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
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..