Quartz at the core of Earth's magnetic field paradox

For centuries, scientists have wondered why Earth has a magnetic field. It's still a mystery, but by using super-high pressures and temperatures to duplicate conditions at the Earth's core, scientists at the Earth-Life Science Institute at the Tokyo Institute of Technology have discovered that there may be quartz crystals there that help explain how the Earth gets the power to generate its field.

As any elementary geology textbook will show, the Earth's interior is a huge ball of molten iron 3,000 km (1,800 mi) down from the surface surrounded by hot plastic rock. The core and the mantle are extremely hot, with temperatures of 3,500 K (5,800° F, 3,200° C) and pressures of 40 to 60 gigapascals (390,000 to 590,000 atmospheres). Scientists believe that the dynamics of this molten core, and the conductive materials within it, generate the Earth's magnetic field, but exactly how is still a mystery.

One aspect of this mystery is what Peter Olson of Johns Hopkins University dubbed "the New Core Heat Paradox." Although it's been found that core rotates slightly faster than the Earth itself, it only circulates at a rate of a few centimeters per year. That might not seem like much, but in geological terms that's almost like the core is churning like a blender.

In 2013, Kei Hirose of the Tokyo Institute of Technology stated that his research had found that convection, with hot buoyant rocks carrying heat from the core to the surface, would have had to have cooled the Earth's core by 1,000° C (1,830° F) over the past 4.5 billion years in order to power the Earth's magnetic field. The problem is that this requires a solid core, but that core is only about a billion years old, while the magnetic field is much older, hence the paradox.

Another problem is that the composition of the core isn't certain. It's 10 percent less dense than pure iron, so there must be lighter elements, like silicon and oxygen, present. In addition, not much is known about what proportions these lighter elements are in and what sort of compounds they form under such extreme high pressure.

To learn more, the Tokyo Tech team placed dust-sized samples in a diamond anvil, where two diamonds are pressed together at the tips to create massive pressures. Meanwhile, lasers were focused on the sample to heat it to core temperatures. Similar experiments have been conducted in the past using two elements, but this time the scientists combined silicon, iron and oxygen in the anvil.

Surprisingly, the iron, silicon, oxygen mixture caused the silicon and oxygen to combine and crystallize to form quartz as it cools, as electron microscope analysis confirmed. This is important because the team says that if the quartz formed at the top of the core, the process would power the core convection to make the core act like a dynamo and power the magnetic field without the interior needing to cool dramatically – eliminating the paradox.

Another thing about the new mechanism is that it helps to explain details of the formation of the Earth and other bodies in the Solar System because, as crystallization continues, the silicon and oxygen will eventually run out and change the composition of the core.

"Even if you have silicon present, you can't make silicon dioxide crystals without also having some oxygen available," says ELSI scientist George Helffrich. "But this gives us clues about the original concentration of oxygen and silicon in the core, because only some silicon:oxygen ratios are compatible with this model."

The research was published in Nature.

Source: Tokyo Institute of Technology