that here in California there are lots of moderate sized earthquakes right off the coast that don’t seem to cause them either. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [December 2, 2005, 9:48pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/4 "2005-12-02T21:48:49Z")
WAG - Insufficient displacement of water. Also, according to your link, the collapse took four and a half hours. Way too slow. Imagine pouring 1,000 pounds of sand into a small pond from a height of 50 feet. Make sure it takes four and a half hours to pour. Now take an identical 1,000 bag of sand and drop it in the bag into the pond all at once. Which one makes a bigger splash? 😃 ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [December 2, 2005, 9:49pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/5 "2005-12-02T21:49:06Z")
I meant a huge _amount_ of the crust… doh! ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [December 2, 2005, 11:24pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/6 "2005-12-02T23:24:35Z")
Note also that the “megatsunami” that some folks say could occur due a sudden collapse of [La Palma](http://en.wikipedia.org/wiki/Cumbre_Vieja) would involve 500 cubic kilometres of land dropping into the ocean suddenly. In comparison, this collapse in Hawaii involved something on the order of 0.05 cubic kilometres or less – at least a factor of 10,000 smaller. ---
**Author:** ![ouryL](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ouryl/32/6067_2.png) [@ouryL](https://boards.straightdope.com/u/ouryL)\ **Post date:** [December 2, 2005, 11:33pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/7 "2005-12-02T23:33:15Z")
However, if you are on the shelf as it falls or are swimming in the water below - bye bye. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [December 2, 2005, 11:39pm UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/8 "2005-12-02T23:39:38Z")
> [@dolphinboy](#): > > I think I know this one. Tsunami’s typically are caused by underwater shifts in tectonic plates or gigantic underwater landslides. If you think about it a huge about of the crust has to move up and down in the ocean to generate a wave that travels thousands of miles. I wouldn’t think a landslide into the ocean would cause anything like a tsunami. Not quite. The requirement for a tsunami seems to be that a large quantity of water must be displaced and a big enough landslide can do that. There is some concern about a volcano on the island of[_La Palma_ in the Canary Islands](http://www.benfieldhrc.org/tsunamis/mega_tsunami_more.htm) which shows signs of starting a large landslide with a volume of perhaps as much as 500 km[sup]3[/sup]. If this happens destructive tsunamis are expected to reach the eastern coast of the Americas. ---
**Author:** ![Doobieous](https://avatars.discourse-cdn.com/v4/letter/d/da6949/32.png) [@Doobieous](https://boards.straightdope.com/u/Doobieous)\ **Post date:** [December 3, 2005, 4:19am UTC](https://boards.straightdope.com/t/hawaii-coastline-collapses-why-no-tsunami/333817/9 "2005-12-03T04:19:45Z")
Ah, but chunks of Hawaii have slid off and caused devastating tsunami. What you’re thinking of was miniscule in regard to causing a tsunami. It’s like glaciers at sea, they really don’t do much but cause big local waves. Here’s a page from the local oceanographic institute here in Monterey that shows a color enhanced image of Oahu and Molokai, showing chunks of rock underwater. Some of the chunks were large enough to be called sea mounts: > **[Submarine volcanoes](https://www.mbari.org/science/seafloor-processes/volcanoes/)** > > Volcanic events and deposits in the ocean are largely concealed from view and more difficult to sample than volcanoes on land. The submarine volcanism group studies processes that form and modify volcanoes in the sea. One of the pieces is almost as wide as Molokai itself (light yellowish green chunk near the upper right corner of the image) Some of the sheer clifs of the older islands of Hawaii are actually the slide scarps (remains of the landslides). The Pali (Cliff) above Kane’ohe bay on Oahu is a good example: > **[Submarine volcanoes](https://www.mbari.org/science/seafloor-processes/volcanoes/)** > > Volcanic events and deposits in the ocean are largely concealed from view and more difficult to sample than volcanoes on land. The submarine volcanism group studies processes that form and modify volcanoes in the sea. This slide caused 23,000 square kilometers (4680 square miles) of debris cover 235 km (141 miles) to be scattered over the sea floor.