Beaches Erode Too Fast for Deep Time

Within a few thousand years, half of earth’s coastlines|
will erode away. How long has that been going on?

New approach estimates long-term coastal cliff loss (Stanford University, 17 April 2023). This press release says that current erosion rates of the cliffs at Del Mar in southern California, measured at 2 to 5 inches per year, have been relatively constant for the last 2,000 years.

The study area was ideal for the researchers’ methodology because the Del Mar beach features a narrow shore platform, the bedrock where tidepools are typically found. Using nine samples of bedrock, the co-authors measured concentrations of the chemical isotope beryllium-10 that track landform exposure to cosmic radiation from space. The data were compared with cliff retreat rates from recent studies based on aerial photography, showing that coastal erosion rates have remained relatively constant over the past two millennia – at about 2 to 5 inches per year.

Jane Willenbring is shown on site taking measurements. For 2,000 years, that multiplies out to 4,000 to 10,000 inches (0.06 mile to 0.15 mile, 278 yards) of coastline retreat in that area. Is it unique to Del Mar?

On sandy coasts, like those spanning much of the eastern U.S., beaches are shaped by waves that pull sand out to the ocean, then re-deposit it on land with the coming and going of the tides. But with rocky coastlines like those along California, once a cliff erodes into the ocean, it cannot be replaced, Willenbring said. Instead, it’s as if the rock becomes air.

Willenbring was surprised to learn through this research that over half of all coastlines on Earth are eroding like California’s.

The press release does not consider what might have happened over millions of years. The paper in AGU’s journal JGR Earth Surface puts such matters into futureware, saying, “Modeling suggests that relative sea level rise rate might exert a control on cliff retreat in Del Mar over long timescales, but more investigation is needed.”

Obviously nobody was around to observe Del Mar before natives reached California, but let’s have a little fun with the measured rate. In a million years at that rate, California’s coast would have retreated 31.6 miles at 2 inches per year (the minimum rate). In 60 million years (the assumed time when an asteroid impact killed the dinosaurs), it would have retreated almost 2,000 miles—most of the American continent (remember, the east coast would have also eroded). That’s at the minimum rate stated in the article (for the higher rate, multiply by 2.5). All of North American should have disappeared well before 100 million Darwin Years when giant dinosaurs were supposed to be roaming the American West.

Old-Earth believers will, of course, object that other processes are going on simultaneously: plate tectonics, orogeny (mountain building), ice ages, sea level fluctuations, and such. It is, however, of interest that these Stanford avoided the implications of their measurements: how could coastlines survive for hundreds of millions of years? One gets the impression that such thoughts are best not mentioned. But we have a calculator.