How long do the earth's minerals last

geology : Journey to the Center of the Earth

The earth is flat. One would inevitably come to this conclusion if one were to draw up a map of knowledge about our planet. The surface with mountains, jungles, deserts and glaciers has been explored by numerous expeditions. In the third dimension, however, the scientists have barely penetrated. With a depth of around twelve kilometers, the world's deepest borehole on the Russian Kola peninsula has just tapped one five hundredth of the Earth's radius.

Thanks to seismic waves, the researchers at least know how things will proceed. Our planet consists of several shells: under the thin earth's crust follows the largely solid earth's mantle, and underneath the outer core, which is liquid. At the very bottom, in the center, there is a solid core.

It is difficult to find out what it looks like on the individual floors. A personal visit, as Jules Verne devised it in his adventure novel, is out of the question because the pressure and temperature increase with every kilometer. Apart from the fact that there are no cavities in the depths - no “earth ship” could withstand these hellish conditions.

Nevertheless, geoscientists can at least travel deeply in their thoughts: With the help of heatable high-pressure apparatus, they simulate the conditions in the deep earth and try to find out what is going on down there using material samples. One of these deep pioneers is Wilhelm Heinrich from the German Research Center for Geosciences (GFZ) in Potsdam. In his laboratory there are various devices with which he can send individual minerals to the core of the earth.

In front on the left, the blue monster, is the elevator for the first stretch. A three meter high punch press that can build up a pressure of 2000 tons. “We can get far into the upper mantle,” says Heinrich. At the push of a button, the scientists' samples are sunk deeper and deeper into the earth, as is the case, for example, with an oceanic plate that is submerged under a continent in a subduction zone: off Chile or Japan or Sumatra.

“With experiments like this, we want to find out how the rocks change in the process,” explains the geochemist. Researchers have long known that the ocean floor releases a lot of water on its way down. Initially from the wet sediment of the former seabed, but also “crystal water” - hydrogen-oxygen compounds that chase various minerals out of their atomic lattice when they get too much pressure and temperature from the outside. The water rises up to the hot underside of the continental plate and accelerates the rock melt there. Magma arises and rises to the volcanoes of the Andes or to Fujiyama.

So far, so clear, says Heinrich. “But how quickly is the water expelled from the submerged plate?” This is an exciting question for geoscientists, because the water content largely determines how easily rocks can be deformed. Computer models, in which tectonic plates crawl across the surface of the earth, submerge themselves and wedge themselves into one another, rely on such information. The “elevator ride” in the Potsdam laboratory not only helps mineralogists with their sometimes somewhat abstract questions, but also seismologists who research earthquake activity in order to ultimately improve the risk assessment for people on the surface.

But how do the scientists succeed, for example, in researching the water release of the rocks in the depths? The minerals - often the tiny puzzle pieces of every rock, which have a certain composition of chemical elements - can contain different amounts of water. “The specific value depends on the prevailing pressure and temperature,” explains the GFZ scientist. "So we take the most common minerals and investigate what properties they have on different levels of the earth and how they react with one another there."

The problem with the large punch press: the researchers cannot observe their sample directly. It is located in a small cage made of gold or platinum, which is enclosed by eight hard metal cubes, on which the dies of the press are finally attached. “Pressure is force per area,” says Heinrich. “If we want to generate high pressure, the area has to be small.” The entire power of the huge stamp is therefore directed onto the gold cage, which is only two to three millimeters in size. It can also be heated from the outside.

For example, if Heinrich wants to know how a muscovite, which is a common mica mineral, fare within a swallowed plate 100 kilometers below the surface, he programs the press to a pressure of 30 kilobars and 600 degrees. This state is kept stable for one to two days so that a chemical equilibrium is established, i.e. the muscovite changes its chemical composition, sometimes forms new minerals and releases so much water that it is optimally adapted to the 100-kilometer conditions is. “At the end of the experiment, we turn off the heating and frighten off the sample,” says the researcher, and taps a finger-thick water hose that leads next to a few cables into the space between the stamps. “If the sample is cooled down slowly, it can happen that the minerals react again and partially absorb the water again.” In the case of sudden cooling, on the other hand, the high-pressure minerals have no time for this reconversion;

The higher the pressure and temperature, the more frequently the researchers observe an adaptation in which the chemical composition is hardly changed. Instead, the atoms simply move closer together and arrange themselves in a different crystal lattice. A well-known example is the carbon mineral graphite, which then becomes diamond.

In the lower mantle, however, two other minerals dominate the scene. According to geoscientists, it is about 80 percent silicate perovskite - a mineral that consists primarily of magnesium, silicon and oxygen - and a good ten percent ferropericlass - a magnesium-iron oxide. Never heard? No wonder, silicate perovskite has never been found on the earth's surface. As soon as it is brought up with one of the great currents in the earth's mantle, the pressure drops and the mineral converts back into compounds that are adapted to the conditions of the shallower layers. In order to find out something about the properties of these exotic substances, scientists first have to produce them in their laboratories before they can be examined.

On the imaginary journey into the depths, the Potsdam researchers come down about 600 kilometers with their blue stamp press. It goes a little deeper with the high pressure press at the Bayerisches Geoinstitut (BGI) in Bayreuth. It creates pressure conditions like those at a depth of 700 kilometers. The scientists don't care about this new top brand. What makes them special is that the press can press differently in all three spatial directions in order to deform specimens in a targeted manner. “Deformation tests under these extreme conditions are unique in the world,” says research director Hans Keppler. He and his team are slowly approaching their performance limits. "Generating high pressure and high temperatures is not that difficult," says Keppler. “The problem is precisely setting and measuring these conditions. Otherwise we will get such big errors that we can save ourselves the experiments. "

That is why the scientists are initially working primarily “in the middle mantle”. There, too, they made an astonishing discovery: Apparently, the high-pressure variant of the silicate mineral olivine, which forms at a depth of 400 kilometers and is called wadsleyite, is somewhat softer than olivine itself. The consequences of this finding are difficult to understand, says Keppler. "It would mean, among other things, that this transition zone has different physical properties than previously thought, which ultimately also leads to different results when modeling the hot currents in the earth's interior."

If the researchers want to go even further, one might think that their apparatuses will have to get even bigger. Paradoxically, however, their “elevator” is getting smaller. The metal construction with a tiny hole in the middle is just the size of a fist. The light from Heinrich's laboratory lamp shines through there, straight through two diamonds that are pressed together. “In there,” says the researcher, pointing to the light dot, “there is pressure like at the boundary between the earth's mantle and core.” If you could put your hand in this layer of earth 2900 kilometers deep, you would put a load of 1000 tons on your fingertip act.

Hauke ​​Marquardt is one of the scientists at the GFZ who are doing experiments with it. He analyzes Ferroperiklas, which, by the way, was produced in the "Mineral Manufactory" at the BGI. With the help of a microscope and a thin needle tip, the researcher first maneuvers a grain of ferropericlass into the sample chamber, then he places the holder with the second diamond over it and threads four ordinary Allen screws into the threaded holes. “The diamonds have to be precisely aligned, otherwise they can break,” he says. Then Marquardt tightens the screws. “It's relatively easy,” he says. "In any case, you don't get the impression of creating pressure conditions like those in the deep mantle of the earth."

In contrast to the experiments with the large stamps, the sample remains visible through the diamonds. The scientists use this window for their analyzes. Using a synchrotron source that emits high-energy X-rays, it is possible to find out, for example, where the atoms are located and what distances they are in the crystal lattice. Several teams are currently working on a method to simultaneously bring the tiny samples to temperatures of well over 1000 degrees using a laser in order to simulate the conditions of the earth's mantle even more realistically.

In his experiments, Marquardt found that ferropericlass crystals, when they are under high pressure, transmit sound waves more quickly in certain directions than in others. The reason for this is a structural change in the iron atoms. Individual electrons move a little closer to the nucleus. These tiny changes can have global effects: at the lower limit of the Earth's mantle at a depth of around 2,800 kilometers, geophysicists have discovered a zone that seismic waves traverse faster than other layers - depending on the direction of wave propagation. This mysterious layer is relatively thin and thus surpasses geophysical madness in higher floors of the earth many times over. Nobody can yet fully explain the phenomenon. "It may be due to a preferred orientation of the ferropericlass crystals in the depth," says Marquardt. "But that's still very speculative."

In general, the knowledge becomes thinner the deeper one penetrates to the center of the earth. As far as the Earth's core is concerned, which begins at a depth of 2,900 kilometers, there is little experimental data so far. They come from experiments with very simple chemical mixtures that the scientists are gradually approaching. Otherwise they only have models of planetary development, interpretations of seismic data and speculations. Not even the temperature is known. Presumably it increases from four thousand degrees on the outside of the core to up to six thousand degrees in the center. It consists of a huge ball of iron and nickel. "That is why a liquid layer closes that contains the same elements and probably also some oxygen and silicon," says BGI researcher Hans Keppler. The hot melt forms large currents. They start at the edge of the hot core, lead outwards, where the melt cools down, and then flow back inwards - similar to a giant lava lamp. Even if this scorching hot zone is inaccessible, it is still an important area of ​​work for geoscientists. The earth's magnetic field, for example, has its origin down there. It is produced by iron that is contained in the hot currents and is moved with them. “The cycle is very stable,” says Keppler. "Apart from brief phases of polarity reversal, the magnetic field has reliably protected us from being hit by high-energy particles from space for billions of years."

There is still little information available about the way in which the innermost part of the earth is connected to our life up here. With the “elevators” in their laboratories, geoscientists will find a few more.

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