Planetary Calderas – Earth

Caldera volcanoes aren’t your ordinary run-of-the-mill fire mountains. They are massive, dangerous, and yet strangely attractive because of the dramatic landscapes they create and the abundant life that thrives on the practical benefits they offer (when they’re quiet).

We live in and around some of the planet’s biggest caldera volcanoes and have complex love-fear relationships with them, from Yellowstone Volcano in North America all the way around the world to Asosan in Japan.
 


A treasure for all of us…until the next supereruption. The million or so Neapolitans who live inside Campi Flegrei’s caldera might disagree with the narrator’s demographics. Aso caldera has a diameter of 24 km. More on Asosan here. To see the whole caldera, you have to get up real high.
 
So what is a caldera, anyway?

Craters vs. calderas

A caldera is a volcanic collapse feature. It’s not a volcanic crater, which builds up during an eruption.

A caldera and a crater are different in the same way a ship and a boat are different. A ship can carry a boat, but a boat that tries to carry a ship, which is much larger, will sink. A caldera can have one or more craters in it, but there can’t be such a thing as a crater with a caldera in it – when the volcano’s magma chamber collapses to form a caldera, any craters are obliterated.
 


 

Classifying calderas

The word “caldera” comes from the Latin word for “boiling pot.” Back in the day, on the Canary Islands, any bowl-shaped or cauldron-shaped land depression used to be called a caldera. Then, as the specialized fields of geology and volcanology developed, earth scientists adopted the term to describe a more or less circular-shaped volcanic collapse depression.

There are several different ways to classify calderas, but let’s not worry about it. Those systems are only for scientific purposes. As laypeople, you and I can see that when a volcano erupts and then collapses, it’s messy, and gets messier still when the same thing happens over and over, as it does at many caldera volcanoes. And then you have to take account such things as weathering and the changes caused by plate tectonics. Sorting out calderas can be confusing even for the experts!

Geologists list calderas by some feature like the chemistry of the lava or the volcano’s tectonic setting. We’re just going to look at the three caldera types that usually form from eruptions: shield, stratocone, and ash-flow calderas. (That last one – the ash-flow caldera volcano – is often what we informally call a “supervolcano.”)

Shield calderas
 


 

Aloha! Mauna Loa is a good example of a shield caldera. Shield calderas are generally bigger than 1.6 km in diameter but smaller than 25 km. Mauna Loa’s caldera is 6.2 km long by 2.5 km wide.

As of this writing, Mauna Loa is inflating slightly, but don’t cancel your Hawaiian vacation plans just yet. This is normal. The Hawaiian Volcano Observatory has Mauna Loa’s alert level set at Green, which means that the volcano basically is quiet.

Scientists think that shield calderas like Mauna Loa usually form nonexplosively. Well, it isn’t actually a gentle process, but it is much quieter than the formation of the other two caldera types. It often happens more slowly, too.

The magma at a shield caldera volcano is mafic, that is, “runny.” This molten mafic rock drains out from underneath the mountain’s summit, either erupting in lava flows down the sides of the volcano or staying underground as magma and moving through cracks to form sills and dikes.

When the magma chamber is emptied out enough to weaken the supports, its roof collapses. One collapse might take days, but more often it happens in slow motion, taking many years to completely fall down. It can happen over and over. A complex of several calderas will develop if the summit inflates and then collapses many times over millions of years.

 

Mauna Loa, Earth's largest shield caldera volcano, built its Moku`aweoweo complex of three overlapping calderas over millions of years.  The most recent caldera formed after a lava eruption on Mauna Loa's flank some 1,000 to 1,500 years ago.

Mauna Loa, Earth’s largest shield caldera volcano, built its Moku`aweoweo complex of three overlapping calderas over millions of years. (USGS via Wikipedia)


 

Moku`aweoweo is impressive, but eruptions at a shield caldera typically aren’t apocalyptic. They are actually very pretty.

Want to see more lava fountains? Check out this rare color footage of a small fissure eruption in Moku`aweoweo in the 1930s.

That’s pretty much what Mauna Loa does with its runny mafic lava, that and sometimes also enormous flows. In or around 610 AD, for example, over a cubic kilometer of lava poured out of a rift system in the volcano’s northeast flank.

(Note: I’m using words like “generally” and “usually” here because, rarely, a few shield volcanoes do have caldera-forming explosive eruptions. Let’s hold off discussing those until next time, when we look at explosive mafic eruptions on Mars.)

Stratocone calderas

Crater Lake in Oregon is a stratocone caldera. On average (PDF), stratocone calderas are a little over 5 km in diameter. Crater Lake is one of the larger ones at 8 x 10 km.
 

Willie Scott, USGS (Source)

Willie Scott, USGS (Source)


 

So what’s the difference between Mauna Loa and Crater Lake?

Setting calderas aside for a moment, Mauna Loa is a shield volcano and Crater Lake is a composite or stratocone volcano.

Here is a PBS video that shows, among other things, how an ordinary stratocone/composite volcano like Mount St. Helens (without a caldera) differs from a shield volcano.
 


 

So how is Mount St. Helens different from Crater Lake? They both are stratocone volcanoes, after all, and they both have big holes in them.

Well, Mount St. Helens actually has a crater in it, not a caldera. In May 1980, an earthquake triggered an enormous landslide that depressurized the molten rock in the conduit, which then exploded. It all happened very quickly and dramatically, but it was directed outward; Mount St. Helens definitely did not collapse in on itself that day.
 

In this USGS illustration, the landslide is green.  On the left, a second or two after the flank slipped, the magma began to explode (red).  The image on the right shows it quickly overtaking the landslide.  (Source)

In this USGS illustration, the landslide is green. On the left, a second or two after the flank slipped, the magma began to explode (red). The image on the right shows it quickly overtaking the landslide. (Source)


 

Outward-directed blast, not inward collapse.

Crater Lake, on the other hand, sits in a caldera that formed when the stratocone volcano (which geologists call Mount Mazama) had a series of big explosive eruptions, 7,000 to 8,000 years ago, that emptied the magma chamber. The chamber’s roof then failed, and the whole summit of the mountain collapsed into the chamber.
 

Subtract the water, trees, and other covering to see the rocky mess Mount Mazama's eruptions left behind 7,700 years ago.  (Krassilnikov et al.)

It’s easier to see if you subtract the water, trees, and other covering to see the rocky mess Mount Mazama’s eruptions left behind. (Krassilnikov et al.)


 

The eruption of a stratocone caldera volcano can eject anywhere from 0.1 to 100 km3 of magma (Mazama’s Crater Lake eruptions blew out some 50 km3). That’s not in the “super” range, but it can be a life-changing event for any living beings within a hundred miles or more. This sort of eruption lasts hours or a few days.

Mount Mazama, by the way, is sleeping right now, but the Cascades Volcano Observatory considers its threat potential to be high.

Ash-flow calderas

Now we’re talking about what you or I would call a supervolcano – calderas like the ones at Aso or Campi Flegrei. On average (PDF), ash-flow calderas are almost 18 km wide. They form in very explosive eruptions that evacuate hundreds to thousands of cubic kilometers of magma from the chamber.

I think this happens over a period of days. However, as we saw in the post about that fossil supervolcano in Italy’s Sesia Valley, earth scientists don’t know very much about this type of eruption, since one has never happened in recorded history.

In addition to a humungous caldera, the landforms an ash-flow caldera-forming eruption leaves behind often include a vast plain (because the magma erupts as tephra and pyroclastic rocks, not as a lava flow).

Sometimes, as in Valles Caldera, a relatively small lava dome will appear when the worst is over – not that there will be anybody left locally to see it. Nor will the rest of the world care very much about the resurgent dome, as everybody will be too busy trying to survive the global effects of this supereruption.

OMG, the end of the world…? Well, not in the long run.

Here is a satellite picture of Ngorongoro, an extinct ash-flow caldera in Africa, with an associated plain formed by huge pyroclastic flows that devastated the region during a caldera-forming eruption some 2.5 million years ago.
 

Ngorongoro is a wildlife refuge and UNESCO World Heritage Site.  That faint horizontal line in the middle of the picture is Olduvai Gorge.  The Serengeti Plain is in the upper left.  (NASA)

Ngorongoro Caldera (the big one in this picture) is a wildlife refuge and UNESCO World Heritage Site. That faint horizontal line in the middle of the picture is Olduvai Gorge. The Serengeti Plain is in the upper left. (NASA, J. L. Whitford-Stark)


 

Everything you see here is teeming with life today.

The big picture

Caldera volcanoes are enormous, and so are the consequences of their eruptions. Things die, but then life reestablishes itself. Soil forms, and the land becomes rich and fertile again. The whole cycle starts over.

Any volcanic eruption will disrupt our technologically advanced civilization if it’s large enough or in the right location. Something like an ash-flow eruption would really do a number on us. But we would survive and eventually come back stronger than ever.

It’s comforting to know, that in just a blink of the eye (in geologic terms) after Ngorongoro had its supereruption, our ancestors were living nearby at what would become Olduvai Gorge…and doing quite well, thank you very much.

It happens, over and over again. Without caldera volcanoes, there probably wouldn’t be life as we know it on Earth.

Today, we look back in time at the relicts of human life near Ngorongoro. We can also look outward, toward places that we would like to visit in the future – our companion rocky planets in the inner Solar System. There are caldera volcanoes out there, too, on Mars and Venus. Next time, we’ll take a look at some of those. Still…

…I can’t help but wonder if extraterrestrial caldera volcanoes can ever be as cool as those we have on Earth, like Taupo. After all, they lack the human touch:

 


I don’t know who made this video, but I think it might have been filmed entirely inside the 35-km-wide ash-flow caldera. The town of Taupo is in there, for sure. And life goes on…

PS: What are calderas on other planets like?

 
 


 
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Categories: Sunday morning volcano

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