“Because we live at the surface, we don’t appreciate that the whole Earth acts as a big system,” says Dr. Elise Knittle from her laboratory in the Earth Sciences Department at the University of California, Santa Cruz. “For example, we live on these big plates that are moving around slowly. Earthquakes, landforms, and volcanoes are surface manifestations of this tectonic activity, and what drives it all is heat from the interior. One main source of this heat is the Earth’s core, which is almost as hot today as it was when the Earth was formed.”
Cross-sections of the Earth show layers as tidy as tree rings: core, mantle, and crust. However, early in the twentieth century, geophysicists realized that in fact the boundaries between these layers are dynamic and complex. Recent evidence shows that activity at the core-mantle boundary in particular has profound thermal and chemical implications for the way the Earth works.
In an article for the journal Science, Dr. Knittle and fellow geophysicist Raymond Jeanloz describe it as “a dynamic boundary between the rapidly convecting outer core and the slowly convecting mantle” which modulates temperature change in the deep Earth. Activity at the core-mantle boundary may also generate “hotspots”—plumes of hot mantle that rise to the surface to create volcanic island chains like the Hawaii-Emperor Seamont chain. Properties of the boundary also influence the Earth’s magnetic field and the paths taken when the magnetic poles reverse.
In order to understand these important functions, scientists need to know what the core-mantle boundary is made of. It’s a challenge: the layer lies almost 3,000 kilometers deep, halfway between the planet’s crust and its center. At the core’s edge, the temperature jumps some 1,000°C. The increase in density is greater than that between air and soil, and the pressures are tremendous—166 gigapascals, to be precise. A gigapascal is a unit of pressure (force per unit area) equaling ten to the ninth pascals, so this amount is “almost 1.4 million times the pressure at the surface,” explains Knittle. Understanding the chemical interactions between the metals that make up the core and the silicate minerals that make up the mantle is essential, and that’s Knittle’s area of expertise. In particular, she studies the chemical and mineralogical nature of the lowermost 200–300 kilometers of the mantle, a separate layer which geophysicists refer to as D” (D double prime). It was her groundbreaking experiment that demonstrated how a chemical reaction, similar to the interaction between iron in soil and oxygen in the atmosphere that gives clay its reddish tint, is taking place between the mantle and the core.
“My research specialty is sometimes called mineral physics, because we focus on measuring the physical properties of minerals,” says Knittle. “We make the link between mineralogy and geophysics. For example, a seismologist studies the speed at which waves move through Earth’s interior. One of the main goals in my field is to try to understand the composition of the materials that correspond to different wave speeds. We try and take this geophysical information and translate it into real rock types in the interior.” Seismic information is Knittle’s evidence. It is the way geophysicists observe what they cannot see or sample.
In her laboratory, Knittle conducts physical experiments that recreate and probe the conditions of the deep Earth. The intense heat and pressure are hard to imagine, let alone simulate. It’s extremely demanding, both technically and conceptually, especially since the limits of current technology make it possible to recreate those conditions only on a minute scale. Knittle’s samples are flecks of mineral that can weigh just a few billionths of a gram and be less than a tenth of a millimeter across. “If I’d known my hands were this steady, I could have been a brain surgeon,” she jokes.
“No one sets out to do this kind of work,” Knittle continues with a smile. “Originally I was an astronomy and physics major. I got some advice that this high-pressure experimentation was an interesting way to look at planetary interiors. I wanted to understand some of the new seismic information coming out about the core-mantle boundary, and I really wanted to work on something completely new. The chemistry of the core-mantle boundary was a total unknown when I started out; no experimental work had been done.”
The real drawing card for Knittle is that “it’s very creative. Instruments are just stuff sitting in your lab,” she points out. “I have to devise what experiments I can do to simulate this region of the Earth. What can I do technically, and what makes sense in terms of what I want to find out? Because the experiments are technically demanding—they don’t work most of the time—I want to come up with an experiment that will give me the most information the couple of times it actually works. So I devised these experiments in which I combined minerals to create these mini-core-mantle boundaries.” That’s only part of the job. “The second challenge is how to analyze the results,” explains the geophysicist. “I need to get chemical and structural information out of a sample so small it looks like a piece of pepper, and I need to figure out the best way to go about it.” Some experiments are harder than others, and “any time you get a difficult experiment, there’s an element of skill and one of chance,” she says. Knittle’s primary tool is called a diamond-anvil high-pressure cell. Small enough to be held in the palm of one hand, the device is essentially a high-tech nutcracker. At the center are two flawless, gem-cut diamonds with surfaces no wider than the heads of pins. They are ground to flat surfaces and mounted opposite each other on a hardened-steel mechanism. A fragment of material about a third of the size of the diamonds is placed between them, and force is multiplied by the mechanism by a factor of 500 to 1,000 when screws or a bolt are turned by hand. Because the area over which the force is concentrated is extremely small, the pressure—force divided by area—on the sample can be tremendous. And while it’s relatively easy to generate enormous pressures for a few millionths of a second with a shock wave experiment, the diamond-anvil cell solves another problem by sustaining the pressure long enough for its effects to be studied.
The fact that the diamonds are transparent enables scientists to observe changes in the color or consistency of the minute speck of rock under pressure. It also enables them to heat the sample through the diamond window. “We heat it with an infrared laser,” explains Knittle. To measure the pressure, “we put little tiny bits of ruby powder in our sample, and focus a different kind of laser, a blue one, onto these ruby chips. Under blue light, ruby fluoresces, and glows a bright red, which shifts with pressure. Then we measure the color of this fluorescence to obtain the pressure, measured in gigapascals.”
“Lab work can be very tedious and time-consuming,” Knittle points out. “Some new techniques don’t work the first fifteen times, yet I’m sure they can work if I just do one aspect of it better. So I have a great deal of patience.” She’s also endlessly curious. “In science you can often get the answer that you want, but it leads you to a new question. I almost never come to the end of a project and think that we’ve gotten all the answers. For example, I document chemical reactions in these incredibly small samples, and then in our imagination we scale them up to what might be happening at the core-mantle boundary. That means we’re going from a 100-micron sample—one-tenth of a millimeter—to something that’s thousands of kilometers across.”
Most of Knittle’s recent work is on subduction zones, trying to figure out what happens to the slabs of oceanic crust that descend into the mantle. “Where do they go?” she wonders. Because some of the new work on the core-mantle boundary suggests that its composition is mixed, Knittle maintains that “the next logical thing would be to recreate why there are areas of melt and areas that are solid. It could be a chemical reason. For example, maybe the regions of melt are the oceanic slab pieces. I’d love to work on that problem, but I’m not exactly sure how to do it.” She pauses. “But I’m thinking about it all the time.”