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These Rocks Are Petrified Lightning - Video học tiếng Anh
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These Rocks Are Petrified Lightning
These Rocks Are Petrified Lightning
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Sous-titres (133)
0:00
Geologists are usually concerned with
0:01
the study of rocks that formed over millennia.
0:04
Those stones can tell us about the rise
0:06
and fall of mountains, oceans, and life.
0:09
But there are a few maverick geoscientists
0:11
that focus their attention on much more fleeting moments,
0:14
studying rocks that formed in a literal flash.
0:18
These are fulgurites, and they’re basically petrified lightning.
0:23
And despite their super-fast formation, they still have the power
0:26
to tell us about major changes taking place thousands of years ago.
0:33
[♪INTRO]
0:35
All over the world, lightning strikes about 45 times every second.
0:40
In the atmosphere, pockets of air are constantly rising and falling.
0:44
And when water is involved, like in tall cumulonimbus clouds,
0:48
the freezing and collision of ice creates charged particles.
0:52
Negative charges accumulate at the bottom of a cloud,
0:55
while the positive charges float to the top.
0:57
And in response to the nearby negative charge,
1:00
the ground becomes positively charged, too.
1:02
Normally, the atmosphere is a great insulator,
1:05
so these charges can keep building and building before grounding out.
1:09
But when they do, they do so in style.
1:12
Voltages of up to one billion volts will flow towards
1:16
the ground in each lightning strike, generating currents
1:19
as high as 30,000 amps for less than a quarter of a second.
1:24
Compare that with a household supply of just 120 volts and 15 amps,
1:29
and it’s clear that this is far more than a flash in the pan.
1:33
Lightning discharges concentrate all of their energy at a single point
1:36
in the ground, instantaneously heating it by thousands of degrees.
1:41
The strike will immediately burn anything flammable,
1:43
or melt anything non-flammable, including rock.
1:47
But since it’s all over in a flash, that melted rock
1:50
quickly cools to normal air temperatures.
1:53
What’s left behind is known as a fulgurite:
1:56
a hollow tube, whose surface is glassy,
1:59
cooled too quickly to create any real crystal structures.
2:02
Beyond their formation in lightning strikes,
2:05
there is very little else that unites the fulgurites,
2:08
since the location, duration, and conditions differ for every one.
2:13
Many are small enough to hold in the palm of your hand,
2:15
but some can reach a meter or more in length.
2:18
The longest that’s been found was in Florida,
2:21
stretching more than 5 meters long.
2:24
And the lightning’s path through the ground determines their shape.
2:27
Water in the ground can have a major effect on its conductivity,
2:31
so the fulgurite left behind can go straight, bend,
2:35
or even branch as the voltage spreads out
2:38
under unique conditions each time.
2:40
Finally, since lightning can strike anywhere,
2:43
a fulgurite’s chemical makeup is determined by
2:46
the composition of the ground where the bolt hits.
2:49
Both sand and rock fulgurites are common, and since silica is a major
2:53
component of both of these, the tubes they leave behind are often
2:56
made from fused grains and a silica glass called lechatelierite.
3:01
But you can also get fulgurites from soil,
3:03
or without any lechatelierite at all if they strike rocks like limestone.
3:08
Being able to hold a bolt of lightning,
3:10
or at least its scar, is pretty cool in itself.
3:14
But geologists studying fulgurites are able to do a lot more with them.
3:18
They’re found wherever they formed,
3:21
and so researchers can use their relative position in rock
3:24
and sediment layers to figure out when they formed too.
3:28
That makes them crucial tools in the unlikely study of paleolightning –
3:33
tracing the geological history of events that are over in a second.
3:37
It might seem like knowing the precise time
3:39
and place of a single lightning strike wouldn’t be very useful.
3:43
After all, lightning isn’t growing mountains or splitting continents,
3:47
however loud the thunder may be.
3:49
But these random, instantaneous events are
3:51
a symptom of much longer-lasting phenomenon.
3:54
Fulgurites, especially when found and mapped,
3:57
can tell us about weather conditions at a particular point in time,
4:01
and by extension, the local climate.
4:03
This isn't always something that’s
4:05
preserved in much-slower-forming rocks.
4:07
For instance, they’ve been helpful in decoding
4:10
the climate history of the Saharan desert in Africa.
4:13
Scientists mapped fulgurites across around
4:16
50,000 square kilometers of central Niger,
4:19
and found them to all date from the Holocene,
4:22
around 15,000 years ago.
4:24
The fact that these fulgurites are there at all
4:27
suggests a climate very different from today.
4:29
These latitudes are now hyper-arid,
4:32
receiving annual rainfall of less than 20 millimeters per year.
4:37
That’s less than New York City can get in a single week in the spring.
4:40
But the presence of fulgurites suggests that,
4:43
15,000 years ago, there were a lot of thunderstorms here.
4:47
And where there’s thunder and lightning, there’s often rain as well.
4:52
The numerous lightning strikes in the area suggest that
4:54
the ground was moist, something that’s also
4:57
supported by evidence of soils and lakes from this time too.
5:01
Researchers found more of the structures
5:03
the farther south they went, suggesting more strikes in that direction,
5:07
which helps explain why this area was wetter.
5:10
Because, to the south of Niger today,
5:12
the Saharan desert gives way to the Sahel,
5:15
a transitional band of only semi-arid climate,
5:19
before getting to the wetter, tropical Savanna further south.
5:22
Critically, the Sahel experiences monsoon rainfall during the summer.
5:26
If the tropical monsoons had stretched farther north back
5:29
during the Holocene, then this could have been
5:32
the source of the Sahara’s moisture and fulgurites.
5:35
And the lightning stones can also help
5:37
to reconstruct just how far north the rain fell.
5:41
Similarly ancient fulgurites found in Libya,
5:43
to the north of Niger, have a little extra sauce in their structures.
5:47
When rocks melt from a lightning strike,
5:49
and then immediately solidify into glass,
5:52
they can trap bubbles of air, which are basically
5:55
tiny snapshots of the atmosphere at that time.
5:58
Researchers have measured the composition of carbon dioxide inside
6:02
those bubbles, and found them to have the characteristic fingerprint of
6:06
an active plant ecosystem – just like that found in the Sahel today.
6:12
With this in mind, it seems that, 15,000 years ago,
6:16
the wetter Sahel climate reached at least
6:18
650 kilometers farther north than it does today.
6:22
In this way, Saharan fulgurites became a key piece of evidence
6:26
in reconstructing a warm wet period during the
6:29
Holocene that scientists called the ‘African Humid Period’.
6:34
Researchers think that this shifting of the climate zones was
6:36
linked to periodic changes in the Earth’s orbit around the sun.
6:40
Essentially, the tilt of the planet’s rotational axis and its elliptical orbit
6:45
aligned to make summers more intense than they are today,
6:48
driving an expansion of the tropics, and the migration of the Sahel.
6:54
So petrified lightning, just a few centimeters long and formed in a flash,
6:59
is capable of telling us about orbital changes over thousands of years.
7:04
And soon, SciShow Rocks Box subscribers
7:07
will get their own fulgurite in the mail!
7:10
Every month, we send out an amazing mineral or fossil,
7:13
in addition to selling some other cool merch for rock hounds.
7:18
Visit Complexly.store/rocks to check it out.
7:27
[♪OUTRO]
