I have always thought that sand was formed by eons of corrosion from the waves…Why then do some beaches have nice smooth sand and others have big rocks
Some beaches have been around longer.
Waves are corrosive?
Erosion
A lot of beach deposition has to do with the amount of wave energy the beach receives.
Faster moving/higher energy water can carry larger rocks, while slower moving/lower energy water can’t carry the large rocks and only carries small sand and silt. The larger rocks drop out quickly.
Beaches with calm, low energy waves will not have larger cobbles or pebbles deposited because the waves will not be able to transport those heavy rocks up the beach. Only the smaller grains of sand will tend to make it up onto the beach. Also check out the sand closely sometime - you’ll find that different parts of the beach have different grain sizes of sand deposited on them. Close to the water level, this will be dominated by wave breaks and water transport of the sediment (sand, silt, or rocks.) Further up, wind becomes important in moving sediment around the beach more often than waves. Some beaches will have exceptions to this where large storms in certain seasons provide waves that reach further up the beach than any other wave, allowing the deposition of large items.
Beaches with crashing high-energy waves will tend to have larger sediment sizes - cobbles, pebbles, or even boulders. The fast-moving high energy waves can transport both larger rocks and the sand and silt. On average, just about any wave will be able to move the sand and silt, scouring it away, but not all waves can carry the larger rocks, so they are more likely to stay.
There’s another factor that allows high-energy beaches to have small sediment sizes: source of new sediments. If the beach has been there for a long time, and there are no longer any cliffs or hills for the waves and wind to erode the rocks off of, then there will be no new large sediments created to deposit on the beach. Then even beaches with huge, crashing waves can have very small sand or silt sediments simply because there’s no place for the larger rocks to come from: they were there a very long time ago, and were smashed to tiny bits over the years.
Currents and topography (both underwater and above sea level) can play a role in the deposition of sediments too, but it’s beyond my ken without any specific examples to work with.
I would think that shore topography plays some role. In Florida, where things are pretty flat, the waves lap up onto the shore, which ramps up out of the sea. In a place like, I dunno, Maine, there are rocky cliffs. The waves aren’t going to hammer them down into a flat beach.
Your source rock type is going to play a role as well since not everything erodes into sand. Granitic, volcanic or carbonate rocks will likely eventually produce a more preferable grain than would a shale source, which not only would be muddy but also likely too fine to withstand the higher energy of a beach environment.
Another clutch of casually brilliant answers to an arcane question. I love this place.
In Hawai’i, dare I say, on islands in general, the sand is deposited by the waves in three directions: Onshore, offshore and alongshore.
Smathers Beach in Key West is shipped in. When a hurricane hits or a bad storm it often carries all the saned away. Then they ship more back in.