Asthenosphere

Part of speech: noun

Pronunciation: /asˈθɛnə(ʊ)sfɪə/

Definitions

1. A zone within the Earth's upper mantle that is partially molten and provides a ductile environment, facilitating the movement of tectonic plates
2. A section of the mantle that lies beneath the lithosphere, known for its semi-viscous characteristics which enable tectonic motion
3. A partially molten region in the upper mantle below the rigid crust that allows tectonic plates to move with flexibility

Etymology: The asthenosphere is a modern scientific coinage, built from Greek parts that never met in this combination in antiquity. Its first half, "astheno-", comes from Ancient Greek "asthenēs", meaning "weak" or "without strength". It is formed from the privative prefix "a-" ("without") attached to "sthenos", "strength". The name thus describes the layer's defining property: it is weak enough to yield slowly under stress. The second half, "-sphere", has a longer road into English. It goes back to Ancient Greek "sphaira", a ball or globe, which Latin borrowed as "sphaera". The word passed through Late Latin and Old French before reaching Middle English as "spere", and it was later respelled "sphere". English then made "-sphere" a productive combining form, so words like "atmosphere" and "biosphere" share the same ending. Put together, the word names a weak shell within the Earth. Its Greek parts show that it was coined from classical building blocks, not inherited from the ancient world, which is why the word reads so transparently to anyone who knows a little Greek.