Somewhere under Tibet

Somewhere under Tibet


[MUSIC PLAYING] My first time ever in
Tibet, I was just excited because I had been brought
up on adventure tales– Edmund Hillary climbing Mt. Everest, and the Maori
expedition, being lost on Mt. Everest. And here was the chance to
come to see the Himalaya and Tibet for myself. And not just traveling,
but traveling with a purpose and a chance
to actually do science. It’s beautiful Tibet. There are the small terraces
with the what or barley that’s ready to be harvested,
and people working over there on the other side of the river. The Yarlung Tsangpo
River rushing down. Big rapids right here. Lots of nice deformed
rocks around us. So it’s a good place. It’s a good place
for geologists. Let’s check if that’s hotter
coming out of the source, then we can feel
in the water, OK? Geophysicist Simon
Klemperer is leading an unusual geologic expedition
across the vast Tibetan Plateau. With geologists from
the US and China, Simon’s group is searching
for hot springs– as many as they can
find across Tibet. At each spring, the group takes
samples of the water and gases bubbling up. By analyzing these
in the lab, they hope to find out where the
springs come from deep below. Down there, two
gigantic tectonic plates crashed together about
50 million years ago. Ever since then,
the Indian Plate has been sliding, or subducting,
beneath the Asian Plate, thrusting up the Himalaya
and the Tibetan Plateau. But in one of geology’s
great mysteries, no one knows what the
collision zone looks like. When we first came
here, there was a lot of debate about the
boundary between India and Asia. Did the Indian Plate
stop under the Himalaya? Did it slide all
the way under Tibet? 25 years ago, no one knew. But Simon and his colleagues
made a breakthrough using seismic techniques. In 1992, we collected the first
reflection profile in Tibet. And we were able to
show through seismology that India dipped at a
low angle beneath Tibet. But we still left
open the question of how far north did India
project beneath the Tibetan Plateau. We’ll probably do the water
sampling in the outlet down there, and do the
gas sampling right here. OK. Why the interest in where the
plates are below the plateau? For one thing, this
plate collision has created Earth’s highest
and largest geologic structure: the Himalaya and the
Tibetan Plateau combined. It is Earth’s number one classic
mountain building example. As this massive block
of rock rose up, it changed the climate
of the entire earth. It created the great
Indian monsoon, which affects millions
of people each time it sweeps across Asia. The Tibetan Plateau
and the Himalaya are also the source of
many great rivers, which supply water to billions of
people in India, Bangladesh, and China. The colliding plates below
have another huge impact on the region’s people– massive, deadly earthquakes,
like the Gorkha quake in Nepal in 2015. It is critical, then, that earth
scientists fully understand the collision of plates below. Yet one of the collision’s
fundamental traits remains unknown: its location. This drove Simon
Klemperer to resort to a radically
different approach in finding the elusive plates. This year’s experiment
was designed in an entirely new way. Instead of using seismology
to try and track India as it extends north
beneath Tibet, we’re going to try and look
for a geochemical signature beneath the Tibetan Plateau. Right, OK. So I’m going to
close this off now. And now I’m going to try
and fill this up with gas. That signature is written
in the gases and water of Tibet’s many hot springs. By reading that
signature in the lab, the scientists will get a
surprisingly detailed glimpse into the earth’s hidden depths. The chemistry can tell you a lot
about the source of the water. So if the water is
very, very clean, it’s probably a
source from rainfall. Whereas some of the
fluids in hot springs can come from very, very deep– like, deep as in the mantle. And in the Tibetan Plateau, that
means possibly 70 kilometers or more. By the chemical
composition of the water, we can tell what
percent of the fluid might come from the mantle. The mantle is the earth’s
second layer down, lying below the rocky crust. What I think is almost most
bizarre about this process is that the bubbles that are
coming out of the spring, some of that gas has come from– may have come from 70
kilometers or 100 kilometers below us, which is what
we’re going to find out when we analyze this in the lab. This is our first
sample, and it’s great. The gas is mostly
carbon dioxide, but it carries a certain noble
gas that the scientists are most interested in: helium. If the ratio of certain
helium isotopes, helium-3 and helium-4, are
above a specific number, then the hot spring is thought
to start in the mantle. If not, then the hot
spring starts in the crust. Simon’s assumption is
that a crust sample could be from the Indian
or Asian Plate, but a mantle sample should be to
the north of the collision zone from the mantle of
the Asian Plate. Taking samples at
many hot springs should tell Simon where the
Indian Plate ends below Tibet. Besides gas samples,
the scientists also take samples of
the spring water itself. With this they can measure
cations and anions, and elements like sulfur,
carbon, and strontium. Yes. These can reveal important new
details of the hidden depths below Tibet, like earthquake
faults, magma pools, and reservoirs of hot water. The scientists had only
been given a few weeks to do their fieldwork, yet
Simon and his group ended up collecting over 80 copper
tubes of gas and water and dozens of samples
in other containers– a huge success. The samples were sent to labs
in the United States and China. It takes months to get
the results, and possibly the long-awaited
answer to the question, where are the colliding tectonic
plates below the Tibetan Plateau? [MUSIC PLAYING]

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