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Geologists are usually concerned with the study of rocks that formed over millennia. Those stones can tell us about the rise and fall of mountains, oceans, and life. But there are a few maverick geoscientists that focus their attention on much more fleeting moments, studying rocks that formed in a literal flash. These are fulgurites, and they're basically petrified lightning. And despite their super-fast formation, they still have the power to tell us about major changes taking place thousands of years ago. [♪INTRO] All over the world, lightning strikes about 45 times every second. In the atmosphere, pockets of air are constantly rising and falling.
And when water is involved, like in tall cumulonimbus clouds, the freezing and collision of ice creates charged particles. Negative charges accumulate at the bottom of a cloud, while the positive charges float to the top. And in response to the nearby negative charge, the ground becomes positively charged, too. Normally, the atmosphere is a great insulator, so these charges can keep building and building before grounding out. But when they do, they do so in style.
Voltages of up to one billion volts will flow towards the ground in each lightning strike, generating currents as high as 30,000 amps for less than a quarter of a second. Compare that with a household supply of just 120 volts and 15 amps, and it's clear that this is far more than a flash in the pan. Lightning discharges concentrate all of their energy at a single point in the ground, instantaneously heating it by thousands of degrees.
The strike will immediately burn anything flammable, or melt anything non-flammable, including rock. But since it's all over in a flash, that melted rock quickly cools to normal air temperatures. What's left behind is known as a fulgurite: a hollow tube, whose surface is glassy, cooled too quickly to create any real crystal structures. Beyond their formation in lightning strikes, there is very little else that unites the fulgurites, since the location, duration, and conditions differ for every one.
Many are small enough to hold in the palm of your hand, but some can reach a meter or more in length. The longest that's been found was in Florida, stretching more than 5 meters long. And the lightning's path through the ground determines their shape. Water in the ground can have a major effect on its conductivity, so the fulgurite left behind can go straight, bend, or even branch as the voltage spreads out under unique conditions each time. Finally, since lightning can strike anywhere, a fulgurite's chemical makeup is determined by the composition of the ground where the bolt hits.
Both sand and rock fulgurites are common, and since silica is a major component of both of these, the tubes they leave behind are often made from fused grains and a silica glass called lechatelierite. But you can also get fulgurites from soil, or without any lechatelierite at all if they strike rocks like limestone. Being able to hold a bolt of lightning, or at least its scar, is pretty cool in itself. But geologists studying fulgurites are able to do a lot more with them. They're found wherever they formed, and so researchers can use their relative position in rock and sediment layers to figure out when they formed too. That makes them crucial tools in the unlikely study of paleolightning -
tracing the geological history of events that are over in a second. It might seem like knowing the precise time and place of a single lightning strike wouldn't be very useful. After all, lightning isn't growing mountains or splitting continents, however loud the thunder may be. But these random, instantaneous events are a symptom of much longer-lasting phenomenon. Fulgurites, especially when found and mapped, can tell us about weather conditions at a particular point in time, and by extension, the local climate.
This isn't always something that's preserved in much-slower-forming rocks. For instance, they've been helpful in decoding the climate history of the Saharan desert in Africa. Scientists mapped fulgurites across around 50,000 square kilometers of central Niger, and found them to all date from the Holocene, around 15,000 years ago. The fact that these fulgurites are there at all suggests a climate very different from today. These latitudes are now hyper-arid, receiving annual rainfall of less than 20 millimeters per year.
That's less than New York City can get in a single week in the spring. But the presence of fulgurites suggests that, 15,000 years ago, there were a lot of thunderstorms here. And where there's thunder and lightning, there's often rain as well. The numerous lightning strikes in the area suggest that the ground was moist, something that's also supported by evidence of soils and lakes from this time too. Researchers found more of the structures the farther south they went, suggesting more strikes in that direction, which helps explain why this area was wetter. Because, to the south of Niger today, the Saharan desert gives way to the Sahel,
a transitional band of only semi-arid climate, before getting to the wetter, tropical Savanna further south. Critically, the Sahel experiences monsoon rainfall during the summer. If the tropical monsoons had stretched farther north back during the Holocene, then this could have been the source of the Sahara's moisture and fulgurites. And the lightning stones can also help to reconstruct just how far north the rain fell. Similarly ancient fulgurites found in Libya, to the north of Niger, have a little extra sauce in their structures. When rocks melt from a lightning strike, and then immediately solidify into glass, they can trap bubbles of air, which are basically
tiny snapshots of the atmosphere at that time. Researchers have measured the composition of carbon dioxide inside those bubbles, and found them to have the characteristic fingerprint of an active plant ecosystem - just like that found in the Sahel today. With this in mind, it seems that, 15,000 years ago, the wetter Sahel climate reached at least 650 kilometers farther north than it does today. In this way, Saharan fulgurites became a key piece of evidence in reconstructing a warm wet period during the Holocene that scientists called the 'African Humid Period'. Researchers think that this shifting of the climate zones was linked to periodic changes in the Earth's orbit around the sun.
Essentially, the tilt of the planet's rotational axis and its elliptical orbit aligned to make summers more intense than they are today, driving an expansion of the tropics, and the migration of the Sahel. So petrified lightning, just a few centimeters long and formed in a flash, is capable of telling us about orbital changes over thousands of years. And soon, SciShow Rocks Box subscribers will get their own fulgurite in the mail! Every month, we send out an amazing mineral or fossil, in addition to selling some other cool merch for rock hounds. Visit Complexly.store/rocks to check it out.
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