by hewhocaves » May 8, 2006 12:23 am
Hey Flash...
the only thing you might want to add to your description is that most of the dissolution occurs at or just below the water table. This Winds up being a very important, but often omitted detail as it can be partly responsible for the gridlike patterns in some caves (along with many caves developing in levels which make no sense otherwise)
The 'average joe' explanation I usually gives goes as follows:
"Big cave systems need a few basic things to form. 1) Rainwater. 2) Limestone. 3) a water table. Now rain, which becomes slightly acidic from rotting plants passes downward to the water table, dissolving little bits of rock along the way. When it hits the water table, all those little reactions which started on their own can come together and collectively eat away far more limestone than they could individually. As they do, they release carbon dioxide into the air, actually changing the atmosphere! Given enough time, large rooms and long passages can be formed this way, confined only by the folding of the layers of rock or the edges of the limestone deposits."
(at this point I usually pause for questions befor continuing...)
"Now, the reason I explained it this way is that a giant cave with no entrance is a pretty useless thing. And even if you could get into you, you'd find a very dull and formationless cave. This is because, for the most part, the environment that makes a cave bigger isn't conducive to making it pretty. But the natural world is really cool in that with just a slight change the exact same preocesses will fix all that up for us. going back to our little cave, we know that little bits of rock are being dissolved as the water trickels downward to the water table. And though this isn't the quickest way to make a cave, over a long long time, it will still make a hole big enough to go through and eventually that hole will come close enough to the surface that a hole will open up and we'll be able to get in! Or maybe a horizontal passage will come close enough to the edge of a hill that a small crack will open to the surface! But the point is, entrances are accidents. Happy accidents, though. Because when they do open up, all that changed atmosphere leaves the cave and the air is back to normal (this happens so well that it's really, really rare to find bad air in a cave).
But the story isn't done yet, because after all that is done, the slightly acidic rain won't stop... it will still keep coming. But this time when it does, something totally different will happen. As it passes through solid rock, it picks up little bits of it as before. But now, when it reaches a room, it immediately drops it load and becomes regular water once again. These little molecules of calcium carbonate (the dissolving part of limestone) arrange themselves (like a natural game of Tetris) into long colums we call stalacties and soda straws. If there's an awful lot of CC, some small amount will splash onto the bottom of the floor, creating a stalagmite. You can see this happen over the course of years rather than centuries by looking under concrete bridges and buildings.
So the same process does double duty - first, to dissolve away the cave and later to make it pretty."
And from that point, I can split the converastion into formations, hydrology, etc... anything.
John
The NSS and WNS: Cooperation, not confrontation.