Here's why it flows more slowly than rock in the atmosphere:
* Extreme Pressure: The D″ layer is at the very bottom of the mantle, directly above the Earth's core. The immense pressure at this depth is incredibly high, effectively "squeezing" the rock. This makes the rock extremely dense and resistant to flow.
* High Viscosity: The D″ layer is composed of partially molten rock, a mixture of solid and liquid components. This molten rock, under immense pressure, behaves with a very high viscosity, making it extremely resistant to flow.
While the D″ layer flows, it does so on a vastly different timescale compared to the atmosphere. While air in the atmosphere can move and change rapidly, the D″ layer flows on a geological timescale, meaning it takes millions of years for significant changes to occur.
Think of it like this: Imagine trying to push a thick, sticky honey through a narrow pipe. The honey (representing the D″ layer) will move, but very slowly due to its high viscosity. In contrast, the air in the atmosphere is like a thin, runny liquid that moves and changes quickly.
