Researchers have unraveled the mystery of two huge blobs in Earth’s mantle, and it turns out they’re quite different. These areas, known as large low velocity provinces (LLVPs), lie beneath the Pacific Ocean and Africa. Rather than being ancient remnants from Earth’s early days, these blobs are likely chunks of crust that sank into the mantle over billions of years.
LLVPs are intriguing because seismic waves—caused by earthquakes—travel slower in these regions, 1% to 3% slower than in other areas of the mantle. This unusual property might impact how heat flows from Earth’s core and could influence the planet’s magnetic field.
Scientists have debated for a long time about the nature of these blobs. Some theories proposed that they might be leftover materials from the planet’s formation, while others suggested they could be remnants of a large impact event that contributed to the moon’s formation.
However, a new study led by geodynamicist James Panton from Cardiff University focuses on the idea that these blobs are actually pieces of oceanic crust. When tectonic plates collide, one can slide under another in a process called subduction, pushing crust down into the mantle. Panton’s research used computer modeling to track how this subducted crust might contribute to the formation of LLVPs.
According to Panton, their findings indicate that the oceanic crust can create LLVPs by itself, without the need for ancient materials at the bottom of the mantle. He mentioned, “They are evolving by themselves, simply through the process of subduction.” However, some ancient material might still exist, contributing to the blobs’ characteristics.
This understanding of LLVPs sheds light on their age. If they began forming with the advent of subduction, this could mean they started appearing shortly after tectonic activity became established on Earth, possibly over a billion years ago. However, the exact timeline of when subduction began is still debated among scientists.
The study also highlights differences between the two LLVPs. The one beneath the Pacific receives more crustal material because of active subduction zones in the Pacific Ring of Fire. In contrast, the African LLVP is older and more mixed with surrounding crust. It has less dense volcanic rock called basalt, allowing it to extend higher in the mantle than its Pacific counterpart.
Looking ahead, one interesting area of research will be the role of hot mantle plumes in driving subduction. These plumes reach from the deepest parts of the mantle to volcanic hotspots on the surface, such as the Hawaiian Islands, and may play a crucial role in the ongoing evolution of LLVPs.
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