In 1909 Croatian geophysicist Andrija Mohorovičić examined seismograms of a shallow earthquake that shook the area around Zagreb. To his surprise the by-then familiar time sequence of P-waves followed by the slower S-waves appeared twice on seismic records up to 800 km away. The only explanation that he could come up with was that the first arrivals had travelled directly through the crust to the detector whereas the second set must have followed a longer path: it had travelled downwards to be refracted to reach the surface when it met rocks denser than those at the surface. His analysis revealed a sharp boundary between the Earth’s crust and its mantle at a depth of about 54 km below what was then Yugoslavia. Later workers confirmed this discovery and honoured its discoverer by naming it the Mohorovičić Discontinuity. Difficulty with pronouncing his name resulted in a geological nickname: ‘the Moho’. It can be detected everywhere: at 20 to 90 km beneath the continental surface and 5 to 10 km beneath the ocean floor, thus distinguishing between continental and oceanic crust.
In the late 1950s accelerating geological and oceanographic research that would culminate in the theory of plate tectonics turned its focus on drilling down to the Moho in much the same way as a lust for space travel spawned getting to the Moon. The difference was that the proposers of what became known as the Mohole Project were members of what amounted to a geoscientific glee club (The American Miscellaneous Society), which included a member of the well-financed US National Science Foundation’s Earth Science Panel. The idea emerged shortly after the Soviet Union had launched the Sputnik satellite and rumours emerged that it was proposing deep drilling into the continental crust beneath the Kola Peninsula. The Mohole’s initial target was the 3.9 km deep floor of the Caribbean off Guadalupe in Mexico and required advanced methods of stabilisation for a new oceanographic ship that was to host the drilling rig.
The Mohole was spudded in 1961, but the deepest of five holes reached only 200 m beneath the sea floor. It recovered Miocene sediments and a few metres of basalt. Deep water drilling was somewhat more complicated than expected and about US$ 57 million was spent fruitlessly. The project was disbanded in 1966 with considerable acrimony and schadenfreude. Nonetheless, the Mohole fiasco made technical advances and did demonstrate the feasibility of offshore drilling. The petroleum industry benefitted and so did oceanography with the globe-spanning deep-sea drilling of ocean floor sediments. The sediment cores produced the 200 million-year exquisitely detailed record of climate change and vast amounts of geochemical data from the basaltic oceanic crust. In 2005 JOIDES (the Joint Oceanographic Institutions for Deep Earth Sampling) had another crack at the Moho. That venture centred on the intersection of the Mid-Atlantic Ridge and the Atlantis Fracture Zone close to the ‘Lost City’ hydrothermal vent. The area around the vent is the site of a huge low-angled extensional fault that has partly dragged the basaltic ocean crust off the mantle beneath causing it to bulge. This provided an excellent opportunity to drill through the Moho. All went well, but 54 days of drilling yielded 1.4 km of basalt but nothing resembling mantle rock. So, again, the Moho had thwarted Science (and research economics). But finally it is beginning to reveal it secrets (see: Voosen, P. 2023. Ocean drillers exhume a bounty of mantle rocks. Science, v. 380 (News) p. 876-877; DOI: 10.1126/science.adi9899
The area around the ‘Lost City’ vent was originally chosen for drilling to examine the chemical processes going on there. Hydrogen emitted by serpentinisation of mantle rocks can combine with carbon monoxide in hydrothermal fluids to create a wide variety of organic compounds, which could be the initial building blocks for the origin of life. As part of the International Ocean Drilling Programme JOIDES decided to launch IODP Expedition 399 to re-examine the area around ‘Lost City’ in more detail. The expedition first tried to continue drilling the 2005 hole, but failed yet again. Finally a new drill site aimed at penetrating the extensional detachment. Within a few days the drill bit punched into mantle rocks and over a 6-week period the expedition had recovered a kilometre of core. The technical accounts for each week of drilling give a flavour of what it must be like to be a part of such a ship-borne expedition as well as describing what emerged in the drill core. It seems like a bit of a jumble, dominated by the mineral olivine– the principal characteristic of the ultramafic mantle – almost pure in the rock dunite and mixed with pyroxenes in various kinds of peridotite. There are also coarse-grained rocks that contain plagioclase feldspar, which cut through the ultramafic materials – gabbros, troctolites and norites. They are relics of intrusive basaltic magmas that did not make it to the seabed. The samples are variably altered by interaction with watery hydrothermal fluids, with lots of serpentine, talc and even asbestos: the drilling presented a health hazard for a few days. The rocks have been metamorphosed under pressure-temperature conditions of greenschist to amphibolite facies and subject to ductile deformation, probably because of the effect of extensional deformation. Whatever, there is plenty of material to be analysed, including for signs of microbial activity. So, the dreams of a 1950s academic drinking fraternity (they were all men!) have finally been realised. But since those pre-plate-tectonic times many geologists have seen and collected much the same, even putting their index fingers on the Moho itself in the time-honoured fashion. Intricate 3-D geology in ophiolite complexes such as that in Oman, provide such opportunities at the much lower cost of air travel, Land Cruiser hire and camping. Indeed what we know of the structure of the oceanic lithosphere – pillow lavas, sheeted dyke complexes, gabbro cumulates and serpentinised ultramafic mantle – has come from such bodies thrust onto continental crust at ancient plate margins. So, why the celebration in this case? They are the first samples of mantle from young oceanic lithosphere; the rocks of ophiolites may not have formed at mid-ocean ridges. These should give clues to the long-term magmatism that has created the vast abyssal basins that the mantle eventually reabsorbs by subduction. Then, of course, there is the link to biogenic processes at constructive margins that underpinned the return to the active hydrothermal venting at ‘Lost City’.
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