New research led by the University of Cambridge has found an unusual pocket of rock at the boundary layer with Earth’s core, some 3,000 km beneath the surface. The enigmatic area of rock, which is located almost directly beneath the Hawaiian Islands, is one of several ultra-low velocity zones, which are so-called because earthquake waves slow to a crawl as they pass through them.
The research ( Nature Communications) reveals the complex internal variability of one of these pockets in detail, shedding light on the landscape of Earth’s deep interior and the processes operating within it.
Earth’s interior is layered like an onion: at the centre sits the iron-nickel core, surrounded by a thick layer known as the mantle, and on top of that a thin outer shell — the crust we live on. Although the mantle is solid rock, it is hot enough to flow extremely slowly. These internal convection currents feed heat to the surface, driving the movement of tectonic plates and fuelling volcanic eruptions.
Scientists use seismic waves from earthquakes to see beneath Earth’s surface — the echoes and shadows of these waves revealing radar-like images of deep interior topography. But until recently, images of the structures at the core-mantle boundary have been grainy and difficult to interpret.
The researchers used the latest numerical modelling methods to reveal kilometre-scale structures at the core-mantle boundary. They observed a 40% reduction in the speed of seismic waves travelling at the base of the ultra-low velocity zone beneath Hawaii. According to the authors, this supports existing proposals that the zone contains much more iron than the surrounding rocks — meaning it is denser and more sluggish.