Looking at the Earth is, in some places, very like looking at the Moon. You've all seen some photo from somewhere and had that thought at one time or other.
Recently, though, the point was brought home in our talk on basaltic flooding by Gary Peterson from San Diego State University. And with that as an inspiration, we were off to look at moonscapes all over northern California and Oregon.
One of our targets was lava tubes. I'm fascinated by rilles, and for years have read about sinuous rilles being primarily collapsed lava tubes that run for hundreds of miles.
I've been willing to take their word for it (dubiously) but what are lava tubes? How can they run so far? Why do they collapse? It wasn't easy to see how they would form, or why they would be so long...
Once you see them, it's pretty easy.
The lava comes from a vent of some sort (there are different kinds, but that doesn't matter for our discussion) and flows in whatever direction is downhill - which is often in many directions, if the vent forms on a plain.
Once the molten rock is out of the vent, it starts to cool immediately ... on Earth, because of the air, the cool earth, maybe rain or just heavy moisture ... on the moon much more slowly because there is no atmosphere to cool the top, so "radiant heating" is about its only option.
But they both cool.
This means the upper and lower surfaces of the flow are cooling faster than the middle part. So you get a crust on top and bottom, much like an Oreo where the biscuits are rock and the creme is molten lava.
This can lead to a number of fascinating landscapes, broken and sharp, glassy ... but it's what's going on underneath that we're after.
Of course, the "creme" in the Oreo keeps flowing, in some places better than others (fluid dynamics, oh well). The faster flowing stuff will, in general, stay warmer as it is being replenished by hotter stuff from behind, and the slower areas are cooling... and when that happens, a flow is insulated top and bottom, left and right.
In other words, it has formed a tube.
This is the Ground State of Lava: its natural tendency.
So there's nothing odd about lava tubes at all: in fact, they are what we should expect.
And when we went to Lava Beds National Park in the northeast corner of California, that's what we saw.
You can just take out your flashlight and climb on down, and go for surprising distances on foot, looking at the flow lines, the strata caused by different flows through the same tube, the smooth areas and rough, the colors, the sandy floors in places... this isn't travelogue (I hope) but if you ever get a chance to hike these tubes, you'll never forget it.
And when you see how often they occur, and how big they can get, and how often they collapse...
Well, of course that's important to us. We can't see a lava tube below the surface of the moon, but we can sure see one that has collapsed and created a trench... and that's just what they do.
When the flow is depleted (perhaps for the last time) the still molten lava will drain out the far end of the tube, until the tail end of the flow gets cool and blocks off the tube.
Time passes. Maybe a meteorite impacts nearby, or maybe there's some seismic activity, but something shakes up the place and the roof, only tenuous at best, simply starts collapsing. As time goes by (or maybe very fast) the whole thing falls like a line of dominos, and we see Schroter's Valley!
One last detail... a lava tube has to be pretty big for us to see it on the moon. Do they get that big?
Here on earth, we've walked through tubes big enough to build a four-lane freeway through with a generous shoulder and center divide. On the moon, though, there's less gravity and the lava is more "liquid" (less viscous) and can run further, and form larger (and longer) tubes.
So, yes. You bet. They could get that big, and once you've seen the blasted lunar landscapes of Lava Beds (and strolled through the underground basaltic burrows) you'll have no doubt: they could be lava tubes.
And you'll never look at a rille up there the same way again. Or, really, down here either.
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