Why are coastal plains narrow or absent along the Pacific mountain system?

Coastal plains are narrow or nonexistent along the Pacific mountain system primarily due to the region's active tectonic environment and the resulting offshore topography.

The Pacific coast is characterized by a narrow continental shelf that drops abruptly into deep ocean waters. This steep transition is a direct consequence of the ongoing movement of tectonic plates, which has caused the land to rise episodically. As the land has been uplifted, wave action has carved notches and terraces into the coastal rock, further defining the abrupt edge between the mountains and the sea.12

Additionally, the geological composition of the coast influences its shape: more-resistant igneous rocks form steep sea cliffs, while softer sedimentary rocks have been eroded to create embayments. The entire region remains subject to periodic tectonic activity, which continues to drive the rise and fall of the coastline, preventing the development of broad, stable coastal plains.1