Coastal bluffs reveal secrets of past

It’s a favored surf spot off La Jolla’s shoreline today, but millions of years ago it was a volcanic “hot spot.”

“It” is the stretch of beach from Scripps Pier north to Torrey Pines that has a very special geology.

“It’s a vertical, volcanic intrusion,” noted Thomas A. Demere, Ph.D., curator of paleontology at the San Diego Natural History Museum. “Distinctively black basaltic rocks deposited there, right out in the surf zone, are 10 to 12 million years old.”

Demere added this remnant volcanic formation lies just beneath the cliff bluffs where the National Marine Fisheries Service Science Center on UCSD’s campus sits. At low tide, standing on the beach in that area looking south toward La Jolla, the linear nature of that volcanic deposit is obvious.

“It’s really quite striking,” Demere added, “quite different from the light brown sandstones that compose the cliffs.”

Geologic “sleuths” like Demere are piecing together the geologic riddle of San Diego’s paleontological history. Evidence buried in, or uncovered by, natural erosion reveals a past topography much different than today, when an ancient oceanic crustal tectonic plate created an archipelago of volcanic islands producing massive volumes of magma that later congealed into rock.

Also recorded in the historical record of coastal San Diego are periods of higher rainfall and subtropical climates that supported coastal rain forests with exotic plants and animals. With the coming and going of worldwide ice ages, San Diego’s coastline endured periods of “drowning,” as well as widespread earthquake faulting.

La Jolla’s downtown Village has its own unique geologic pedigree, Demere said.

“La Jolla is built on a series of sea floors that are related to climatic fluctuations over the last 120,000 years,” he said. “Scripps Park down by the Cove on that nice broad, flat surface is a sea floor 85,000 years old. The flat surface on Prospect Street, the central portion of La Jolla Village, is another sea floor 120,000 years old.”

Terraced sea floors like those in La Jolla are the consequence of ice ages and intervening periods of global warming, in roughly 100,000-year cycles that caused wide discrepancies in sea levels.

“The peak of the last ice age, 18,000 years ago, sea level was up to 400 feet lower than it is today,” noted Demere.

Natural wave action led to the carving out of platforms resulting in the current topography.