Visitors commonly note the rounded appearance of the back side of Half Dome. It was not polished by glaciers but rather fractured off in sheets much like how one might peel layers from an onion. This process occurs because these rocks formed at ~20–25 km depth, but have experienced a removal of confining pressure because they are now exposed at the surface (think of how a sponge expands once you remove your hands pressing down on it). What is this process called? (Hint: Look these words up in a glossary.)