Geologists hope to date Mount Calavera, Morro Hill's volcanoes

NORTH COUNTY ---- Area geologists are poking into part of Carlsbad's Mount Calavera and Oceanside's Morro Hill hoping to find out how long ago these two ancient volcanoes tossed ash and rocks into the air.

"There's really quite a difference between 20 million and 30 million years in what was going on (geologically) in this part of the world," said Tom Demere, curator of paleontology at the San Diego Natural History Museum. "That's what makes these little volcanoes so interesting."

He and colleague David Kimbrough of San Diego State University have taken samples of Mount Calavera ---- a distinctive skull-shaped mountain in Carlsbad's far northeastern edge ---- with a goal of age-testing them later this year.

Their initial estimate is that Calavera's volcanic "plug" or core throat area, is 28 million years old, Demere said Thursday.

If so, that might make it the oldest of its type in the San Diego region, he added.

He isn't the only person eyeing Mount Calavera with interest.

MiraCosta College geology professor John H. Turbeville has hosted talks and walks about the peak's volcanic roots for several years.

He also has plans to do his own age-dating of the volcanic material.

On Wednesday night, he told a standing-room-only crowd at the Agua Hedionda Lagoon Discovery Center that he finds Calavera fascinating even though its long-ago eruption was tiny compared with the 1883 eruption of Krakatau in Indonesia.

Krakatau was so impressive that "when it erupted, they heard it in Europe 10,000 miles away," he said as several audience members gasped.

Its eruption rated a six out of eight on the Volcanic Explosivity Index scale, or the volcanic equivalent of the earthquake assessment system known as the Richter scale.

For comparison, tiny Mount Calavera is estimated to have been only a .5 on the volcanic scale, Turbeville said.

Still, it tossed out rock chunks and plenty of ash. Demere said he believes that ash deposits he has seen in Chula Vista and National City may have come from Calavera's eruption.

"The chemistry is right," he said as he discussed the makeup of the ash materials.

Northeastern Oceanside's Morro Hills, which has lava flows as well as a volcanic plug, and Mount Calavera aren't the only evidence of ancient volcanic activity in the San Diego region.

There is widespread evidence of volcanic activity from 140 million years ago, when dinosaurs walked the Earth.

However, those are scattered rock pieces in places such as San Elijo Hills rather than an intact throat of a volcano, Demere said.

More recent volcanic plugs than the Morro Hill and Calavera spots also exist.

Ones in Jacumba Valley are estimated to be 16 million to 18 million years old, Demere said. There's also a little volcanic deposit just north of Scripps Pier known as Dike Rock that dates from 10 million years ago, he added.

Because they're older, Calavera and Morro Hill could provide clues about what was happening in the area geologically between 20 million and 30 million years ago, Demere and Turbeville said.

At that time, the region looked much like it does today, but there was much more movement of the Earth's crust in this area, Demere said.

Turbeville told the crowd Wednesday night that Calavera was created the same way that the now-volcanically active Cascade Range of the Pacific Northwest began: Offshore land was shoved under a continental crust, melted under tremendous heat and pressure and then found its way back to the surface through cracks in the Earth's surface.

Source: North County Times