The mystery of too-deep earthquakes

Look at the depth distribution of earthquakes on Earth (Fig. 1):

Fig. 1: Depths of earthquakes on Earth. Shallow earthquakes (0-60 km) are in red, intermediate-depth earthquakes (60-300 km) in purple and deep earthquakes (>300 km) in blue. Data from the International Seismological Centre.

In general, earthquakes are located at the boundaries between tectonic plates. Shallow earthquakes (< 60 km) happen at all plate boundary types, but intermediate (60-300 km) and deep (> 300 km) earthquakes mainly occur in subduction zones, where one plate moves beneath another. Because these earthquakes are located either within the subducting plate or between the two plates, they get deeper and deeper the further they are from the surface trace of the plate boundary. Because the plate located west of South-America moves towards the east and is subducted under South-America (Fig. 2), the earthquakes on the west coast of South-America get deeper from west to east (Figs 1, 2). Continue reading

Breaking rocks: a closer look

When I described in my last post how rocks can be broken up by volume-increasing reactions happening within them, I left you with several open questions in the end. One of them was whether reaction-driven fracturing can also occur when there is no stress from the outside and no fracture to start with. It is easy enough to imagine that minerals that grow in a crack may push against the walls of the crack, move them apart and cause further fracturing. But for this first crack, with which everything starts, we certainly need some forces from the outside that make the rock break. Or do we really?

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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”.

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Breaking rocks

When rocks are broken, they are not necessarily useless. Quite the opposite – geologists are often happy when their rock samples show signs of breaking. This does not mean you should go into our offices and smash all the pieces of rock you see there (we will not be happy then), but if we can relate a crack to the natural process that caused it, we might be able to puzzle out one more piece of a rock’s history. Some geologists become so fascinated with broken rocks that they behave in a way their non-geologist friends deem worthy of a picture: KD1_fig_1 Continue reading