BAN #361: The sheer power of an erupting volcano, Name an ALMA dish

27 September 2021   Issue #361

September 27, 2021

[The planetary nebula M 2-9, winds from a dying star. Credit: NASA / ESA / Hubble Legacy Archive / Judy Schmidt]

[Note: This is an extra-long issue due to the more newsy nature of it; I wanted to infiorm you about the volcano while it’s still erupting, and the ALMA contest has a quick deadline. Paying subscribers help support me to do this sort of thing, and I appreciate it with the fiery energy of lava fountain.]

A Bit o’ Science

The entirety of science is too much for one sitting. Here’s a morsel for you.

On September 11th, a series of thousands of tiny earthquakes started rumbling under the surface of the Cumbre Vieja volcano that makes up the southern end of La Palma, one of the Canary Islands off the coast of Morocco. These swarms generally indicate magma movement, and sure enough, a little over a week later the volcano started erupting.

Thousands of people have evacuated and while there has been extensive damage to structures, as I write this there have been no deaths mostly due to volcano scientists understanding the situation, and the people there trusting the scientists.

The eruption is nothing short of magnificent, and there are a few live cameras showing it 24/7. There are several live streams of the event and I recommend finding one and watching; it’s mesmerizing. Because they’re live the feeds change a lot, so search on YouTube and you’ll find one. This previously recorded one has some amazing views, as does this one.

I was watching it last week and it was showing a wide enough field of view that the lava fountain spewing out of it was almost entirely visible. I noted with awe just how high the fountain was… and realized I could calculate it!

An object traveling only under the influence of gravity, say a ball thrown into the air, will move in what’s called a ballistic arc, though in reality it’s an extremely narrow ellipse. If the solid ground weren’t in the way it would orbit the center of the Earth! But in reality it leaves your hand, goes up at a certain initial velocity, slows as it goes higher, reaches the top of its arc when its velocity is 0, then falls back down. It accelerates on the way down until it hits the ground with very close to the initial velocity (barring air resistance and such).

If you do the math (just a bit of Newtonian mechanics; I remember learning this in I think 9th grade but you can find derivations online) you find that the distance something travels under the influence of gravity (in this case, the height of the arc) is equal to ½ x acceleration x time^2. The acceleration due to gravity is about 10 meters per second per second.

[From one of the Cumbre Vieja live streams. Credit: TV Canarias]

So all you have to do to get the height of the lava fountain is time how long it takes a lava bomb (one of the “sparks”) to fall in seconds, square that number, and multiply by 5 (½ x 10) to get the height in meters.

Timing a number of bombs that reached near the top of my screen (so roughly the same altitude) I got an average time of about 10 seconds. That means the height those bombs reached was 10^2 x 5 = 500 meters.

Five. Hundred. Meters. Holy crap.

I don’t know how big those bombs are, but even a small one would easily weigh a kilogram; no doubt some are much, much heavier. Imagine how hard it would be to throw a rock the size of a baseball a distance equal to five football fields and you start to get a picture of the energies involved for just one small blob of lava.

But this volcano is casually flinging thousands of tons of material that far all the time. Every second of every day for over a week now.

How much energy does it take to do this? I’ll spare you the math (but if it takes 10 seconds to reach the peak, the initial velocity must be 100 meters per second, and then use that to get kinetic energy which is ½ x mass x velocity^2) I get that it takes 5,000 Joules per kilogram of material to launch it up that high. But what’s that in more graspable terms?

Assuming a fountain pours out 100 tons of material per second (I’m guessing here but that jibes with other lava fountains), that means the volcano is erupting with the energy of about 120 kilograms of TNT detonating per second.

Now, I could be way off in my estimates (and who knows, my math too though I triple checked my numbers) but this sounds about right. For the first episode of “Bad Universe” we detonated the equivalent of a ton of TNT in the New Mexico desert, and it made a helluva bang and a big hole (about 20 meters across). The volcano is doing this every eight seconds or so. In a single day the energy expended is equivalent to about 10,000 tons of TNT detonated. That’s just a bit less than half the explosive yield of the Trinity atomic bomb.

Yeah, OK. Cripes. And this is a relatively gentle eruption. Imagine what Mt. St. Helens or Pinatubo or the Lombok eruption in 1257 was like.

Volcanoes are powerful. Do not screw around with them.

This is a terrifying event and I’m disturbed by the disruption to everyone’s lives there, but I’m also glad that so far everyone on the island of La Palma is safe. I attended a meeting on La Palma in 1998 and stood on the bigger volcano at the north end of the island, and that gave me quite the appreciation for it, as well as for the natural beauty of the island. The last I heard, the astronomical observatory there is safe, too. I hope it stays that way, and that the loss of life stays at zero as well.

Do you like what you’ve read? Then please share this issue with friends and family!

Blog Jam

What I’ve recently written on the blog, ICYMI

[Art showing the Lucy spacecraft passing an asteroid. The mission launches in less than a month! From Tuesday’s article. Credit: Southwest Research Institute]

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

I love this kind of thing: The ALMA observatory, a collection of 66 dishes in the Chilean high desert, is holding a public contest to name the dishes! Voting opens today and will close at 03:00 UT on September 30th.

They’re doing this to celebrate the 10th anniversary of the first observation made by ALMA (of the wonderful Antennae Galaxies). ALMA is one of my favorite observatories. It stands for the Atacama Large Millimeter/submillimeter Array, which looks at long-wavelength light from the sky (longer wavelengths than infrared by shorter than radio). It’s very sensitive to warm (well, warm to astronomers, but just a few dozen degrees above absolute zero) gas and dust, for example, so it takes amazing images of galaxies and sites of star formation. It’s revolutionized our understanding of such things.

In fact I worked with the folks at ALMA and the National Radio Astronomy Observatory to write the Top 10 ALMA observations of 2019, and a similar one in 2020. I’ve certainly covered the science from ALMA on the blog many many times.

[A part of the ALMA dish array on the Chajnantor Plateau in Chile. Credit: Pablo Carrillo (ESO/NAOJ/NRAO)]

So the chance to name one of the dishes is exciting. There are two categories of names: astronomical objects, and words in the Atacameño Kunza language (the historic tongue of the people who live in the area ALMA is located). I love this idea. What words would be appropriate? I can think of a few: sky, beginnings, altitude (or height; the observatory is at 5,000 meters elevation!), knowledge, wonder…

You can find details on the contest at the link above. I urge you to give some thought to this and submit your ideas! ALMA is a terrific observatory and is run by terrific people. This is a lovely idea, and a great way to connect with the public.

And congratulations to everyone involved with ALMA on ten years of opening up the Universe!

Et alia

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