A mineral called garnet plays an unexpected role in forming one of Earth's most dramatic internal boundaries, according to research addressing a long-standing puzzle in geophysics.
The 660-kilometer seismic discontinuity marks a sharp boundary separating the mantle transition zone from the lower mantle. Seismic waves traveling through Earth behave differently across this boundary, creating a detectable signal that reveals its presence. Scientists have long known this boundary controls heat and material circulation through Earth's interior, driving plate tectonics, volcanic activity, and planetary evolution.
Researchers attributed the boundary primarily to the breakdown of ringwoodite, a high-pressure form of olivine that transforms into bridgmanite and ferropericlase at extreme depths and temperatures. Yet seismic observations beneath subduction zones and mantle plumes showed complex structures that the ringwoodite explanation alone could not adequately describe.
The new work reveals that garnet, another mineral abundant in the mantle transition zone, significantly contributes to the boundary's formation and properties. As garnet transforms into different crystal structures under the intense pressure and temperature conditions at 660 kilometers depth, it influences how seismic waves propagate and creates the layering patterns detected beneath subduction zones.
This finding resolves inconsistencies between laboratory measurements and real-world seismic data. Subduction zones, where tectonic plates collide and descend into Earth's interior, show particularly complex seismic patterns that garnet transformations help explain. Similarly, mantle plumes rising from deep within Earth display distinctive seismic signatures linked to garnet's behavior at the transition zone boundary.
Understanding the 660-km boundary matters because it controls how Earth's interior convects. The boundary affects how efficiently heat escapes from the core and how easily material circulates vertically through the mantle. This process influences surface phenomena ranging from volcanic hot