Friday, February 6, 2015

Walking on Santa Lucia Granodiorite sand: the white sand of Carmel Beach

South of Pebble Beach on the Monterey Peninsula: Carmel Beach
Carmel Beach with Arrowhead Point
Carmel Beach interfaces California's Carmel-by-the-Sea with the Pacific Ocean. A sandcastle contest is held annually at this beach, during which amazing architectures and sculptures are constructed from its white sand. The public beach arc between Pebble Beach and Carmel Point attracts locals and tourists on sunny as well as foggy days for relaxing shoreline walks—combing over silvery white Santa Lucia Granodiorite sand.

Granodiorite is a rock rich in quartz. It is similar to granite, but granodiorite contains more plagioclase feldspar (calcium and sodium feldspar) than orthoclase feldspar (potassium feldspar). Granodiorite is an intrusive igneous rock that forms in magma chambers underground and slowly cools, resulting in the formation of visible crystals—visible to humans once the rocks rise to the surface. Santa Lucia Granodiorite formed more than 80 million years ago along the west coast of North America. It then rose to the surface over millions of years. Granodiorite rocks that became available to the erosive action of marine waves were and are transformed into gravel and sand. 

The Shifting Sands of Carmel Beach originate from Santa Lucia Granodiorite. An interpretive panel at the western end of Carmel's Ocean Avenue explains:

Carmel's unique white sand comes from offshore rocks called Santa Lucia Granodiorite. Ocean waves pound these rocks into fine sand granules that wash onto the beach. The power of the waves changes with the season, alternately removing and replacing the sand.

The shifting of the intertidal-zone sand by the seasonally changing activity of ocean wave is enhanced by the foot traffic of beachcombers and beach volleyball players. Overall, the sand shifting is naturally balanced: High-energy winter waves scour sand from the beach and lower-energy summer waves return sand back to the beach. However, severe winter storms sometimes remove so much sand that underlying bedrock is exposed. Therefore, sand is artificially redistributed. The panel labels this process as Beach Housekeeping, performed in the following way:

Carmel's beach sand is essential for protecting the shoreline from erosion. Bulldozers push sand uphill every spring to cover exposed rocks and counteract the downhill movement of sand from foot traffic. By mid-September, the sand is returned to the lower beach, ready to absorb the impact of winter waves.  

Keywords: California coast, granodiorite erosion, destructive waves, constructive waves, beach engineering.

More about granodiorite
[1] National Park Service: Granite and Granodiorite FAQ [http://www.nps.gov/goga/forteachers/granite-and-granodiorite-faq.htm].
[2] Encyclopaedia Britannica: Granodiorite [www.britannica.com/EBchecked/topic/241730/granodiorite].
[3]: World Heritage Encyclopedia: Geological History of Point Lobos [community.worldheritage.org/article/WHEBN0026945648/...].

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