| --- On Wed, 2/2/11, Lin Kerns <linkerns@gmail.com> wrote: >A few comments on this post: > According to vulcanologists in Indonesia, Anak Krakatau is a young, Strombolian volcano and indications are that it is not in danger of exploding. In fact, they stress that as long as people maintain a distance of 2 kilometers from the cone, everyone should be safe. The nearest habitation is 42 kilometers away and even if A K blew, volcanic bombs would not reach where people live. >According to these same Indonesian volcanologists, Anak Krakatau will not cause a tsunami. We should put these comments in perspective. The likelihood of a large, explosive eruption like the 1883 eruption that produced the tsunamis is very, very low at present. That does not mean that it will NEVER happen. Krakatau had a previous large explosive eruption in 416 AD. This left a caldera about the same diameter as the one left by the 1883 eruption. The complex sits above a magma chamber that tends to be a little over 4 miles in diameter. The magma chamber goes through a cycle. It starts with injection of basalt and some of the basalt makes it to the surface and causes the type of eruptions that we see today at Anak Krakatau. This volcano built a cone big enough to reach sea level in roughly 40 years. Off the top of my head, the caldera was about 900 feet deep. The frequency of eruption at Anak Krakatau indicates a fairly large magma supply to the magma chamber. With time, the magma that remains in the magma chamber will begin to differentiate, even as more basalt enters the system from below. Eventually, the volume of differentiated magma will become large enough and viscous enough that an explosive eruption occurs. Then the whole cycle starts over again. The last cycle was about 1400 years long. To generate that size eruption again will certainly take more than 250 years. Of course, there are no guarantees that Krakatau will continue in this way. There are a number of factors that go into whether a larger eruption takes place or if the cycle is disrupted. >Yellowstone is NOT due for another eruption. In fact, geologists are uncertain whether given the current data that Y will even erupt at all again. The current rise of the surface in the caldera is something that has been occurring for thousands of years. The caldera rises and falls in a cyclical pattern. No one knows when the next eruption will come, but currently the quakes that occur at the park do not indicate magma is rising. When or if that time comes, then we'll have plenty of warning. Don't believe the stupid movies out there that would have you heading towards the caves and do not trust the alarmists. Well, the geophysicists are hedging their bets. They say the earthquake swarms COULD be from changes in the hydrothermal system. The fact of the matter is that there is a large, active magma chamber under the caldera and to the east. Since the last large, caldera-forming eruption there have been a number of small, moat filling eruptions, the last about 70,000 years ago, I believe. There is evidence of some magma injection to the magma chamber because they have detected the release of magmatic gases (sulfur dioxide) that indicate fresh magma. It just hasn't been in "large" volumes. As for there being plenty of warning, there may or may not be plenty. Predicting eruptions depends upon having experience with that type of eruption previously. We've never seen a large caldera-producing eruption (and like the mythic purple cow, we never hope to see one). Because we haven't seen an eruption anywhere near this size, we don't know for sure what the precursors are or how long before an eruption they may occur. I'm not very familiar with it, but it is my understanding that the eruption of Chaiten, Chile, in 2008 occurred with very little warning, for example. This was the first rhyolitic eruption since Novarupta in 1912. The large eruptions at Yellowstone are rhyolitic and rhyolitic eruptions are fairly infrequent. I worry that the realization that the large calderas "breathe" may actually cause volcanologists to be complacent and miss some precursors. That's just me. Diorite |
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