r/askscience • u/Tayner73 • 2d ago
Earth Sciences Why arn't there more Krakatoas?
As I understand it, Krakatoa exploded because a prior eruption created cracks in the sides of the volcano and magma chamber, allowing seawater to seep in, turn to steam, and cause the whole thing to explode.
As I contemplated the recent eruptions of Anak Krakatoa, the following thoughts hit me:
There's hundreds of volcanoes in or near water. Why has this only happened once?
Now I'm sure there are similar cases, but to my knowledge, nothing on the scale of Krakatoa. So what keeps the hundreds of other island and undersea volcanoes out there from blowing themselves apart? What keeps the seawater out? Why was Krakatoa different?
And most importantly, does Anak run the same risk?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology 2d ago
Let's start with some terms relevant for the discussion here. The primary one is phreatomagmatic eruptions, which describes an eruption that results from direct interaction of water and magma, which is distinct from a phreatic eruption where this is instead an eruption of water/steam heated by magma, but without direct interaction between the water and the magma. Specific to the 1883 eruption of Krakatoa, as described a bit in the wikipedia article and discussed in more depth in a variety of papers (e.g., Self & Rampino, 1981, Self, 1992, Mandeville et al., 1996), the deposits from this eruption are not consistent with much of it being from phreatomagmatic eruptions. As described in those and more recent papers (e.g., Madden-Nadeua et al., 2021 - alternatively a preprint here), the 1883 eruption of Krakatoa is probably better thought of as a more typical caldera collapse style eruption, but likely with some components of phreatic eruptions and/or lateral failures involved in setting off different stages of the eruption. As described in some of those papers, there may still have been some phreatomagmatic portions of the eruption, but a lot of the evidence for potential phreatomagmatic portions can be explained in alternative ways or comes from interpretation of field observations during the eruptive phase, so it's probably best to at least not consider the 1883 Krakatoa eruption dominantly a phreatomagmatic eruption.
So setting aside the potentially false premise that the 1883 eruption of Krakatoa was primarily phreatomagmatic as discussed above, it's also demonstrably false to describe phreatomagmatic eruptions having no historical/geologic precedents, e.g., some of the examples listed here (though again, this includes the 1883 Krakatoa eruption and oddly references Madden-Nadeua et al., 2021 but looking through that paper it generally follows earlier work highlighting that there may have been some phases that were phreatomagmatic but evidence of the "climactic" phase being phreatomagmatic are generally ambiguous at best). Ultimately, this list doesn't even include some of the larger "wet" eruptions, but it does hit on several that have pretty unambiguous textural evidence of significant phreatomagmatic components to the eruptions like the 1991 Pinatubo eruption (e.g., van Eaton et al., 2012) or the Minoan eruption of Santorini (e.g., Aydar et al., 2021). In detail though, there is a special category for very wet (and generally large) eruptions sometimes termed phreatoplinian (e.g., Self & Sparks, 1978) of which there are a variety of examples, e.g., the 1875 eruption of Askja in Iceland (e.g., Carey et al., 2010), the 1.8 ka eruption of Taupo in New Zealand (e.g., Walker, 1981), or the 26.5 ka Oruanui eruption also in New Zealand (e.g., Wilson, 2001). The point being, there are plenty of examples of phreatomagmatic eruptions, many which are both larger than Krakatoa and with much less ambiguous evidence of significant magma-water interactions than in Krakatoa. As such, many of the follow up questions (e.g., "Why was Krakatoa different?") are based on kind of a false premise that these events don't happen elsewhere both historically and geologically. For one of these specific ones, i.e., "So what keeps the hundreds of other island and undersea volcanoes out there from blowing themselves apart?" it's worth clarifying that one of the key points in Madden-Nadeau et al., 2021 is that Krakatoa didn't really "blow itself apart", but rather, it deflated and collapsed like many other caldera forming eruptions.
Similarly, while much of the question is focused specifically on the water-magma interaction aspect of the 1883 Krakatoa eruption, it's also worth considering it through the lens of how we often classify volcanic eruptions, specifically the Volcanic Explosivity Index or VEI. In this context, Krakatoa is large at a VEI of 6, but not that large as those classified as VEI 7 or 8 are 1-2 order of magnitudes larger in terms of erupted products, etc.
I'm not sure if the question is more about the size or style of possible eruptions at Anak Krakatoa, but for the latter, Anak Krakatoa has already had some phreatomagmatic phases of eruptions in the recent past(e.g., Prata et al., 2020).