Question

why is upper mantle a viscous layer​

Answer 1

Answer:

Because of the temperature difference between the Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle.

Answer 2

The upper mantle is a viscous layer because of the temperature difference between the Earth’s surface and outer core and the ability of the crystalline rocks at high pressure and temperatures to undergo slow,creeping,viscous-like deformation over millions of years,there is a convective material circulation in the mantle.