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volcano of the week
Oct. 21st, 2020 11:12 pm-- which I am sure I have mentioned hereabouts before, but it came up as Incidental Material when I was teaching on Monday and some of my students were actively enthusiastic about it, which I was proper delighted by seeing as being told about this volcano in my first term of university made A Big Impression On Me --
-- is Ol Doinyo Lengai, at the end of the East African Rift Valley in Tanzania (the Smithsonian has lots of photos), which erupts really weird lava. This is -- in brief -- because it's sat on top of an old, thick, stable craton, so it's had plenty of time to be gradually metasomatised, i.e. to accumulate lots of Weird Shit. Being toward the end of the East African Rift, the reason for melting is that the crust is very slowly pulling apart, so it's very slowly melting at very low temperatures as a consequence of pressure decreasing faster than the rocks can cool down. At 500-600°C (or equivalent at appropriate pressure), silicates -- minerals based on SiO2 -- don't melt: you gotta get up to around 1300°C for that, give or take. Which means only the weird shit manages to reach its melting point.
Which is why Ol Doinyo Lengai erupts, essentially, baking soda. It doesn't even manage to glow red: it appears black on eruption, and cools to a fairly pale grey-white, a lot of the time, as you can see in those Smithsonian aerial photographs.
And. The thing is. The lava then washes away every time it rains, because it just dissolves. (This is why it came up in teaching: we were talking about ionic versus covalent bonds, and why salt forms aqueous solution but oils don't, and what the melting temperature of NaCl is versus the melting temperature of sugar, so... obviously I got on to "okay, so where do we see something like NaCl actually being erupted?")
And washes into Lake Natron, where the intensely alkaline brine encourages halophile ("salt-loving") organisms, including cyanobacteria that photosynthesise... using a red accessory pigment.
The intensely inhospitable-to-most-mammals environment makes it prime breeding ground for flamingos.
Who consume the algae.
And that's why flamingoes are pink.
-- is Ol Doinyo Lengai, at the end of the East African Rift Valley in Tanzania (the Smithsonian has lots of photos), which erupts really weird lava. This is -- in brief -- because it's sat on top of an old, thick, stable craton, so it's had plenty of time to be gradually metasomatised, i.e. to accumulate lots of Weird Shit. Being toward the end of the East African Rift, the reason for melting is that the crust is very slowly pulling apart, so it's very slowly melting at very low temperatures as a consequence of pressure decreasing faster than the rocks can cool down. At 500-600°C (or equivalent at appropriate pressure), silicates -- minerals based on SiO2 -- don't melt: you gotta get up to around 1300°C for that, give or take. Which means only the weird shit manages to reach its melting point.
Which is why Ol Doinyo Lengai erupts, essentially, baking soda. It doesn't even manage to glow red: it appears black on eruption, and cools to a fairly pale grey-white, a lot of the time, as you can see in those Smithsonian aerial photographs.
And. The thing is. The lava then washes away every time it rains, because it just dissolves. (This is why it came up in teaching: we were talking about ionic versus covalent bonds, and why salt forms aqueous solution but oils don't, and what the melting temperature of NaCl is versus the melting temperature of sugar, so... obviously I got on to "okay, so where do we see something like NaCl actually being erupted?")
And washes into Lake Natron, where the intensely alkaline brine encourages halophile ("salt-loving") organisms, including cyanobacteria that photosynthesise... using a red accessory pigment.
The intensely inhospitable-to-most-mammals environment makes it prime breeding ground for flamingos.
Who consume the algae.
And that's why flamingoes are pink.
