Why are there gravel beaches? In thousands of years of waves crashing on the beach shouldn’t all the gravel have become sand?
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Sometimes this just depends how old the beach is – some gravel beaches are just less far through that process than sandy beaches: the pebbles have spent less time in the sea rather than on the land (as part of cliffs or rocks, for example) compared to a sandy beach, so they have had time to get worn smooth by the waves but not to be worn all the way down to sand.
However, there’s also different types of sand – some types are formed from worn rocks and some from crushed shells and coral – being softer and usually thinner than rock, shell and coral break down into sand more quickly. In the same way some types of rock break down more quickly than others and you might see a sandy beach where the sand is one colour but the bigger rocks on the beach are a different colour as they’re from rock layers that eroded less quickly. In that case the rocks would often be in long thin stripes through the sand.
And finally, the types of waves on the beach and the shape of the land as it meets the water (how long or steep the beach is, whether there are cliffs that the waves can reach etc) can have an effect too – especially on beaches where there is a mix of sand and stones to begin with.
Some types of waves race up the beach with plenty of energy, carrying sand from the sea bed or an estuary up to the top of the beach, and lose energy as they go – when they lose energy they can no longer carry sand particles as easily so they dump sand on the beach as they recede, and the sand stays on where it’s dropped and over time the beach builds up or forms dunes at the top.
Some waves break at the top of the beach with plenty of energy and pick up sand and smaller stones from the top of the beach which they carry back down to the sea bed or out to sea on a river, but they don’t have enough energy to pick up rocks or heavier stones so the sand washes away and only the pebbles remain on the beach.
I wondered about this as well for a very long time. There’s a sign on a sand beach in Maine That explains the curious fact of why this sand beach exists in Maine where most of the beaches are rocky.
It said most gravel beaches are at northern latitudes. Sand is often made of crushed shells. Cold water dissolves more CO2 making it more acidic and dissolves away the sand.
This particular beach where the sign was placed received warmer water and thus the sand was preserved and able to accumulate on the beach.
Most of the sand on beaches off coral reefs is fish poop, not rocks ground up by the force of the waves.
One thing to consider is the changing sea level. Just 20 thousand years ago, at the top of the last Ice Age, the average sea level was 130 meters (400 feet) below what it is today. The ice melted and the sea level rose by about 10 thousand years ago, so all the modern beaches only became beaches that long ago. In terms of geology, not that much time has passed to break down all the gravel everywhere.
Another thing to consider is that beaches aren’t permanent, they are being constantly replenished, for example through rivers. That is, the sand on a beach is being constantly destroyed and washed away by the sea, but a nearby river is bringing a constant supply of fresh sand as replacement. In fact, damming up a river can cause a beach to deteriorate and disappear in as a short time as a few decades.
Sand on beaches comes mainly from shells of marine animals, not from the soil. As a side note, the sand in the deserts is different than the beach sand in that respect.
Those pebbles on the beach are coming from the soil that gets eroded and washed in the sea. They are pretty “young”, since revealed by the water, maybe just a few years/decades.
Rocks in the soil cannot be carried by waves with the same speed as dirt, so they remain behind for a while. Washed and rounded off…
Simple answer!! Envi scientist here 🙂 the gravel has been delivered to the beach by a process called long shore drift from a river which is situated down-drift of the beach. It’s not a case of the rocks not eroding (they do this by rolling and chipping bits off more so than waves on their own), but rather a case of the material being freshly delivered to the beach. You’re seeing sand in its first stage of life.
Take a look at New Zealand’s gravel beaches along the east coast of the South Island, you will see them most commonly north (down drift) of the major braided rivers, and you’ll see sandy beaches in sheltered areas. The sand is delivered from off shore as it’s easier for the waves to delivery this kind of lighter material for long distances to sheltered coves – there’s less energy in the wave which means it doesn’t have the power to lift heavy boulders/gravel, hence why sand travels more readily and settles in places where wave energy is lower.
Not all beaches have been where they are for thousands of years. Somewhere you need hundred year old beaches too or you can never have thousand year old beaches.
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I live in Naples and we have coral and shell beaches that regenerate their own extra fine sand fairly quickly, but even every 5 years or so we bring a couple truckloads of sand from a few miles east over the the popular beaches as they get worn because we have so few waves to carry new sand in shore.
As you walk down the beach on the little brittle shells that wash up, they crunch beneath your feet and you can see the layers of broken pieces basically turning to sand before your eyes. It’s very cool. I also understand that in terms of bringing in sand, we do it far less than other areas and only need to do it because of our lack of tide.
I haven’t seen this mentioned yet, but time of year definitely plays a role as well. Generally, winter storms are more energetic, and they carry smaller grains of sand out to sea a little bit and deposit them on a sand bar, leaving the heavier gravel and rocks behind. Time of year corresponds to storm wave energy. You can see how the shape of the beach changes in the linked profile.
[beach profile](https://opentextbc.ca/geology/wp-content/uploads/sites/110/2016/07/differences-between-summer-and-winter-on-beaches.png)
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