In Geology 101, the interior of the Earth is divided into neat layers, like a sugar-coated puzzle. But it turns out that parts of the planet’s middle layer might look more like peanuts in a sea of caramel. Seismic data reveals that there may be chunks of oceanic crust stuck deep in the planet’s liquid mantle, creating large chunks in one of those smooth layers.
The authors of a new study have discovered these “peanut chunks” inside the gooey mantle under East Asia. Their findings, in addition to being deliciously intriguing, could have implications for models of oceanic crustal formation and movement.
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How did these pieces of oceanic crust get into this layer? The lithosphere is the rigid outer layer of the Earth, encompassing a fissured crust and a warm upper mantle. The warm mantle swirls and circulates, moving the crust to the surface, causing the oceanic crust to plunge into its depths – a process called subduction – and triggering the rise of vast plumes of magma to the Earth’s surface.
“Earth is energetic, manifested by the tectonic movement of the lithosphere and the underlying convection in the deep mantle, ”said Jikun Feng, lead author of the study and postdoctoral researcher at China University of Science and Technology.
But geologists know very little about the behavior of deeper regions of the mantle, despite its likely impact on mantle circulation.
The team wanted to create a more detailed picture of the mantle structure and composition and how it relates to mantle circulation, especially in the transition zone between the upper and lower mantle. Feng and his colleagues focused on an area under China, where the crust of northern China sits on a piece of Pacific oceanic crust that is buried deep in the mantle. This region of the Pacific tectonic plate is considered “stagnant” because it does not sink beyond the transition zone and instead appears to float within the mantle. They wanted to better understand what is happening in the transition zone inside the mantle and how the stagnant plaques might affect circulation.
Traditionally, seismologists have studied the structure of the mantle using seismic waves (waves that pass through the Earth) produced by large earthquake, said Feng. However, these earthquakes do not happen everywhere, all of the time. To work around this limitation, Feng’s team used an existing network of more than 200 seismometers to record ambient seismic noise, or small daily vibrations unrelated to specific tremors.
Seismic waves can reveal “the mantle’s deep circulation imprint,” Feng told Live Science. This is because seismic waves travel differently through materials of different densities and properties. And these properties can change or be modified by other phenomena, such as the descent of oceanic plates. The rising mantle plumes also disturb the interior of the Earth and lead to different seismic measurements.
In the new study, the researchers stacked the seismometer readings from these instruments to see how the seismic waves behaved in the mantle at the transition zone, where the upper and lower mantle meet. (The lower mantle is warmer, deeper, and under more pressure than the upper mantle.)
They found a marked discontinuity, or change in seismic wave velocity, in the mantle at a depth of 410 miles (660 kilometers), or at the bottom of the transition zone between the upper and lower mantle. Based on these waves, they concluded that part of the oceanic plate had “regrouped” at the base of this area and was preventing the Pacific plate from diving further. The team hypothesized that when the oceanic slab encounters denser rock at this depth, it ceases its descent into the mantle and instead spreads laterally into the transition mantle. The bonded slab then separates chemically into different mineral compositions. This chemical separation creates a “coarse” region of the mantle with a complex structure, which differs slightly from the rest of the mantle material, which is pyrolite (a rock that is about three parts peridotitis and part of basalt).
“Our results provide direct evidence of segregated oceanic crust trapped in the mantle transition zone,” said Feng.
The new work provides insight into mantle circulation, including how stagnant plaques might behave in the transition zone, Feng said. He noted that understanding the nature of mantle heterogeneities “can provide essential information on the mantle circulation process and ultimately on the evolution of our planet.”
Their findings were published on May 5 in the journal Nature Communication.
Originally posted on Live Science.