The evolution of sand

It is omnipresent in our community. It cannot be escaped. On the streets, on your feets and in your sheets. In your car and your carpet. It is everywhere. Some people dislike it, while others celebrate it as it represents our privileged geographical location. It is summertime, beach time — sand is everywhere. North County’s beach sand is an important natural resource. The story of sand involves geology, oceanography, ecology, meteorology and politics (which we will not be discussing here).In geologic terms, “sand” refers to the size of individual particles: smaller than pebbles and larger than silt. A sand particle measures between 1/16mm and 2mm in diameter. The main mineral components of beach sand include quartz, feldspar and hornblende. Compared to varied assortment of Anza Borrego sand, Encinitas sand is well-sorted, containing mostly quartz.Quartz is the most abundant beach sand mineral because it is resistant to chemical weathering and thus able to withstand the tumultuous journey from mountain to beach. Beach sand also contains crustacean shells, plankton skeletons and decomposing marine plant life. Plus a diverse ecosystem.

The sand on our beaches began its most recent incarnation as igneous rock, building the mountains in eastern San Diego County. Fingers of hardened lava, called plutons, were uplifted by tectonic forces to create the Palomar, Cuyamaca and Laguna Mountains. Consider these gigantic, granitic rocks thrust up, exposed to wind and rain. Some freeze and thaw; erosion slowly works from boulders to cobbles to pebbles to sand.

All the while, gravity carries the ever-lighter particles further down toward the sea. Rivers and streams gorge canyons, cascading downward. They carry the sediments that millions of years ago built our coastal cliffs. The mighty ocean marches in to reclaim the land. Wave action along the cliff bottom erodes large masses of the sedimentary rocks. The rocks fall to the beach and are gradually worked into sand. Rivers continue to deliver fresh cobbles, pebbles and sand through lagoons to the beach. This is natural beach replenishment.

The littoral or intertidal zone is the area between the lowest low tide and highest high tide. This is the zone where life first began its adventure onto land and is now a playground for millions of humans around the world. Sand is obviously vital to the littoral zone. Wave action and the longshore current work to perpetually change the shape of beaches by moving sand along the coast. Surfers can attest to the power of the longshore current as we are swept a mile down the beach when there is a steep angled swell in the water.

In summer, when the South Pacific is most active, swells carry sand from Baja along the longshore current, making a deposit on our beaches. During the winter, North Pacific storms form closer to our shore and carry tons of sand south. These winter swells excavate the intertidal reef system revealing tide pools and changing the shape of the beach.

Eventually, every grain of sand on our beaches will return to the internal furnace of the Earth to become magma and then mountains again. Along the way, it will be glued together (lithified) with other sediments to become different rock. Or forced deep underground where heat and pressure cause metamorphosis into different rock then thrust upward where erosion begins again building new beaches that will become new mountains … the rock cycle!

 

Cobbles, pebbles and sand in the littoral zone or eroding sea cliffs of sedimentary rock, which naturally replenish the beach with sand. Photos by Kyle Stock

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