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Why do some sinking plates get stuck halfway down?
Earth ScienceStandardAbout 3 min
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Standing on the ground, it's hard to imagine that hundreds of kilometers below, huge rock plates are slowly moving. Scientists have found that some plates, after subducting into the mantle transition zone (about 410–660 km deep), can stall there for tens of millions of years, like a bottle getting stuck at the neck. Why don't they sink further? What secret lies behind this?
To answer this, scientists needed to "see through" the Earth. They used high-resolution seismic imaging to analyze how seismic waves change speed as they pass through the mantle, mapping its fine structure. At the same time, they ran rock physics numerical simulations in the lab to test rock properties under different conditions. Using these two methods, the team focused on a seismic discontinuity about 700 km below the mantle transition zone beneath North China.
This discontinuity shows features consistent with water-free basaltic mélange in the central region (about 400 km across), but the surrounding area shows an anomalous downward deflection of about 10 km. Scientists explain this as caused by water-induced delay of the post-spinel phase transition. This delay effect can contribute 97.5% of the buoyancy needed for plate stagnation, while under water-free conditions it contributes only 27.5%. In other words, water makes the plate "lighter" and easier to get stuck.
This study provides the first systematic explanation for why oceanic plates can stay in the mantle transition zone for long periods, highlighting water as a key controlling factor. This not only helps us understand the dynamics of plate subduction but also has important implications for mantle material cycling.
Currently, this research is mainly based on seismic imaging and numerical simulations, lacking direct observational evidence. Moreover, the study area is limited to the mantle transition zone beneath North China, so it may not apply to other regions globally. Scientists estimate that more observational data from other areas are needed to verify whether this mechanism is universal.
To understand this article, you need two concepts: 1. Seismic discontinuity: When seismic waves pass through the Earth, they encounter boundaries where composition or structure changes abruptly, causing a sudden change in wave speed. This boundary is called a discontinuity. By analyzing seismic waves, scientists can infer rock properties deep underground, similar to using sonar to map the seafloor. 2. Phase transition: Under high temperature and pressure, minerals can change their crystal structure, known as a phase transition. For example, olivine transforms into a denser spinel structure under high pressure. This change affects rock density and buoyancy, influencing plate motion.
Note: 97.5% is the relative contribution of water-induced delay to buoyancy, not that water provides all the buoyancy; other factors also contribute a small part.
Did you know? The mantle transition zone is about one-tenth of Earth's radius deep, where pressure can make rocks harder than steel!
This article was researched and written from the following materials:
Primary research: Nature CommunicationsOpen link
News report: IT之家31 August 2026Read the original
AI editorial note: this article was rewritten by AI from the materials above and fact-checked. It is not a reproduction of the source article.
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