Jeopardy! category: CAVES (20-5-2015)
$800 clue: In Lebanon’s Jeita Grotto, one of these hangs 27 feet from a cave ceiling
The gist: Although it’s clearly the best way to do it, I can’t open with the mnemonic because that would give it away. I’ve always remembered it as “stalactites fall down, like trouser tights, and stalagmites climb up, like mites do.” For some inexplicable reason, that works for me. An objectively better way is that stalactite has a ‘c’ for ‘ceiling’ and stalagmite has a ‘g’ for ‘ground,’ but hey, whatever works, right?
Both stalactites and stalagmites are (most often) examples of speleothems (aren’t these names great?), large mineral deposits in caves that form, over geological timescales, from flowing water that deposits layer after layer of calcium minerals along its path. Accounting for the majority of the rocky stalactites in the world, and certainly the most picturesque and iconic ones, are those found in limestone caves, where the calcium minerals are easily dissolved and subsequently deposited by water flows. The process begins with water carving a channel through the cave ceiling until it can drip down to the floor. Once it forms a breach, the water begins to deposit rings of calcium that grow on one another, forming long, narrow, hollow formations known as “soda straws.” If these get plugged up, the water finds a new way to flow, ending up on the outside of the straw, depositing its rocky cargo in the more familiar conical configuration. These sorts of stalactites grow mind-numbingly slowly, usually around 0.005 millimetres a year, much thinner than a human hair – the fastest can grow up to 3 millimetres a year.
But there are many other types of stalactites that can form much more quickly. In caves with active lava flows, stalactites of exotic shapes can form through the same process but with the water replaced with molten rock, growing from nothing in a matter of hours, days, or weeks. Ice stalactites – commonly known as “icicles” – form and fall routinely during cold weather; this time, it’s the calcium that’s replaced with water. Stalactites can even form on concrete in modern buildings, if the water supply is calcified and there’s a route for the water to flow through the concrete.
And one more fun fact: the ancient Roman writer Pliny the Elder (23 CE – 79), in his omnibus (and dubiously accurate) work Natural History, got it right on the money when he wrote that in a certain Macedonian cave, dripping waters harden into “pendentes” (“hangers” or “things that hang down”). Way to go, Pliny. You got one right!
The clue: It’s all but spelled out for you – it’s in a cave, and it hangs. And of course, the spelling is the crux of this clue anyway. Just need to remember if it’s stalactite or stalagmite.
In Jeopardy!: Stalactites are in 13 clues in the J!Archive, beating their ground-dwelling cousins by two, although they also often appear together. Limestone is the correct response in three of them. Other common themes are caves and words like “hang” or “drip” or others that imply coming down from on high. Again, refer to above for the handy mnemonics so you’ll never get them mixed up again, if you ever have in the past.