What is Earth’s thickest major internal layer?

Earth’s mantle is the planet’s thickest major internal layer.

It lies between the crust and the core and extends to a depth of about 2,900 kilometres. The mantle makes up most of Earth’s volume and is composed mainly of silicate rocks rich in magnesium and iron, including peridotite in much of the upper mantle.

Although often described as solid rock, the mantle can flow extremely slowly over geological time. Heat-driven convection and other forms of mantle movement help drive plate tectonics, volcanic activity, and the recycling of oceanic crust.

The mantle is different from Earth’s crust, which is relatively thin, and from the metallic core. Its boundary with the core is called the Gutenberg discontinuity, while its boundary with the crust is the Mohorovičić discontinuity, or Moho.