Breaking rocks from the inside

Did you ever forget a beverage bottle in the freezer? If not, you can be both glad and sad now: Glad, because you did not have to clean up the resulting mess (which can be quite substantial, especially if it was not just water that you forgot in the freezer). Sad, because you have missed a great opportunity to observe reaction-driven fracturing in your own kitchen. However, some people experienced it and even took pictures (figure 1):

Fig. 1: The volume increase during the phase transition from water to ice (from left to right) caused fracturing of a bottle of Julebrus, Norwegian “Christmas Soda”.

On the left, you see a nicely chilled bottle. On the right, a bottle in which the liquid became a bit too cold, froze, and broke the bottle apart. Why is the second bottle destroyed? The reason is very simple: When water turns into ice, it takes up more space, because the density of ice is lower than that of water. The growing ice pushes against the glass bottle, and finally breaks it. The reaction of water to ice causes the fracturing of the bottle. Therefore, we call this process reaction-driven fracturing. The same happens in rocks: If there is a small crack or hole in a rock, it can get filled with water. When this water freezes during winter, it pushes against the sides of the crack, and makes it even larger.

The change from water to ice is a phase transition. Water and ice consist of the same chemical elements (H2O), but these elements are arranged differently in the liquid and the solid state. Minerals can undergo phase transitions as well. Quartz and coesite, for example; both have the chemical composition SiO2, but spatially the Si and O atoms are arranged differently. At high pressure, in coesite, Si and O are closer together than at low pressure, in quartz, so that the same number of atoms takes up less space. Imagine a rock deep within the Earth, where the pressure is high. It contains SiO2 in the coesite form mixed with other minerals. When this rock now comes up to the surface (because it is part of a growing mountain chain, for example), what would happen? The Si and O atoms in the coesite would notice that they are not being squeezed together anymore, would move apart a bit, and the crystal would turn into quartz. It would undergo a phase transition from coesite to quartz. If the volume of the quartz crystal is higher than that of its precursor coesite, what does that mean for the other minerals surrounding it? The quartz would push against them. Therefore, just as the glass bottle with the frozen beverage, the surrounding minerals would break, and a lot of small cracks would form around the quartz crystal. For geologists like me, this is extremely helpful, because it allows us to reconstruct part of a rock’s history. If we see a quartz crystal with these kinds of cracks around it, we can assume that it used to be coesite one day, and that the rock was previously under high pressure.

Both the water to ice and the coesite to quartz transitions are reactions during which the composition of the material does not change: H2O stays H2O, and SiO2 stays SiO2. However, reaction-driven fracturing also works when the composition changes as well. A common process in “sea rocks” is the chemical reaction of olivine with water to serpentine, brucite and magnetite (called serpentinisation). During this reaction, the volume of the minerals can increase by up to 55%! The exact volume change depends on the composition of the olivine that is hydrated, but even an increase of, let’s say, 10% is quite significant. Therefore, when the new minerals grow, they push on their surroundings and make them break (figure 2), just as the quartz does.

Fig. 2: The volume increase during the serpentinisation of olivine caused microfracturing of the surrounding mineral. (Microphotograph: © 2015 Bjørn Jamtveit, University of Oslo)

Something I have not mentioned so far is how the water got to the olivine in the first place and how the serpentinisation reaction was initiated. The easiest way to transport water into a rock is through fractures. So we need to break the olivine rock from the outside (for example by tectonic movement) to get water into it. The reaction of olivine with water will then break the rock even more from the inside. This means that even more water comes in, even more reaction happens, and even more fractures form. Sounds simple enough, doesn’t it?

But (of course there is a “but”), what if there are no cracks to start with? Is reaction-driven fracturing possible without any stress from the outside? How does a crystal even push on something? What if the growing minerals simply fill the existing fractures, and thereby prevent water from coming into the rock?

Sounds a bit more complicated? It is. I will try to give parts of the answers in my next posts, but do not expect all of them! If everything was clear, I might not have a PhD position now to look for clues and propose explanations.

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