When lightning strikes the ground, the result is a little tube of melted minerals called a fulgurite. In recent years fulgurites have become a staple at rock shops thanks to collection efforts in the sandy wastes of the Saharan countries. In 2007, this geological obscurity exploded as a scientific star as well.
A lightning bolt can deliver about a gigajoule, enough energy to power an all-electric home for a week (some 300 kilowatt-hours). When this kind of energy enters the ground, it vaporizes and melts the soil into a fragile hollow tube of glassy froth that may extend several meters deep. Much depends on the nature of the soil. The best fulgurite specimens come from clean quartz sand. On solid rock, fulgurites look like greenish, gray or black scabs. They get their colors from other elements in the rock-forming minerals.
Similar rocks are known from two other causes besides lightning. Sand swept up in the Trinity atomic-bomb test, in the New Mexico desert, melted and rained down on the test site forming a green glass called trinitite. And impacts by meteorites can produce fused sand and other melted bits known collectively as tektites. The clear green variety called moldavite is the most famous of these.
Testing Bottled Lightning
A Mexican scientist named Rafael Navarro-González realized that fulgurites should contain samples of air in their glassy bubbles. With the latest spectrometers, even fantastically small samples can be analyzed. Not only that, but the glass-walled bubbles should preserve their gases for geologically interesting periods of time. Navarro-González had several questions he could answer with fulgurites.
- First, the bubbles should preserve samples from the lightning bolt itself. Lightning is so energetic that it shreds the air's oxygen and nitrogen molecules (O2 and N2) into free atoms, which create nitric oxide (NO). This interests ecologists and geochemists because breaking nitrogen out of its inert molecule is essential for life, but very difficult without certain bacterial enzymes. Before these bacteria arose, lightning would have been the world's main source of active nitrogen. And NO is an important gas in the life cycle of ozone, making it relevant for climate-change modeling. But the details of NO production are not well known.
- Second, the bubbles should preserve samples of the air in the soil, which would include carbon dioxide. By sifting and weighing the carbon atoms, it should be possible to say something about the conditions at the time lightning struck. No one had tried the experiment before.
- Third, the glass itself could be dated by another laboratory method. The natural radiation in the environment damages quartz glass with atomic-scale flaws, similar to the delicate color change that old glass gets after years of sunlight. Stray electrons are caught in the flaws. A heat or laser treatment releases the stored electrons and causes the material to emit light. This luminescence method allows various surfaces to be dated. As with the first two questions, luminescent dating had never been reported before from fulgurites. Indeed, no lab dates of any kind had been published on fulgurites.
Fulgurites: New Archives of Old Data
Navarro-González and his research team captured and analyzed the gas from fulgurite bubbles and measured luminescence using heat treatment (thermoluminescence). They published their results in the February 2007 Geology. Fulgurites, it turns out, have things to tell us about lightning, ancient climate and paleoecology.
- Nitric oxide was found in the bubbles. This is supporting evidence for the argument that NO forms in the lightning channel, not from shock waves nearby. In a word, the work is done by the lightning, not the thunder.
- The bubble gases were shown to come from the soil and its organic content, not the atmosphere. The carbon atoms match the isotopic profile of so-called C4 plants, which thrive in hot dry places. Because lightning is absent from this part of the Sahara today, the fulgurites testify that the area once had a climate like the Sahel to the south, where lightning and grasslands are common today.
- The luminescence dating gave an age of about 15,000 years, in the latest Pleistocene Epoch. Until this study, we had little hard evidence that lightning had even existed in the geologic past, let alone any knowledge of its patterns in time and space.
This study reveals new facts about the ancient Sahara and the chemistry of lightning. But much more important is the proof of concept—fulgurites are archives of ancient lightning, atmospheres and ecology.
PS: Advances in lab instruments always lead to advances in science—part of the genius of scientists is seeing what would be worth putting into the new instrument. For instance, Galileo learned of the newly invented telescope and quickly used it to observe the sky. The mass spectrometer that Navarro-González used so well has brought out equally interesting information from fossil teeth, the internal zonation of zircon crystals, and much else in geology today.