# Volcanoes: how close is too close? **URL:** \ **Category:** Factual Questions\ **Created:** [July 3, 2008, 2:06pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187 "2008-07-03T14:06:40Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [July 3, 2008, 2:06pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/1 "2008-07-03T14:06:40Z")
I’ve just read [this MSNBC](http://www.msnbc.msn.com/id/25499980/) article. At a distance of 40 miles, is Popocatépetl really close enough to devastate Mexico City? Sure there might be ashfall, but surely it’s too far away to directly affect the city with mudslides et al? ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [July 3, 2008, 2:15pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/2 "2008-07-03T14:15:58Z")
[Mt St Helens](http://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens) > [@](#): > > A volcanic ash column rose high into the atmosphere and deposited ash in 11 U.S. states.[citation needed] At the same time, snow, ice, and several entire glaciers on the mountain melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly fifty miles (eighty kilometers) to the south. I don’t know what the drainage pattern looks like between Popocatépetl and Mexico city, but the capital is within the historic range of lahar flows. ---
**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:** [July 3, 2008, 4:36pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/3 "2008-07-03T16:36:47Z")
[QUOTE=Quartz] Sure there might be ashfall, but surely it’s too far away to directly affect the city with mudslides et al? [/QUOTE] Mt. Rainier created the [world’s largest lahar](http://www.emporia.edu/earthsci/student/zellers1/rainier.htm) some 5,600 years ago, covering some 212 square miles. When the volcano blows and sheds her cubic mile of glacial ice and snow the next time around, there will be little to slow down the lahars from reaching Tacoma, if not Puget Sound itself. Ashfall is another story entirely. When Mount St. Helens blew in 1980, Spokane, Washington, had a water shortage because of the amount of water they used washing down their vehicles, homes and businesses. When the Yellowstone Supervolcano blows, being on Planet Earth will be too close. You might want to build that getaway summer home on the shores of the Sea of Tranquility about now. ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [May 23, 2023, 7:15am UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/4 "2023-05-23T07:15:51Z")
> **[Millions in Mexico warned to prepare for evacuation as Popocatépetl volcano...](https://www.cnn.com/2023/05/22/americas/mexico-volcano-popocatepetl-latam-intl/index.html)** > > Millions of people in Mexico have been warned to prepare for a possible evacuation after increased activity from the country’s most dangerous active volcano, which has been spewing ash into several nearby towns since last week, according to... ---
**Author:** ![md-2000](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@md-2000](https://boards.straightdope.com/u/md-2000)\ **Post date:** [May 23, 2023, 1:01pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/5 "2023-05-23T13:01:03Z")
> [@Duckster](#): > > Ashfall is another story entirely. When Mount St. Helens blew in 1980, Spokane, Washington, had a water shortage because of the amount of water they used washing down their vehicles, homes and businesses. ANd Mt. St. Helens is about 50 miles away from Portland. They key obviously is how much comes out of the volcano. The occupants of Herculaneum and Pompei were done in by a pyroclastic flow of superhot gases coming down the mountain, yet vessels out in the bay a ways survived to write about it (except the old guy who had medical problems, leaving his nephew to detail the result.) But if the volcano dumps a huge load of ash nearby, the first problem will be if the building roofs can take the weight - particularly those with flat roofs. The later problem would be clean-up. Can the average car drive through a foot of ash, even if it’s cooled off? It would take how many dump trucks how many weeks to make the city passable again? (As I recall, Mexico City is not known for abundant water and being in a mountain bowl, not a lot of places to wash the stuff away to.) Where will the money come from to do all this? ---
**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\ **Post date:** [May 24, 2023, 6:58pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/6 "2023-05-24T18:58:12Z")
Pyroclastic flows (superheated gases and debris) from volcanoes have been known to travel up to 50 miles (Krakatau) or even 100 miles based on historic evidence. [Think You Can Drive Fast Enough to Escape an Erupting Volcano? | WIRED](https://www.wired.com/2016/03/think-can-drive-fast-enough-escape-erupting-volcano/#:~:text=The%20common%20response%20is%20that,150%20kilometers%20(~100%20miles)). That’s the nastiest type of volcanic eruption\*, and evidently much faster than flows of lava or mud. \*St. Pierre in Martinique was wiped out in 1902 by just such an event. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [May 24, 2023, 7:51pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/7 "2023-05-24T19:51:55Z")
> [@Duckster](#): > > > [@](#): > > > > Sure there might be ashfall, but surely it’s too far away to directly affect the city with mudslides et al? > > Mt. Rainier created the [world’s largest lahar](http://www.emporia.edu/earthsci/student/zellers1/rainier.htm) some 5,600 years ago, covering some 212 square miles. area = pi \* radius^2 212 = pi \* radius^2 212/pi = radius^2 sqrt(212/pi)=radius I’m getting about 8 miles. A 40 mile gap seems relatively safe from lahars. But, that is specifically lahars and ignoring all other sources of danger. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [May 24, 2023, 8:00pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/8 "2023-05-24T20:00:23Z")
> [@Sage\_Rat](#): > > > [@Duckster](#): > > > > Mt. Rainier created the [world’s largest lahar](http://www.emporia.edu/earthsci/student/zellers1/rainier.htm) some 5,600 years ago, covering some 212 square miles. > > area = pi \* radius^2 > 212 = pi \* radius^2 > 212/pi = radius^2 > sqrt(212/pi)=radius > > I’m getting about 8 miles. A 40 mile gap seems relatively safe from lahars You’re assuming lahars are circular, which turns out to be a very bad assumption. Lahars are flows of a slurry of pyroclastic material and water/mud, usually funneled down a river valley and then perhaps running out over a plain. Much more a linear feature than a circle. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [May 24, 2023, 8:03pm UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/9 "2023-05-24T20:03:33Z")
Hm…true. It looks like a tentacle model would be more appropriate: > **[The Mountain is Out (To Get You)](https://www.southsoundmag.com/arts-entertainment/the-mountain-is-out-to-get-you/article_aadb0bd9-2418-572e-9052-73ae3a02fd4e.html)** > > One could argue our adoration of Mount Rainer has clouded our vision to the danger that it poses. Ignore my post. ---
**Author:** ![CairoCarol](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cairocarol/32/3825_2.png) [@CairoCarol](https://boards.straightdope.com/u/CairoCarol)\ **Post date:** [May 25, 2023, 7:39am UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/10 "2023-05-25T07:39:02Z")
> [@Jackmannii](#): > > (Krakatau) @Jackmannii - just posting to thank you for using the accurate name of the volcano in question. It’s hard to fault people too much for calling it “Krakatoa,” but forward-facing writers should do better. ---
**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:** [May 26, 2023, 2:19am UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/11 "2023-05-26T02:19:45Z")
> [@md-2000](#): > > ANd Mt. St. Helens is about 50 miles away from Portland. Mount St. Helens is north northeast from Portland. Prevailing winds are west to east. And the 1980 blast opened the crater facing east. Yes, it’s possible Portland would experience some ash. Portland’s volcanic dilemma is Mt Hood, due east of the city. The lahars and pyroclastic flows could easily destroy the much of the fresh water reservoirs used to supply the city with fresh water. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 26, 2023, 6:23am UTC](https://boards.straightdope.com/t/volcanoes-how-close-is-too-close/455187/12 "2023-05-26T06:23:06Z")
> [@Duckster](#): > > And the 1980 blast opened the crater facing east. Nitpick: looks **[more north to me](https://www.google.com/maps/place/Mt+St+Helens/@46.202326,-122.2182927,17439m/data=!3m1!1e3!4m6!3m5!1s0x54969956568a2691:0x69ddb4f4b6cf94c7!8m2!3d46.1914006!4d-122.1955508!16zL20vMDk0eHo?hl=en&authuser=0&entry=ttu)**.