The Mohorovicic discontinuity is a strange region, and could hardly be more important to us
The paradox of planet Earth is that almost all of it is unknown. The European Space Agency has just underlined this with the most accurate gravimetric satellite map so far of a universal feature called the Mohorovicic discontinuity. This strange region – the Moho for short – is a kind of basement floor, 70km or so below the Himalayas, a trifling 10km or so below the ocean floor. It marks the boundary between the brittle crust of the continents and seabed, and the dense and very different mantle that makes up 68% of the planet. It could hardly be more important to us.
Nobody knew of its existence until 1909, when a Croatian seismologist called Andrija Mohorovicic noticed that earthquake waves accelerated as they reached a certain depth. It was the first evidence that the Earth had a crust, above a mantle that extended for nearly 3,000km to the planet's metal core. The mantle is an agency in the steady making of the living world. Through submarine volcanic vents along the mid-ocean ridge, it delivers fresh basalt to resurface the planet's oceans every 200m years, and to drive the moving pavements on which the continents ride at a few centimetres a year, occasionally colliding to throw up features such as the Alps and the Tibetan plateau. These same forces built the Andes and the Rockies, and power the volcanoes that yearly discharge massive quantities of new water, gas and minerals to the biosphere. Earthquakes, too, are a reminder that the mantle is active, and determined to go on pushing us around. The great seams of concentrated mineral wealth – from the copper, tin, silver and gold that enriched the first civilisations to the rare earths and fissile elements that power new technologies – are ancient casual side-effects of the same process.
The Moho is so important that, 50 years ago, geologists backed an international effort to drill down to it, taking with them the novelist and amateur oceanographer John Steinbeck. The Mohole was abandoned as seemingly impossible: no ocean drill had ever got below 2km. The initiative has now been revived by European oceanographers who want to probe the mantle where the ocean crust is thinnest, should anyone deliver the finance, the political will and the technology.
Even in 1961, the idea was not new: Arthur Conan Doyle sent his Professor Challenger drilling to the mantle in 1928, in a short story called When the World Screamed. Challenger discovered that, beneath its scaly mineral carapace, the planet was a living thing, which reacted violently to the insult. Doyle's fantasy was prescient: the world is dynamic, and forever renewing its skin, and the creatures on that surface are alive ultimately because the mantle is febrile and active. There certainly is a case for going deeper.