I've always found the idea of an unstable system fascinating, even before I knew the name to put on it. As a kid I liked to do things like build towers of stones and see how high I could get them before they'd teeter and collapse, and got quite good at creating a multi-tiered house of cards. (Can't do it any more -- I drink too much coffee to have the steady hands I did at age twelve.) What I found interesting was that up to a point, such systems tend to self-stabilize; touch your tower of stones gently, and sometimes it'll jostle a bit then settle back into its original position. But introduce too much energy into it, and it destabilizes fast. After that, every bit of the collapse feeds more energy into the process, until all you have left is a pile of chaotic rubble.
This phenomenon of a tipping point -- the point where the system crosses the line between stable and unstable -- is a special case of a wider phenomenon called hysteresis, which is the dependence of a system's state on its history. If something has started a trend in the past, sometimes it takes far less energy to keep it going than it did to get it started in the first place. Think, for example, of popping the cork on a champagne bottle. The amount of force you have to exert to push the cork up the bottle neck stays the same until... suddenly... it doesn't. Once the frictional force between the cork and the neck is exceeded by the force exerted by the pressure in the bottle, the system changes state fast.
Bang.
Lots of systems act this way, but none quite as alarmingly powerful as a volcanic eruption. Take, for example, what happened to Anak Krakatau, an island in the Sunda Strait in the Indonesian archipelago. This island was the site of the stupendous 1883 eruption of Krakatau (more commonly, but less correctly, spelled Krakatoa), one of the largest in recorded history. But volcanoes seldom stop at one eruption; the magma chamber feeding them doesn't just empty and go away. The same processes that caused the first eruption eventually rebuild the volcano and generate subsequent outbursts. Anak Krakatau ("Child of Krakatau" in Indonesian) emerged in 1927 from the giant caldera left by the eruption forty-four years earlier, and continued to grow and produce steam, ash bursts, and lava flows afterward.
Then in 2018, the entire island collapsed. I'm not overstating. It lost two-thirds of its above-sea-level volume, and the summit dropped from 338 meters above sea level to 110. This sudden cave-in generated a two-meter-high tsunami that killed over four hundred people and displaced forty thousand, mostly along the coastline of Sumatra and Java. Geologists knew the potential of the island to generate another deadly eruption, and even that there was a potential for collapse, but no one saw it coming on the day it happened. No warning, everything's quiet, then...
Bang.
The sudden collapse of Anak Krakatau was the subject of a paper this week in Earth and Planetary Science Letters which studied the lead-up to the event, looking at whether there were signs in the preceding months that might have tipped geologists off to what was going to occur. And... scarily... there weren't. Just like the cork in a champagne bottle giving you no warning when it's going to pop. The authors write:
The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals... [Our research] suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short- and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.
This put me in mind of another geological phenomenon that results from a similar kind of champagne-cork effect; kimberlite eruptions, which I wrote about here last year, and which apparently have the same no-warning-then-boom behavior. (These are the eruptions that produce diamonds -- and, once you read my post, you'll be glad to hear that they are thought to be a feature of Earth's distant past, and very unlikely to happen now.)
It's easy for us to look around and think everything we see -- not only the geology, but the climate, the global ecosystem, society itself -- is stable, and any perturbations will set up a feedback that will return everything to "normal." The problem is, for a lot of systems, there is no "normal." They're stable up to a point -- but if pushed beyond that point, unravel fast. Some of these phenomena, like the caldera collapse that struck Anak Krakatau four years ago, are powerful and unpredictable, and other than evacuating people, there wouldn't have been anything we could have done to prevent it even if we had known. But we'd damn well better not close our eyes to the analogy between this event and the bigger picture. It's easy and convenient to believe that "everything will be fine because it's always been fine," but that kind of thinking gives people license to keep poking at things, heedlessly pushing on the superstructure and acting like it has infinite resilience.
Then, without any warning, where you had an orderly stone tower, all you have left is a pile of rocks, dust, and debris.
Since reading the classic book by Desmond Morris, The Naked Ape, when I was a freshman in college, I've been fascinated by the idea of looking at human behavior as if we were just another animal -- anthropology, as it were, through the eyes of an alien species. When you do that, a lot of our sense of specialness and separateness simply evaporates.
The latest in this effort to analyze our behavior from an outside perspective is Pascal Boyer's Human Cultures Through the Scientific Lens: Essays in Evolutionary Cognitive Anthropology. Why do we engage in rituals? Why is religion nearly universal to all human cultures -- as is sports? Where did the concept of a taboo come from, and why is it so often attached to something that -- if you think about it -- is just plain weird?
Boyer's essays challenge us to consider ourselves dispassionately, and really think about what we do. It's a provocative, fascinating, controversial, and challenging book, and if you're curious about the phenomenon of culture, you should put it on your reading list.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]
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