In Geology 101, the Earth’s interior is divided into neat layers, like a sugar-coated jaw crusher. But it turns out that parts of the planet’s middle layer may look more like peanuts in a caramel sea. Seismic data reveals that there may be hunks of oceanic crust stuck deep inside the planet’s floating mantle, creating large lumps in one of the slippery layers.
The authors of a new study discovered these “peanut snuff” inside the sticky mantle of East Asia. Their findings could, in addition to being deliciously exciting, have implications for models of how the sea crust is formed and moves.
How did these lumps of oceanic crust get into that layer? The lithosphere is the Earth’s rigid outer layer that includes a cracked crust and a warm upper mantle. The hot mantle curls and circulates and moves the crust on the surface, causing the oceanic crust to dive into the depths – a process called subduction – and trigger the swelling of large magma plumes against the earth’s surface.
“the earth is energetic, manifested by the tectonic motion of the lithosphere and the underlying convection in the deep mantle, ”said Jikun Feng, lead author of the study and a postdoctoral researcher at the University of Science and Technology of China.
But geologists know very little about how the deeper regions of the mantle behave despite its likely impact on the mantle’s circulation.
The team wanted to create a more detailed picture of the mantle’s structure and composition and how it relates to the mantle’s circulation, especially in the transition zone between the upper and lower mantle. Feng and colleagues focused on an area beneath China where the North Chinese crust sits on top of a piece of oceanic Pacific crust buried deep inside the mantle. This region of the Pacific tectonic plate is considered “stationary” because it does not sink past 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 stationary plates can affect circulation.
Traditionally, seismologists studied the structure of the mantle using seismic waves (waves traveling through the earth) produced by large earthquakeFeng said. However, these earthquakes do not happen everywhere all the time. To circumvent this limitation, Feng’s team used an existing series of more than 200 seismometers to detect ambient seismic noise or small, everyday vibrations that were not bound to specific emotions.
Seismic waves can reveal “footprints of deep mantle circulation,” Feng told WordsSideKick.com. This is because seismic waves move differently through materials with different densities and properties. And these properties may change or be altered by other phenomena, such as the descent of oceanic plates. Rising mantle feathers also disturb the interior of the earth and result in various seismic measurements.
In the new study, the researchers stacked the seismometer readings from these instruments to see how seismic waves behaved in the mantle at the transition zone where the upper and lower mantles meet. (The lower mantle is warmer, deeper, and under more pressure than the upper mantle.)
They found a sharp discontinuity or change in the velocity of seismic waves inside the mantle at a depth of 610 kilometers or the bottom of the transition zone between the upper and lower mantle. Based on these waves, they concluded that some of the oceanic plate was “assembled” at the bottom of this zone and prevented the Pacific plate from diving further. The team assumed that when the oceanic plate meets closer rock at that depth, it ceases to descend into the mantle and instead spreads laterally within the transition mantle. The attached plate then differs chemically in different mineral compositions. This chemical separation creates a “clumsy” region of the mantle with a complex structure that differs slightly from the rest of the mantle material, which is pyrolite (a rock that is about three parts peridotite and some basalt).
“Our findings provide direct evidence of separated oceanic crust trapped in the mantle transition zone,” Feng said.
The new work provides insight into sheath circulation, including how stagnant plates can behave within the transition zone, Feng said. He noted that understanding the heterogeneities of the mantle “can provide critical insight into the mantle’s circulation process and ultimately the evolution of our planet.”
Their findings were published May 5 in the journal Nature communication.
Originally published on WordsSideKick.com.