Listen to part of a lecture in an Earth Science class.

P: You might not think that studying lightning activity in a given place would be helpful for understanding changes in environmental conditions over time. In fact, though, the frequency of lightning strikes is a factor in determining whether an area can support life at all. That's because lightning provides plants and animals with crucial access to nitrogen, an element they can't live without, in a form they can use.

See, even though nitrogen is the most abundant element in Earth's atmosphere, most living things can't absorb it in its molecular form, because its atoms are so tightly bound together. The electrical energy in lightning breaks the molecules apart and the atoms bind with oxygen atoms. This form of nitrogen then falls to Earth and gets taken up by plants, which are eaten by animals, with the nitrogen eventually cycling back into the atmosphere.

The frequency of thunderstorms and lightning strikes is so important that we now have a device in space, a sensor that keeps track of lightning. Not that it keeps track of lightning strikes per se, rather, it detects momentary changes in the brightness of clouds. Now, if for any given location we could compare current thunderstorm activity with that of the distant past, well, that would be really useful for formulating a history of environmental changes there.

That's why it was such a big deal when geology researchers got hold of a fulgurite from the Sahara Desert. Because fulgurites are petrified lightning, lightning in solid form. A fulgurite is a hollow glass tube that's created when lightning strikes sand or sandy soil. The extreme heat melts the sand and fuses it into glass. Fulgurites are usually only a few centimeters in diameter, but they can extend well below the surface, branching out like tree roots. Each one's a permanent record of the path taken by a lightning bolt inside the ground.

The one that I'm talking about was found in the Libyan desert, one of the driest parts of the Sahara. Fulgurites are pretty rare, but they've long been popular with collectors because, you know, they just look so cool. But to find one in the Libyan desert of all places, the lightning sensor I mentioned confirms that there are no thunderstorms in this tropical desert at any time of year. Yes, Beth.

S: If fulgurites are under the sand, how can collectors find them? P: They're uncovered when the sand is eroded by wind. S: But glass is so fragile, wouldn't it break? P: It could, but this one survived intact for 15,000 years.

We've determined its age using a technique called thermo-luminescent dating, which was a major breakthrough all by itself. Now thermo-luminescent dating is complicated, but here's the basic idea. At the moment the lightning strikes, energy gets trapped inside the glass as the glass is being formed. Then when we heat the fulgurite in a laboratory, that trapped energy is released in the form of light, and by measuring this light, researchers can calculate how long ago the fulgurite was formed, in this case, approximately 15,000 years ago.

How ingenious is that! I mean, consider, until fairly recently, an ancient fulgurite could only be dated indirectly by examining the sediment it was buried in. We used to just assign the fulgurite to whatever geological era the surrounding material was thought to date from. So if only our Saharan fulgurite could talk, huh, and tell us what was going on in the Libyan desert at that time.

Well, the researchers did get it to talk, in a sense, by investigating tiny gas bubbles embedded in the glass. Gases can remain inside glass for millions of years without changing. When researchers opened the bubbles and analyzed the gases, they found nitrogen, of course, also large quantities of carbon. Now, carbon in the soil mostly derives from organic matter that comes from plants and animals.

Nowadays, the Libyan desert hosts very few plant and animal species, only those few that are adapted to extremely dry conditions. But the carbon content of the soil in the ancient Libyan desert, as we see from the fulgurite, turns out to be similar to what exists today in the region just south of the Sahara. That region has enough rainfall to support plants like grasses and trees, as well as a wide range of animal species.