Academic English for International Exams

W: We tend to think of lightning as something temporary. It is a flash of electricity that disappears almost as soon as we see it. However, lightning can leave evidence of its presence. When lightning strikes sand, its intense heat melts the sand and turns it into glass. Pieces of this lightning-created glass are called fulgurites, or petrified lightning. They are interesting to scientists for two reasons.

The first reason is because scientists can test fulgurites to determine their age. They can do this by heating fulgurites to make them glow. Over time, radiation causes more and more defects in the fulgurites. These defects glow when heated. Older material will glow brighter than newer material because it has more defects. Hence, scientists can determine the approximate time that the lightning hit the sand in the past. Now, why is determining the age important? Because climate scientists also study fulgurites to get clues about the climate in the past. When fulgurites form, they trap tiny bubbles of gas within them. Since they've already tested the fulgurites' age, scientists can study these bubbles of gases to understand what the climate was like at a specific period in the past. They can learn if the climate supported plants and animal life, or if it was too harsh. For example, fulgurites from the desert in Niger show that 15,000 years ago, the area could support shrubs and other plants. Today, there is only sand there.