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).

  • Why do rocks pop?

How it is possible that a rock pops? Or more scientifically, what is the geological process at the origin of these spectacular jumps? Scientists interpreted it as the result of the presence of gas at high pressure in the rocks escaping under lower pressures, when the rock goes from the bottom of the ocean to the surface. Let’s compare these explosions with popcorn genesis and see how a corn pops!

A kernel of corn consists of 1) a very small amount of water and oil in the centre covered by 2) a layer of starch, all enclosed by 3) an outer envelope yellow, harder, rigid and waterproof shell that surrounds the grain. When you heat the corn on your stove, the water within the kernel boils and turns into steam, like that coming out of a tea-pot put on a hotplate. The steam is pressurised because the water vapour occupies more volume than liquid water but cannot cross the impermeable barrier formed by the shell! Therefore, because the hull’s volume remains constant, the pressure increases, like in a pressure cooker on a hotplate! Around 180 °C, the pressure of the gas water continues to increase and becomes too high. The outer shell is then subjected to high pressure (up to 9 atmospheres), and breaks suddenly, causing the explosion: Visually the grain bursts, jumps and you can hear a nice POP! This corresponds to a rapid expansion of the steam and a sudden drop in pressure inside the kernel. That’s the genesis of popcorn. As for the production of the soft and delicious white starch foam… this is another story!

Popping rocks lie on the seafloor under the pressure of some kilometers of water. When scientists collect those rocks using a dredge and bring them fastly (at the geological scale) to the surface, the pressure now sustained by the rock is much lower. However, the pressure in the vesicles becomes higher on the glassy wall. You can compare this phenomenon to hiking in the mountain with a packet of crisps. You can see that the packet of crisps blows up when you reach the top of the mountain where the atmospheric pressure decreases progressively. However, the glassy wall of vesicles cannot be deformed and when the internal pressure becomes high enough compared to the external one, the breaking point is reached and the explosion occurs!

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Figure 2: Sketch of blowing packet of crisp during mountain hiking. source: bretagnemontagne.wordpress.com
  • Why is there gas in that rock?

Fresh submarine basalt is a solidified volcanic rock formed by black glass embedding minerals such as olivine, pyroxene and plagioclase. The formation of this rock results from a volcanic eruption occurring on the seafloor, where an elevated water pressure prevails. An almost instantaneous cooling of lava from 1200 °C to near freezing occurs in contact with the cold seawater. You can observe some rounded vesicles in these rocks (Fig 1), single bubbles like those in sparkling water. The vesicles are filled with magmatic gases coming from the exsolution of gas dissolved in the lavas before the eruption.

Lava contains dissolved magmatic gas (H2O, CO2, H2S, noble gas, etc..). The exsolution of gas from the magmatic liquid is triggered by the decrease in lithostatic pressure (i.e., in the  shallow crust) during its rise from depth toward the surface. To understand that process, just think about a closed bottle of sparkling water, where the internal gas pressure reaches 5 times the atmospheric pressure. It is not possible to see a bubble until you release the pressure by opening the lid: PSCHHHT!!! The pressurized gas escapes to the outside, the pressure applied on the water decreases and the dissolved gas exsolves, forming individual bubbles. This process is also familiar to scuba divers as a potential cause of decompression accidents during underwater ascent: they have to rise to the surface (with lower water pressure) step by step to prevent gases dissolved in their blood and tissues by high pressure forming bubbles inside their body upon depressurisation.

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Figure 3: Photo before (a), during (b) and after (c) the opening of a sparkling water bottle showing the CO2 degassing after removing the bottle lid. (d) Photomicrograph of a vesiculated popping rock (Sarda and Graham, 1990) and (e) photo of a dredged pillow lava. source: insu.cnrs.fr

Therefore, the fast solidification when the lava reaches the seafloor allows trapping of the exsolved gases as bubbles behind the basaltic glass wall and prevents them from escaping into the ocean.

  • Why are these rocks interesting to scientists?

These rocks are really interesting because of their richness in gas compared to other basalts emitted on Earth. Popping rock have ~ 20 % of vesicles filled with CO2 (Fig. 3.d), while 1) the classic underwater basalt have few vesicles and low gas content (Fig. 3.e) and 2) the basalts emitted on land have 1 % of vesicles filled mainly with water. The low gas content of standard basalts (on land or submarine) suggests that the magma lost its volatile content during its ascent toward the surface (like the bottle of sparkling left without a lid, Fig 3.c), while on the higher gas content of the popping rocks seems to indicate a fast rise of magma towards the surface, without any storage in a magma chamber preventing the gas from escaping. Therefore, those rocks are the witnesses of the parental magma which has kept its primary volatile content, while other basalts partly lost this precious information.

Finally, popping rocks are really interesting because volcanic gases that are trapped in their bubbles did not escape during the volcanic processes. Because these lavas are the product of partial melting of deep rock, their gas content and their isotopic signature could give researchers more information about the inventory of gases within Earth, but also help them to better understand the history and formation of our planet, including the origin and history of the atmosphere and oceans and the functioning of the global carbon cycle (carbonation post).

 

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