Scientists probe ‘mystery of the Moho’

Prof Chris MacLeod leads team on a cruise to the Indian Ocean to drill into the Earth’s interior

An international team of scientists, co-led by Professor Chris MacLeod from the School of Earth and Envrionmental Sciences, will set sail for a remote location in the Indian Ocean as part of an expedition to test a 100-year-old theory that currently underpins our understanding of the Earth’s structure.

Professor MacLeod and his team, from the International Ocean Discovery Program (IODP), will set sail aboard the JOIDES Resolution research vessel and drill through the Earth’s crust at a target site on the South West Indian Ridge submarine mountain chain.

They will be looking for evidence that will overturn currently accepted theories describing the nature of the Earth’s outermost layers, and could have profound implications for our understanding of the Earth’s crust and the extent of life on the planet.

The Earth’s interior is layered, like an onion, with four distinct sections: the crust; the mantle; the inner core; and the outer core. Between the outermost layer – the crust – and the mantle, a boundary exists, called the Moho.

The Moho is named after the Croatian seismologist Andrija Mohorovičić who first noticed in 1909 that seismic waves from earthquakes changed speed about 50 kilometres below the planet's surface. He hypothesised that this sudden change must mark the boundary between a crustal layer above and the mantle below, an idea that has become widely accepted.

Yet Professor MacLeod and his team question these assumptions. They believe that the Moho in the oceans does not always mark the crust-mantle boundary. They propose that the seismological boundary may instead represent the lowest point at which seawater has seeped into the mantle through cracks, transforming the mantle rock, known as peridotite, into a different type of rock called serpentinite, through a process known as serpentinisation.

Professor MacLeod said: “For decades we have accepted without question that the Moho represents the boundary between the igneous ocean crust and the mantle, yet it is equally plausible that it instead marks a serpentinisation front. The seismic characteristics of the crust and serpentinite are essentially the same, so seismology alone cannot distinguish them.

“If the lower ocean crust may instead be formed of serpentinite, the implications are profound – we will need to rip up the textbooks. We suspect the igneous ocean crust is much more variable in thickness and structure than we previously thought, and in places we think it is completely absent.

“If so it completely changes our estimates of the elemental composition of the Earth’s crustal layer. Two-thirds of the surface of the planet is paved by ocean crust and a significant – but unknown – proportion could be made up of this serpentinite seafloor.

The research team plan to drill a very deep hole into the crust at a site on the South West Indian Ridge in the Indian Ocean, known as ‘Atlantis Bank’, where they believe conditions are optimum to drill a hole that can ultimately penetrate the Moho.

Using the latest high-tech drilling equipment during the two-month IODP Expedition 360, the team aim to drill at least 1.3 km below the seafloor to start to explore how the lower crust is generated, and how much seawater penetrates at depth.

They will return to the site subsequently to deepen the hole and achieve the ultimate goal of drilling a ‘Mohole’ right the way through the Moho and into the mantle.

At the end of January, Professor MacLeod will hand over the ship to Cardiff University’s Professor Ian Hall in Mauritius. Professor Hall, who is Head of the School of Earth and Environmental Sciences, will be the Co-Chief Scientist on a separate expedition studying the world’s largest surface ocean boundary current off the coast of South Africa and its effect on global climate.

The UK’s participation in the International Ocean Discovery Program is funded through the Natural Environment Research Council.