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I believe in his book “Scale” Geoffrey West explains that the “crinkliness” of coastlines is primarily a function of the slope of the land, such that the steeper the slope the longer the actual coastline. I think he includes a mathematical expression for the relationship but I can’t recall it off-hand.

Frost wedging has a large impact in the poles, water melts gets into cracks, freezes and then expands causing more fractures to be infilled and cracked. The erosion rate between the tropics and the poles is probably close by I imagine the tropics have more erosion present. I'll be back to update this comment when I can read up more. Source, also a b.s in geology

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Is the weathering “heavier” in the tropics or have the coastlines just had longer to smooth out? Glacial periods near the poles are somewhat frequent on geologic timescales, so the water there hasn’t had as much time to erode the coastlines compared to the tropics, but the erosion “rate” is probably similar between the two regions, no?

That being said, vegetation is not necessary for the formation of a delta, and can in fact enhance shoreline rugosity in a delta by enhancing channel stability.

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Rocky coasts (high and low relief) result from resistant geology (to the erosive forces of sea, rain and wind), often in a high-energy environment, whereas coastal plain landscapes (sandy and estuarine coasts) are found near areas of low relief and result from supply of sediment from direct terrestrial and offshore.

Because of the way that glaciers and waves shape the land.

In the polar regions, glaciers advance and retreat over thousands of years, carving out fjords and other features in the coastline. The glaciers erode the land, creating valleys, and deposit the rock and sediment they have picked up in the process, building up landforms like moraines. This process creates a crinkly or jagged coastline.

In tropical regions, waves are the main agent shaping the coast. The waves erode the land primarily through a process called longshore drift, in which waves hit the coast at an angle and push sediment along the shore. This process creates a smooth coastline because the waves tend to erode the land evenly, not carving out fjords or creating other distinct landforms.

Climate and sea level changes also play a role. In warmer climates, sediment is transported more quickly, leading to a less-pronounced coastline and sometimes sediment deposits forming barrier islands or lagoons

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Coasts can be erosive or depositional., old or young. Young coasts of any type will likely be jagged (look at pacific islands when they first break thru sea level). Then if the coast is old, it can be jagged if it’s erosive or smooth if it has sediment (coral reefs count too!) being deposited.

That's not entirely accurate.

For example, the Marquesas are just south of the Equator and they have crinkly edges.

Meanwhile, Wrangel Island up in the Arctic Ocean is pretty smooth and rounded. Same with Bouvet Island down by Antarctica.

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