Debunking Caldera Myths

Post by cbus20122:

Before this post gets read, I would like to note that I am not a scientist or geologist. If any information is inaccurate in this post, I would like to encourage the more scientifically inclined to correct me and inform readers if there are any inaccuracies!

Caldera Volcanoes.. The Mythological Beast of Volcanology

Image Wikimedia Commons : Aniakchak Caldera – Alaska

If you’ve ever paid attention to volcanoes, there is a good chance you’re familiar with what a caldera is. For those who are new to the terminology, a caldera is a collapse structure that forms when the magma chamber below a volcano empties, leaving the overlying rock to subside into the ground. Calderas are to volcanoes what an atom bomb is to explosives. They’re the largest, most destructive, and rarest variety around, and because of that, they’re incredibly interesting.

Caldera forming eruptions are interesting and notable to scientists and casual observers alike since they’re both rare, and incredibly powerful. In fact, some caldera-forming eruptions can be so powerful, that they’ve been associated with global climate change, and small-scale extinction events. Due to their potentially cataclysmic nature, there is a lot of misinformation and doom & gloom in the press and media.
Chances are, you’ve heard the title “supervolcano”. The term “supervolcano” was coined by the media to describe the largest caldera-forming eruptions on earth. Ever since the inception of the term, it’s been used to describe any massive volcanic eruption, the likes which haven’t been seen in the modern era. So what are some common myths about calderas and supervolcanoes? Read the guide below!

Debunking Myths Associated With Calderas

Myth – There Are Only 6-7 Supervolcanoes on Earth

Somewhere along the line, the media decided that there were less than 10 supervolcanoes on earth. This myth is a bit difficult to dispel, because there is no real cutoff between “supervolcano” and “really large caldera” as it’s not a true scientific term.

Campi Flegrei in Italy is frequently described as a supervolcano, yet it’s not even 1/10 the size of Lake Toba. If we were to assume that Campi Flegrei is a proper supervolcano, then that means there are over 100 known supervolcanoes on our planet, and it would be on the lower end of the size spectrum. If we’re defining “supervolcano” by capability of producing a VEI – 8 eruption, then it’s true that there are only a few volcanic systems with this capability.

Myth – All Calderas form from explosive eruptions

While more calderas form as a result of a violent eruption, some caldera systems form from a gradual subsistence. Hawaiian volcanoes have calderas that formed slowly following the gradual effusion of basaltic magma, which caused a gradual drop in the size of the magma chamber. Subsistence calderas form most often in mafic shield volcanoes, which are common in oceanic hotspots such as the Galapagos, or the Hawaiian Islands.

Myth – Volcanoes that have had a violent caldera forming eruption are extremely violent by nature

Caldera forming eruptions are more of a cyclical process then they are indicative of a Volcano’s overall nature. Even extremely violent and active volcanoes such as Krakatoa show that they’ll stay active with small-scale eruptions post-collapse. A caldera-forming event typically happens only after a volcanic system has been “plugged” up for a long enough time, allowing pressure to build and magma to evolve to a degree that it can erupt in a dramatic fashion. For some volcanoes, this takes a very long time, others like Krakatoa can recharge much quicker. Some caldera volcanoes will create multiple massive caldera-forming eruptions. Others will only go massive one time, then they’ll sprout several smaller volcanoes after the initial caldera collapse event.

It’s also important to note that there are different varieties of explosive calderas. Caldera volcanoes formed from andesitic arc-volcanism behave in a much different fashion than Caldera volcanoes that form from basaltic rift-oriented volcanism, which typically erupt effusive basalt eruptions, but can create massive rhyolitic eruptions on rare occasion. These caldera systems are usually indicative of a large heat source (basaltic magma) transforming country rock into Rhyolite (the most explosive variety of magma) which later erupts after being disturbed by a fresh injection of basaltic magma.

Myth – Supervolcanoes Are Formed By Hotspots

The largest caldera systems in the world all have a few things in common, yet being hot spot volcanoes is not a similar trait they share. In fact, Yellowstone is the only supervolcano that is known to be formed in association with a mantle plume (hot spot), whereas most other supervolcanoes are located in subduction arcs. What they do have in common is extremely hot and shallow heat sources, typically produced by continental rifting. Rifting occurs when land pulls apart due to largely tectonic reasons. Rifting lowers underlying pressure and thins the surface, which in turn pulls magma and hot rock closer to the surface. Eventually, these large shallow heat sources melt and evolve country rock (often granite) into our familiar friend Rhyolite. If you accumulate enough Rhyolite, let it evolve for a long enough time, then set it off with a fresh injection of magma, you have the ingredients for a massive eruption.

For Yellowstone, the heat source comes from the mantle plume, instead of a rift-oriented heat source (although it’s likely some rifting is occurring as well).

Google Earth Overlays For Caldera Systems – Calderas Outlined in Green or Red (screenshots)

Ecuador has quite a few massive caldera systems, with the Chacana caldera being the largest

Of the 11 large calderas in Kamchatka, the smallest is still 10 square KM..

Cbus20122

Santorini – End of Civilisation

Santorini - The most beautifull harbour to enter in sunset on Earth. Sunup is equally stunning. Nea Kameni in the middle.

Short background

Few volcanoes have such an ominous reputation as the Island volcano of Santorini. And it is in one way a justified reputation; after all it is one of the few volcanoes that have been the end of the civilization as we knew it when it happened.

Today’s island group of Santorini consists of a large caldera ring constructed of no less than six (6!) independent calderas constructed during massive caldera forming explosive eruptions. The largest island is to the east and is named Thera, than follows clockwise Aspronisi (small) and Therasia. Inside of this large caldera basin lays the two islands of Nea Kameni and the adjacent smaller Palea Kameni.

Against normal belief the islands are of a non volcanic origin. Instead they started as a mainly limestone based island as the sea bottom was pushed up by the same powers that made the Alps into the rather craggy mountain chain that much later gave name to the Alpinist tribe.

Later the Hellenic Trench Subduction Zone got active as the African Plate slammed into Europe and got pushed down under Europe. That little event started quite a bit of volcanic activity. Today Santorini, together with its neighbor Colombo Reef, share the distinction of being active Greek volcanoes with Kos, Methana, Milos, Nisyros and Yali.

Image by Nasa.

Pre 1610BC caldera formations

The first mega cycle of caldera formations happened about 600 000 years ago. It was most likely the largest eruption at Santorini and is still responsible for the largest of the calderas. The next caldera formation came 360 000 years ago and had a different magmatic combination than the previous. It was highly siliceous and created highly evolved magmas through crystallization, this type of siliceous magmas are still the norm. 180 000 years ago it was then time for the southern caldera to form, then came the Skaros Caldera 70 000 years ago. After that came the Cape Riva Caldera about 21 000 years ago, the youngest caldera is of course the 3 600 year old Minoan Eruption Caldera.

Caldera formation process of Santorini

During a mega-cycle Santorini has series of cycles with smaller eruptions that build up a volcanic edifice. These eruptions are normally unimposing events ranging between VEI-2 and VEI-3 judging from historical records and excavations. But it is probably not impossible for a VEI-4s happening during a cycle. During the cycle the magma chamber evolves and becomes continuously larger with time. This gives more time for crystallization to happen to the cilicic magmas which gives more and more evolved magmas as the cycle continues. So any VEI-4 or larger would most likely come late in the cycle. The current cycle between caldera formations started in 187BC with an island forming event.

The different Caldera forming events have given differently coloured layers. The most recent is whitish.

1610BC Caldera formation

Few eruptions have such a reputation as the 1610BC Minoan Eruption. It has been given the blame of the biblical story of Noah, the sinking of Atlantis, the disappearance of the language of Linear-A, and of course the fall of civilization as that time knew it. It was not only the Minoan culture at Crete that started to dwindle, also the Hittite, Babylonian and Egyptian cultures showed a rather marked decline during the time after this. The only people that oddly enough seemed to benefited where the 12 tribes of pre-Israelites that formed Israel in the ensuing power vacuum. Jolly good that did for them, as soon as the Hittites, Babylonians and Egyptians got back on their feet they made short order of Israel. Sense moral is that no volcano may ever protect you from biblical revenge.

Technically the 1610BC event was a VEI-7. It is believed to have begun as a VEI-6 ultra-plinian eruption, with a massive effusive component that caused the by then very large magma chamber to subside enough for the ocean to fall into the half empty chamber. When we are talking about the magma chamber of 1610BC we should remember that it does not exist any longer. The size of that chamber was in the order of a 1 000 cubic kilometers or more judging from the erupted material, and the eruptive standard value of 5 to 10 percent of the magma being ejected. Today’s chamber is probably around 1 to 5 cubic kilometers.

As the water came into contact with the open magma chamber a violent Ultra-plinian VEI-7 event took place. It caused wide spread tsunamis and heavy weather affecting atmospheric conditions.

Santorinis normal eruptions

During the period from 187BC and up to 1950 Santorini have had 10 sub-aerial eruptions. And at least four separate island forming events. Two islands have later been subsumed by Nea Kameni. Neither of these eruptions has been spectacular in any sense of the word. In addition there has probably been sub-aquatic eruptions inside the caldera ring, but those has either not been written down, or was too deep to make a surface impact. The big exception is of course the Colombo Reef (6,5km NE of Thera) eruption of 1650AD which formed an ephemeral island.

Other than being small in scale, the eruptions are normally prolonged in time. Eruptions lasting between one and two years are not uncommon.

Photographer unknown, postcard that I bought the first time I sailed to Santorini.

Signs of an upcoming eruption

Up until a couple of years ago Santorini was subsidizing, this together with cyclic swarms of earthquakes over at Colombo Reef lead researchers to believe that an eruption was up and coming at that location, and not at Santorini.

About a year ago a marked inflation started at Nea Kameni together with increase in CO2, SO2 and the Santorini marker gas of Radon. Together with an increase in seismic activity lead to the scientists starting to keep a bit of a closer watch on the volcano.

Lately we have seen quite a few more signs of an upcoming eruption. We have now had swarms of earthquakes inside the caldera at the right depth, microseismics that remind of popping giant popcorn in a spectral analysis, and of course harmonic tremoring lasting from hours to days.

We should though know that this has happened before in volcanoes of this type without anything happening. But let us say that the likelihood of an eruption happening has risen quite a lot.

What will an eruption look like?

Most likely an eruption now would be a dome forming event at Nea Kameni. The beginning will be explosive, and then trending to effusive as lavas are extruded. Most likely it will be a VEI-2 event, but a VEI-3 is not out of the question. The eruptive period is hard to predict, it could be anything from one month to four years. There is really no saying which. And historical records do not give much help; it is equally possible that the eruption will be short as long. Playing hard and fast with statistics here would only give silly results due to lack of substantiating occurrences.

Are you starting to get the thing with the Santorini sun yet? Nea Kameni out in the bay.

One thing is for sure, there will be no end of civilization as we know it, this time around.

CARL

A special thanks to Summer who fixed the code in the last post, and told me how to not bungle it up again.

Update

The earthquakes that some believe to be at Santorini are not really at Santorini, they are located 12 to 25 kilometres south to south west of Santorini. They are located at a faultline that is cause by the subduction of the African plate under the European plate. The same process that help sprout new members of the alpinist tribe.

It is quite possible that the tremoring noticed is also related to this, about that we will hav to wait and see.

Image by EMSC-CSEM. As you can see this quake is 12km SSW of Santorini. This quake was one of the more northern in the Swarm. It is located in a fracture zone between two faultlines.