The mystery of too-deep earthquakes

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

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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

Another way to cook rocks: in water!

Remember the blogpost where experiments were compared to cooking? Well, we can take that parallel even further and use more water to heat the rock with! As I mentioned in a previous post, rocks at the seafloor can get altered when fluids move through them. We call these hydrothermal systems, which can have spectacular venting sites on the seafloor. One way to find out more about the process behind these systems is studying natural examples that underwent hydrothermal fluid circulation; another way is by trying to reproduce these processes in the lab. But how can we do this when water is involved? Continue reading

An adventure on the JOIDES Resolution: One year later

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Fig. 1 – (Top) One of the many magnificent sunrises observed by scientists on board the JR; (Bottom) View of the derrick, tower that holds the drill string, from the Bridge Deck, and (Top-left) all Expedition 360 participants. Images credits: William Crawford, Exp. 360 Senior Imaging Specialist; Jiansong Zhang, Exp. 360 Education/Outreach Officer.

Earlier this year Barbara wrote about ‘Life on board of a scientific drilling vessel’. That interview gave some hints in the unique experience my colleagues and I shared on board the Joides Resolution. Now, you might wonder what Joides Resolution (JR) exactly is. The JR is a drilling vessel dedicated to scientific research on ocean and ocean crust dynamics. Different disciplines are involved, from geology (to elucidate the formation of the oceanic crust), to climate change science (to understand how the Earth handled past climatic events), oceanography (to study global water circulation), or microbiology (to track extreme life in rocks forming the ocean floor).Cores of rocks are drilled under the ocean floor, giving scientists a glimpse into Earth’s dynamics. The JR works for the international research program IODP (International Ocean Discovery Program), a marine research collaboration that aims at recovering data recorded in seafloor sediments and rocks, and monitoring subseafloor environments. Continue reading

Rocks never forget!

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Figure1. Elephant rock, Castelsardo, Sardinia, Italy (picture by Vid Pogačnik)

Did you know that some rocks can have an incredible “magnetic memory”? The age of rocks can vary from seconds to billions of years, and besides their sometimes very old age they store information that is useful to reconstruct the history of our Planet.

We commonly use the word “memory” referring to our computer storage capacity or our own ability to remember. Rocks store information, but unlike us they are able to do it over longer periods of time. The oldest memory we have is limited to what humankind experienced but some rocks are much older than humans. Therefore it is really important to be able to extract their memories in order to better understand what we didn’t experience ourselves.  

This “magnetic memory” relates to certain minerals in rocks (e.g. magnetite, hematite) able to record the direction and the intensity of the Earth’s magnetic field when they form.

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Surprise: Catching bugs in rocks!

X-ray tomography is a powerful technique that allows us to see very tiny details inside a rock. However, the image acquisition is usually just a starting point for the image analysis. In order to get quantifiable information, one has to develop specific image processing algorithms. In the porous medium research, one of the most important processing step is the development of the task oriented image segmentation algorithm.

While trying our segmentation algorithm on a 3D image of a sedimentary rock, we found some curious piece of a former life! The “worm” you can see in the video is an orthoceras — an ancient mollusk that is often found in sediments.

This carbonate rock has been cored in the Miocene carbonate platform of Llucmajor, in Majorca. The rock has suffered a re-equilibration from aragonite to calcite (dissolution of aragonite and crystallization of calcite). This reaction led to the formation of porosity (grey parts of the picture). In this case the spatial distribution of the pores has been controlled  by the pre-existing structure of the rock. This process allowed the preservation of the shape of the fossil, even after re-equilibration and recrystallization into calcite. That is why we can see the orthoceras, although its skeleton has undergone chemical alteration.

Figure 1. This video is a series of 2D slices of a 3D volume. No orthoceras is actually swimming here

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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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Rocks that pop!

  • Discovery

In 1972, the scientists onboard the French research vessel Jean Charcot, during the “Midland” cruise made an amazing discovery: Rocks that pop! From the seafloor in the Atlantic Ocean they retrieved some basaltic glassy pebbles that exploded noisily, much like firecrackers and jumped merrily to a height of up to one meter on the ship deck. A decade later, another geologic expedition aboard the RV Akademik Boris Petrov made the same surprising discovery from a complex region of the Mid-Atlantic Ridge that contains vast areas of lava flows (see previous post) as well as heavily faulted terrain with intact blocks of deep crust. These rare forms of lava rock are really interesting because of their spectacular behaviour but mostly because of their richness in gas and information they provide on the deep Earth.

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Figure 1: a) Photo of a popping rock. Volcanic glass in black and rounded vesicles. b) Photo of a thin section of popping rock (Sarda, 1990).

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