Petroleum exploration geoscientists high in the Romanche Alp of Switzerland. The green rocks formed from iron and magnesium-rich magma and are representative of what the upper mantle (the lower part of the tectonic plates) is made of, several kilometres beneath the ocean floor. These rocks became exposed at the surface following a period of extreme stretching and thinning (‘hyper-extension’) of the earth’s crust deep beneath the Jurassic ocean that once existed between Europe and Africa. This caused cracking and fracturing of the crust allowing seawater to penetrate the mantle leading to a process known as serpentinization in which these mantle rocks were turned into beautiful serpentine minerals (as occur on the Lizard, Cornwall - see photo below). Finally, the land masses each side of the Jurassic ocean collided, squeezing and uplifiting the oceanic material caught in between to form the Alpine-Himalayan mountain belt. Serpentinization is a precursor to continental splitting, the birth of a new ocean. The seismic profile below from the hyper-extended Iberia ocean margin shows low-angle normal faults emplacing rift sediments against mantle rocks in similar configurations to the examples shown here from the Alps. Oil and gas companies are interested in understanding these processes because some of the largest remaining oil reserves are located in deep water where the crust has been similarly stretched and thinned e.g. offshore areas of Brazil, Suriname, Ghana, Newfoundland, Gulf of Mexico.

The serpentinization process is an extraordinary phenomenon - it weakens the lithosphere thus serving as a nucleus for continental splitting, it involves carbonation reactions that have the potential to sequester globally significant volumes of CO2, it is a non-biogenic source of methane on Earth and potentially on other planets, it is strongly exothermic thus providing a source for the hot (~260degC) acidic (pH~2) water at oceanic vents, and finally serpentinization is implicated in the origin of life. In her book The Solid Earth Mary Fowler estimates every 10 Million years an amount of water equivalent to the entire volume of the ocean basins circulates through the hydrothermal cells that drive serpentinization.

You can find out more about the geologic settings in which energy companies explored for oil & gas at the feather edge of the continental crust in this slide pack produced in 2013 for an internal workshop when I worked for British Gas. All the data shown are in the public domain.