Question

in 1928, geologist arthur holmes came up with an idea that the continents were anchored to the earths core by thick, strong columns of solid rock stable and interconnected breaking up into smaller and smaller pieces on plates \floating\ on the mantle

Explanation:

Arthur Holmes proposed the idea of continental drift. He suggested that continents are on plates that "float" on the mantle. This is a key concept in geology related to plate tectonics.

Answer:

on plates "floating" on the mantle