How Caves are Formed

Dr. George Veni discusses how caves are formed with Ann Miller.

How Caves are Formed (Windows Media) | How Caves are Formed (Real Media)

Transcript


A view of lighted formations inside Longhorn Caverns


Transcript

(Ann)
Good morning, I'm Ann Miller and today I have with me Dr. George Veni. We're so happy to have you this morning with us to tell us a little bit more about caves and this is another man who has been studying caves for a very long time. How long have you been studying caves George?

(George)
About 28 years.

(Ann)
Oh, alright we have some real experts with us this morning. Um, we heard just a bit ago from Jim that there are lots of different types of caves. Are there lots of different types of caves in Texas?

(George)
There's several different types of caves in Texas. There's caves that form in different types of rocks. What I want to focus on are the caves that form in the rock called limestone because they're the most common caves. They're usually the longest, the biggest, the thing that people are most familiar with. But caves are basically any naturally occurring underground cavity that people can fit into.

(Ann)
Ah ha.

(George)
And if we can't fit into it it's just a hole in the ground (laughter). Caves are basically a bigger hole in the ground. Um, so er…

(Ann)
We have a little model here. I think the folks can see that shows some caves.

(George)
Yeah. This is a…this is a model of what we call a karst landscape. Well, er the theme of what we're talking about today is karst. In karst basically describes an area, a landscape much like we can talk about swamps, or mountains, or prairies. Karst is an area that's formed primarily by dissolving away the bedrock and limestone is what we call a soluble rock, meaning that it dissolves away pretty easily. It dissolves more easily than it mechanically breaks down. And so typically the way caves tend to form, at least limestone caves, is that water, rainfall comes down from the sky and picks up carmadioxide gas that's naturally in the atmosphere and then when it moves into the soil there's more of that gas in the soil and it makes it acidic. It makes the water slightly acidic. And so it begins to move down, as you can see here on the front part of the model, it'll move through fractures and as it moves through the fractures it slowly dissolves open these fractures making them bigger so they can carry more water. Eventually, for instance here's a piece of rock here that we have, you can see that it's eaten out, that it's dissolved away to form lots of little holes in it that the water flows through. As time goes on, and we're talking about thousands and thousands of years, what happens is the holes start to get bigger and bigger. They become more selective in terms of which ones are the ones that are really going to get enlarged and eventually the holes become big enough, as we see here from the model in front, they become what we call a cave. What we're sitting in here right now, Longhorn caverns. Something big enough for us to walk in, crawl, not run through but we could if we really wanted to in this cave because the floors are so flat and level but it's not a good idea.

(Ann)
So the cave we're in was made in a Karst landscape.

(George)
Yeah

(Ann)
OK. So we've got limestone all around us?

(George)
Yes

(Ann)
OK. But there are other caves in Texas that are not limestone. Is that correct?

(George)
The most common type in Texas that isn't limestone is are gypsum caves and they occur in west Texas. They're still karst because gypsum is a very soluble rock, it dissolves very easily. In fact, it dissolves more easily than limestone and you only find gypsum caves in very dry climates because if you were, let's say, in the eastern part of the country were it rains a lot the gypsum rock would dissolve away very fast.

(Ann)
Oh, so totally. So you wouldn't even have a cave left. So we have the gypsum caves out in west Texas, we have the limestone caves here in central Texas. Is that correct? This is a beautiful cave we're in right here. How old do you think this cave is? How long has it been here?

(George)
Trying to determine, trying to figure out how old a cave is is really difficult because the cave is the absence of the rock, the cave is nothing. So you're trying to figure out when did the rock go away?

(Ann)
Yeah.

(George)
And so it's a really tough er tough prospect. I would guess, we don't have enough information, but I would guess that this cave began to form about a million years ago. The rock is hundreds of millions of years old. The rock was here a long time before the cave actually began to form in it.

(Ann)
OK and it's still growing or it's still being formed. Is this a live cave or not?

(George)
The part of the cave we're in right now is pretty inactive. It's not really growing very rapidly right now. There's parts of the cave further down that way which are a lot more active and are still growing. They have water flowing through them regularly and so those parts of the cave are still pretty active and growing. This part here isn't growing very rapidly right now.

(Ann)
OK. Well, um I know that you have some favorite caves in Texas. I know that not all caves are the same size or have the same structures. Are there some caves, for instance this cave has a lot of water marks all over it, it looks like there's been water flowing through. Is that pretty normal for caves?

(George)
Definitely because caves are formed by water, so this cave, at one time, was filled with water and it was moving through here and as an underground river. Now sometimes people think that that means there was a flood of water rapidly moving through but in this case the water was moving very slowly through this cave. It was a lot of water, moving slowly over a long period of time that slowly dissolved out and created this cave.

(Ann)
Well, it is a beautiful cave and I've been to the caverns of Senora myself and I know that instead of the structures you're seeing here you see a lot of crystals in that cave. And what kind of rock, its still limestone?

(George)
It's still limestone but what you're seeing are speleothems which are crystal deposits that form in caves and we'll be talking about those later on today.

(Ann)
OK, good.

(George)
And we can talk more about those then. But that cave has a lot of speleothems, here we have very few.

(Ann)
OK. Well we will look at more cave structures later on. I really appreciate you being with us this morning and now we're going to go over to Cappy who is with some folks who can tell us more about habitat in caves.

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