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?
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.