Why are the west coasts of North and South America commonly erosional coasts?

The west coasts of North and South America are classified as erosional coasts primarily because they are situated on the leading edges of lithospheric plates.1

Unlike trailing-edge margins, which are typically broad and characterized by sediment accumulation, leading-edge margins are tectonically active and generally feature narrow continental shelves and steep slopes. These regions are often backed by rugged mountain ranges and are dominated by exposed bedrock rather than abundant sediment deposits. Because these coasts lack the significant sediment supply required for long-term depositional growth, erosional processes—such as wave action against steep, high-relief topography—predominate. While these areas are subject to constant erosive forces, the actual rate of shoreline retreat is often moderated by the resistance of the underlying bedrock.12