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Why are continental shelves narrow near mountainous coasts and wider near plains?

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Map graphic showing the boundaries of the five oceans-Pacific, Atlantic, Indian, Arctic, and Southern-using color-coded regions. Four different map projections are included: North Pole view, South Pole view, International Date Line view, and Greenwich Meridian view, each displaying the oceans' extents around the continents. — ocean Earth's oceans and their marginal seas cover nearly 71 percent of the planet's surface.

Continental shelves are essentially extensions of the continental landmass beneath the ocean margins. Their width and characteristics are primarily determined by the tectonic setting of the coast and the accumulation of sediment.12

Near Mountainous Coasts: These areas are typically located on the leading edges of tectonic plates, such as the Pacific coast of North and South America. These margins are often tectonically active and characterized by steep slopes that may drop directly into deep-sea trenches. Because these regions are often subject to high-energy environments and lack the broad, stable coastal plains necessary for massive sediment accumulation, the shelves remain narrow, steep, and rough.231
Near Plains: These shelves are generally found on the trailing edges of tectonic plates, such as the Atlantic coast of North America. These are tectonically stable regions where large amounts of sand, silt, and mud from the continent have accumulated over millions of years. This consistent deposition of sediment creates broad, comparatively level, and expansive continental shelves.231