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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The abyssal crazy world

As part of the “Abyss” group, we might wonder what things look like down there, in the deep ocean. As you probably already experienced, diving in water comes with (uncomfortable) changes of temperature and pressure. And that’s only a few meters! The conditions keep changing going deeper in the water column (more than freezing toes!). In oceans, the abyssal waters represent the part lying between 2000 m and 6000 m under sea level. At these depths, the temperature is constant around 0-4°C, the pressure is up to 200-600 atmospheres, and there is no light. And light is not only useful to see around but it is also the energy for photosynthesis and hence life sustenance at the surface of the Earth. Yet, although very poorly known, these depths allow life to exist. And what comes out of discoveries is sometimes very interesting or unexpected!

Life has to adapt to these difficult conditions of low temperature, high pressure, absence of light and scarcity of nutrients. The result is not exactly what we are used to, evolution sometimes leads to cool physical and morphological features! Let’s have a look at some inhabitants of the abysses.

Beyond 100 m in the dark cold water, plants disappear, life in the deep sea is 100% animal, likely because photosynthesis is impossible. With disappearance of light at depth, numerous species evolved to be blind or, conversely, grew big, globular eyes in the attempt to catch any remaining light like our very cute friend in Figure 1.

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Figure 1: The extreme growth of the eyes allows the capture of every bit of light! http://www.astronoo.com/en/articles/bioluminescence.html

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