Download Mantle Reading

Could you dig a hole all the way to the Earth’s mantle?
by Patrick J. Kiger
#1: Introduction
If your family took you on a vacation to the seashore when you were a small child, you probably remember
the exhilarating feeling of digging into the wet sand with a plastic shovel. As the hole got bigger and deeper, you
naturally wondered what would happen if you just kept digging and digging. How deep could you get? Would you
really eventually pop up out of the ground somewhere in China, as your big sister or brother tried to get you to
believe? Unfortunately, you never got to find out, but still, somewhere in the back of your mind, you've kept
wondering about what would happen if someone dug a really, really deep hole.
Well, you may not have to wonder any more, if an international team of scientists who call themselves the
2012 MoHole To the Mantle project succeed in their quest. They're counting upon international support for a $1
billion effort in which a Japanese deep-sea drilling vessel, the Chikyu, would burrow into the bottom of the Pacific
Ocean to dig deeper than anyone has ever gone before. The plan is to go right through the Earth's crust, the rocky
top layer of the planet, which is 18 to 37 miles (30 to 60 kilometers) thick on land, but as little as 3 miles (5kilometers) thick at its thinnest spots on the ocean floor. If the Chikyu's drill rig breaks through a transitional
boundary called the Moho, it would reach the Earth's mantle, the mysterious 1,740-mile (2,900-kilometer) thick
layer between the crust and the planet's hot, molten core.
Unlike your childhood fantasy, the scientists don't have any ambitions of boring a tunnel all the way
through the planet. That probably isn't even possible, since the enormous heat and pressure inside the Earth would
make crawling down such a passageway impossible, even if it somehow didn't collapse. But just reaching the mantle,
a layer about which we know relatively little, and retrieving a sample would be a scientific achievement of such a
magnitude that some have called it geology's version of the moon landing. In this article, we'll explain the difficulty
of digging such a deep hole, and what we might gain from it.
#2: What Is the Earth's Mantle?
It's amazing to think that we may be spending $1 billion to drill through the Moho, when you consider that
a century or so ago, we didn't even know that boundary existed. In 1909, Andrija Mohorovičić, a Croatian
researcher, discovered that about 20 miles (50 kilometers) inside the Earth, the waves triggered by earthquakes
traveled faster than they do closer to the surface. While scientists had at least a vague notion already that the
Earth had layers, Mohorovičić's work suggested that there was a clear boundary between the crust and a layer
beneath it that had different composition and physical properties. In his honor, we now call that boundary the
Moho.
Since then, scientists have managed to learn a bit more about the mantle, the layer that lies beneath the
Moho, which amounts to 83 percent of the Earth's volume and 67 percent of its mass. The easiest way to
understand this is to think of the Earth as a chocolate éclair. The thick outer layer of glazed chocolate and baked
dough is solid but elastic. That's the crust. Underneath that, though, there's a lot of viscous, gooey stuff. Of
course, that's a limited analogy, because the Earth isn't cream-filled. Instead, the mantle is made of molten, fluid
rock called magma. Some of that magma is ejected by volcanoes, so we know that in the upper part of the mantle —
that is, the top 620 or so miles (1,000 kilometers) — it seems mostly to be composed of oxides of silicon,
magnesium and iron, with smaller amounts of aluminum oxide, and calcium oxide thrown into the mix.
That said, our knowledge of the mantle is fairly limited. Scientists can't go down and look at it, and they've
never had a pure sample taken directly from the deep to analyze. That's what the 2012 MoHole to the Mantle
project hopes to accomplish.
#3: How Hard Is It to Dig That Deep?
It's going to be pretty tough. We know this because scientists actually tried to do it once before. In the
early 1960s, they drilled five holes into the ocean bottom near Guadalupe Island in the eastern Pacific Ocean at a
depth of 11,700 feet (3,566 meters). The deepest hole only penetrated 600 feet (183 meters) into the crust, just
past the sediment on the surface into a sub-layer of hard rock. Unfortunately, they didn't get much farther. Some
members of U.S. Congress thought digging to the mantle wasn't worth the cost, and in 1966, they canceled the
project.
Nearly a half-century later, scientists are hopeful that the U.S., Japan and other countries will pool their
resources to cover the cost. But the physical challenges of drilling to the mantle remain pretty daunting. Even if
scientists find the thinnest possible section of the crust on the ocean bottom, that still means drilling through at
least several miles of solid rock. To make things more difficult, as they drill deeper into the Earth, they'll
encounter extreme temperatures, possibly in excess of 1,000 degrees Fahrenheit (538 degrees Celsius), and
fantastic amounts of pressure — as much as 4 million pounds per square foot in the vicinity of the mantle. With
that crushing force squeezing the equipment, it's going to be a challenge to keep it running, let alone to push
material that's being excavated back up to the surface, so that scientists can get the samples they want.
On the plus side, though, in the past 50 years, thanks to deep-water drilling by the oil industry, drilling
technology has advanced significantly. We've got improved drill bits, tools and instruments that are much more
able to withstand heat and pressure. And thanks to GPS and other advances, it's much easier to keep a drilling ship
in exactly the same spot in deep water. Researchers also now know more about the ocean crust and how it is
formed, and about the differences between the crust and mantle, according to Damon Teagle of the National
Oceanography Center in Southampton, England, one of the leaders in the project. "We have a much better
understanding of what we are trying to do," he explained in a 2011 interview.
If the scientists don't encounter unforeseen snafus -- which is a big if, of course -- it could take them
between 18 months and two years to drill down to the mantle. They hope to start in 2013 or the following year and
complete the project before the end of the decade.
#4: What Will We Learn From Digging Down to the Mantle?
Hopefully, a lot. As we explained previously, knowledge about the Earth's mantle is pretty limited, because
we can't go there, and we've never had a pure sample of it. Instead, scientists have tried to figure it out by
studying seismic waves and examining the molten rock that flows out of volcanoes. They've also tried to glean clues
about the mantle's composition by studying meteorites, which are forged from the same space debris as our planet.
But all those sources leave a lot of questions unanswered. If scientists eventually get some of the mantle to
study, they stand to gain some new insights about how the Earth was formed billions of years ago, how it developed
into the core, mantle and crust, and how plate tectonics began. If they can learn more about the precise mix of
chemicals and isotopes in the mantle, they can get a better sense of how the mantle transfers chemicals to the
surface.
More important, they may learn exactly how movement of the mantle's fluid rock affects the Earth's
crust, in particular how the tectonic plates push and pull against one another. Knowing more about the mantle and
how it interacts with the crust might someday even help us to predict events such as earthquakes and volcanic
eruptions.
But one of the most tantalizing possibilities is that scientists might actually find life deep inside the Earth.
We're not talking about the monsters that Jules Verne imagined in "Journey to the Center of the Earth," but
rather tiny, primitive organisms called extremophiles, which have evolved to resist extreme pressures and high
temperatures. Scientists already have found such organisms in the deepest ocean floor. If they're able to exist
even deeper in the Earth, scientists speculate that such organisms might contain unique enzymes or other
characteristics that researchers could put to use in developing biotechnology. Even more importantly, they might
help us to understand the physiological limits of life.
Name: ________________________________________ Date: ________ Period: _____
Digging to the Mantle Jigsaw Activity:
Read your selected section and fill in the designated information box. Make sure you are thorough as
you will share this information with a group and must give them all the required information.
#1: Explain what scientists are trying to do? What is #2: What is the mantle? How do we know it exists?
the plan?
What do we know about it?
#3: What happened when we tried to dig before?
What are the difficulties scientists will face? What
new knowledge do we have today that will be
beneficial?
#4: Describe at least four pieces of information we
could learn about if we are able to obtain samples.
Analysis:
After reading the article and discussing it with your group do you think it is important to study our Earth?
Think of some reasons why or expand on the reasons given in the article and explain the importance of
knowing that information. Write your answer on the back of this paper.