Why are there gravel beaches? In thousands of years of waves crashing on the beach shouldn’t all the gravel have become sand?

Question

Why are there gravel beaches? In thousands of years of waves crashing on the beach shouldn’t all the gravel have become sand?

In: 6381

Answer 1

I am going to go for a top-level comment here and simply mention that, yes, tens of thousands of years of erosion will break gravel down into sand. Most places where there is gravel though, is because there has not been tens of thousands of years of erosion at that spot. The earth experienced a huge change in sea level some 10,000 years ago or so (not an instantaneous process but rose several hundred feet/100 meters in a few thousand years about that time frame). Most shorelines today are young (roughly 6000 years or thereabouts).

In many places, the regions were glaciated and the sand was all pushed away so there weren’t beach sands nearby which could migrate along with the rising sea level. The ground was down to bedrock, so beaches had to start from scratch. Elsewhere, the sand was deposited by the glaciers and entire regions are massive sand zones (think Long Island or Cape Cod).

On top of this specific problem, there is an energy issue which matters. Even in places where erosion is happening and cobble-gravel should be (is) breaking down into sand, the wave interaction energy is relatively high and sand (and smaller) material gets carried away. The general result is deposition of that sand up the coast (down-direction of the prevailing current) and as bars in slightly deeper and thus calmer water. You end up with a steady-state condition in the gravelly zones where new cobble/gravel rock is being created from nearby exposed bedrock, and whatever cobble/gravel is breaking down is being transported away and sand is slowly accumulating elsewhere.

But the main reason for lack of sand in gravelly regions is one of time. As in, there has not been enough to make the loads of sand needed for sandy beaches. It is not always glaciated regions that have a lack of sand either, it is often regions where uplift or land rejuvenation occurs for tectonic or volcanic reasons. The land isn’t stable so the land doesn’t sit in place long enough to develop sand deposits.