Where Does Sand Come From?
Most of the sand on California beaches started as mountain. Rocks inland get broken down by physical and chemical weathering - freeze-thaw cycles cracking stone apart, rain dissolving minerals, roots prying open fractures — and the resulting sediment works its way downhill over thousands to millions of years. What survives the trip depends on what the rock was made of.
Quartz makes up most of mainland beaches because it’s extremely hard and chemically stubborn. Softer minerals like feldspar dissolve or crumble along the way. By the time sediment reaches the coast, quartz is often the last man standing, which is why sand from San Diego to Santa Cruz looks more or less the same despite coming from very different source rocks.
Rivers are the main arteries in this system. Weathered sediment washes downstream, bumper-carting against riverbeds and other particles the whole way, getting smaller and rounder with every mile. A grain of sand on your beach might have started as part of a granite cliff face a few hundred miles inland. California has over 1,500 dams, and most of them are trapping sediment that would otherwise end up on the coast. With waves that keep pulling sand offshore, and rivers not restocking it the way they used to, the beaches run a deficit. Cliff erosion chips in locally, waves eating into coastal bluffs and dropping material directly into the surf zone and oblivious sunbathers, but it doesn’t come close to replacing the river supply.
In tropical environments, however, a huge portion of beach sand comes from biology. Parrotfish scrape algae off coral with their beak-like teeth, chewing up chunks of coral skeleton in the process. What comes out the other end is white sand. A single parrotfish can produce several hundred pounds of it per year. Hawaii’s green sand beach at Papakolea is olivine, a mineral in the local basalt that weathers out faster than the surrounding rock and concentrates on the shoreline. Those white Caribbean beaches are largely crushed shells, urchin spines, coral fragments, and literally-rock-hard fish poop.
Once any of this sediment reaches the coast, waves decide what stays. High-energy beaches exposed to open ocean swell lose their fine grains offshore, currents carrying the small stuff away and leaving coarser, heavier material behind. Sheltered beaches don’t generate enough force to move the fines, so they keep them. Push this far enough and sand disappears entirely. The rockier stretches around San Diego are hard local geology meeting persistent swell with no major river nearby to resupply. Whatever fines existed got stripped out long ago, leaving cobble that clacks together in the backwash, clipping your ankles as you time your paddle.
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