Where Does Sand Come From?

On the geological origins of my favorite California beaches

“Sand is overrated. It’s just tiny little rocks.” From Eternal Sunshine of the Spotless Mind

Sand is essentially composed of minuscule rock fragments. It represents one stage of the continuous rock cycle that has been molding Earth's surface for approximately 4.5 billion years. Rather than being overrated, I believe that recognizing the significance of the rocks beneath us enhances our experience at the beach.

Sources of Sand

I initially thought that sand primarily originates from coastal erosion and the mechanical breakdown of rock by ocean waves. While this is indeed one source, it's not the only one.

At Black Sands Beach in the Marin Headlands, powerful waves relentlessly strike a coastline primarily made of basalt, alongside a mix of conglomerate rock, graywacke sandstone, greenstone, limestone, red chert, and gray quartzite. The result is a beach with dark, multicolored, coarse sand grains. I appreciate this location for its raw energy—where the wind and waves are palpable, the earth's layers are visibly tilted from tectonic forces, and you can truly grasp how these natural elements shape the landscape. Walking barefoot on the rough grains offers a delightful exfoliation experience.

Sand can also stem from the exoskeletons of deceased marine creatures. For instance, beaches in Tankah, Mexico, and Elafonisi, Greece, consist largely of eroded remains of sea animals. All sand contains some degree of crushed shells, coral, sea urchin spines, sponge spicules, or other skeletal remnants mixed with rock particles.

Another source of sand is human refuse, particularly discarded glass. The prevalence of sea glass has diminished recently due to collection for personal use and the rise in plastic production.

Moreover, a significant contributor to sand is the erosion of rocks that have broken away from inland mountains, finding their way to streams and eventually reaching the ocean. The relentless forces of rain, wind, and ice erode the landscape, causing outer layers to detach and wash away. Jon Erickson emphasizes the vast scales of mass and time involved in geological processes.

“Some 3 billion tons of rock each year are dissolved in water and carried by streams to the sea. This is sufficient to lower the entire land surface of the Earth by as much as an inch in only 2,000 years.” From “Rock Formations and Unusual Geological Structures,” by Jon Erickson.

Additionally, it's estimated that around 25 billion tons of rock fragments are transported by stream runoff into the ocean annually. Once in the streams, these fragments undergo further erosion from water and collisions with other stones. Some rock components dissolve, while others break down into small particles that remain suspended in the water until they settle on the ocean floor. Larger fragments, however, sink to the riverbed, moving only when the river's velocity is sufficiently high, such as during storms.

Only certain rock components possess the strength necessary to travel to the ocean in sizes greater than 1/16th of a millimeter. Quartz, known for its durability and low chemical reactivity, is the most prevalent mineral that reaches the ocean as sand-sized grains, explaining the abundance of sparkling, light-colored quartz sand on many beaches.

The beach's slope and the force of incoming waves dictate which grain sizes will accumulate along the shore. Generally, a steeper incline correlates with higher wave energy and larger grain sizes.

At Seabright Beach in Santa Cruz, the sand is a soft, dirty blonde composed of finer grains. It was surprising to learn that this beach was artificially created just 55 years ago. Once a narrow stretch of sand adjacent to mudstone cliffs, Seabright Beach expanded by approximately 400 feet when the Santa Cruz Yacht Harbor jetty was constructed in 1963. The San Lorenzo River carries sediment from the Santa Cruz Mountains to the ocean, where some grains are pushed back towards the shore by waves. The construction of the jetty has effectively redirected these sand grains to help form Seabright Beach.

The sheltered area and gentle slope created by the jetty also favor lower wave energy and finer sand grains, making it a pleasant beach for walking barefoot and an inviting spot for swimming.

During my time at UC Santa Cruz, I frequently visited Seabright Beach for exercise, running along the shoreline through shallow waves and then jumping into the ocean to cool off. I’ve noticed that my mood often reflects the energy of the beach I’m at. At Black Sands Beach, I feel invigorated and alert, while at Seabright Beach, I experience calmness and serenity.

Through the Hourglass and Back Again

The question, "how long does it take to form sand?" is complex. What do we define as the beginning of this process? Since sand arises from rock erosion, we might consider the age of the surface rock. For instance, the rocks at Black Sands Beach were uplifted above sea level 3-4 million years ago, but fossil dating indicates these rocks were formed between 80 to 200 million years ago. Thus, it took over 80 million years for some of that rock to become sand. Moreover, the materials that formed those rocks have existed since the Earth's inception.

Since the Earth cooled enough for its earliest rocks to solidify from magma over 4 billion years ago, rocks have perpetually cycled through various forms. To borrow a concept from Battlestar Galactica, the sand grains on our beaches are not at the end of their life cycle; these tiny rocks have likely transformed from sand before and will do so again. Sand grains accumulate in layers on the ocean floor, and when buried deeply enough, pressure converts sand into sandstone. From there, sandstone can descend further, where heat and pressure recrystallize quartz grains into quartzite. If it continues deeper, the heat may melt the rock, recycling it into magma. Alternatively, tectonic uplift might raise the sandstone or quartzite to the surface, where exposure to the elements starts the cycle of erosion anew.

Geologists analyze rocks to comprehend Earth's history, relying on the principle that slow geological processes and cycles shape our planet.

“In the Eastern view of the world, time is a hoop: the ‘ever-circling years’ repeat themselves endlessly. In the Western world, we are accustomed to thinking of time as linear, ‘progressing’ from one point to another. Nothing can repeat itself in this view, because even if all other parts were the same, time would have changed. Such a view makes it difficult for us to comprehend eternity, but it allows us to measure time as if it were units on a ruler.” From “California Landscape: Origin and Evolution,” by Mary Hill.

As sand creation is an ongoing process embedded within the larger rock cycle, we might only need to consider erosion rates when assessing how long it takes to create sand. We could express this as the volume of sand produced annually from each source. However, this remains a challenging question to answer, as erosion rates are influenced by numerous factors including topography, weather, and human activity.

Even measuring the seemingly straightforward source of sand, cliff erosion, proves to be quite variable and more challenging than anticipated. To gain insight into erosion rates, I examined a recent study that utilized airborne LiDAR datasets taken along California's central and southern coastline from 1998 to 2010. LiDAR employs pulsed laser light to scan the earth and create a 3D representation for measurement and analysis. Comparing LiDAR data over time allows researchers to calculate coastal retreat rates. According to the study's author, Adam P. Young, the average retreat rates for cliff faces and cliff tops were 0.04 and 0.12 meters per year, respectively, with some hotspots experiencing retreats of up to 4 meters annually.

This data, however, does not provide insights into the volume of eroded rock that ultimately becomes sand, nor does it clarify the timeline for this transformation. It does, however, suggest that the California coast is gradually succumbing to the ocean's embrace. Perhaps, one day, my current home in California's flat central valley will again be submerged, with waves crashing against the Sierra Nevada's cliffs, just as they did 15 million years ago.

Land that erodes into the ocean is not lost; it merely transitions to another phase in the rock cycle. Jon Erickson notes that “the rise of active mountain chains such as the Himalayas is matched by erosion so that their net growth is almost zero.” There exists a balance between rock creation and destruction, which I find profoundly reassuring.

References

Alden, Andrew. “Geological Outings Around the Bay: Rodeo Beach.” KQED (2012). Link to article. Elder, William P. “Geology of the Golden Gate Headlands: Field Trip Guidebook.” Link to pdf. Erickson, Jon. Rock Formations and Unusual Geological Structures: Exploring the Earth’s Surface. New York: Facts On File, Inc., 1993. Griggs, Gary. Introduction to California’s Beaches and Coast. Berkeley: University of California Press, 2010. Hapke, Cheryl. “Geology and Coastal Hazards of the Northern Monterey Bay, California: Field Trip Guidebook.” Prepared as part of the California Shore and Beach Preservation Association Conference (2000). Link to pdf. Hill, Mary. California Landscape: Origin and Evolution. Berkeley: University of California Press, 1984. Perlman, David. “Ancient sea life thrived in Central Valley.” SF Gate (2009). Link to article. Young, Adam P. “Decadal-scale coastal cliff retreat in southern and central California.” Geomorphology 300 (2018) 164–175. Link to article.

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