Round Mountain, near Jacumba, can be viewed from Interstate 8
By Thomas A. Deméré, Ph.D.
Although San Diego County is currently experiencing a period without active volcanism, this has not always been the case. As long as 125 million years ago, during the early part of the Cretaceous Period, subduction-fed magmatism was extruding tremendous volumes of andesitic lavas and ash to form an impressive chain of volcanic islands.
Similar to the modern Japanese islands, these Cretaceous volcanic islands were separated from the adjacent continent by a broad inland sea. Continued subduction-fed magmatism along the western margin of the North American mainland during the middle portion of the Cretaceous Period (125 to 90 million years ago) failed to reach the Earth’s surface and instead congealed at depths of 10 km or more within the crust to form tremendous volumes of plutonic igneous rocks. As more and more magmas formed and cooled in the crust, tall mountains were created that perhaps rivaled the Andes of modern-day South America.
All of this magmatism and mountain-building caused heating and alteration of the older island-arc volcanic rocks. Subsequent large-scale erosion during the early part of the Cenozoic Era (approximately 65 to 55 million years ago) removed large portions of the Cretaceous mountain range to expose the mountains’ core. This period of erosion was followed during the Eocene Epoch by high global sea levels and deposition of thick layers of marine and fluvial sedimentary rocks. The sedimentary rocks buried the hills and valleys of the older eroded landscape. During the later part of the Eocene (approximately 42 million years ago) distant volcanoes sent clouds of ash into the area that settled into coastal waterways to form altered deposits called bentonites. Sometime later, perhaps during the later part of the Oligocene Epoch (approximately 29 million years ago) renewed regional magmatism caused by shallow plate movements resulted in the extrusion of dacitic volcanic flows and ash from local volcanic vents.
A period of quiet followed until the middle Miocene (approximately18 million years ago) when basin-and-range tectonics took hold and numerous volcanic vents began to form and extrude basaltic flows, ash, and pyroclastic debris onto the countryside. Local volcanism ceased in the coastal portion of our county approximately 10 million years ago with small-scale fissure eruptions of basaltic rock. However, some form of volcanic activity continues to the present day in the eastern portion of Imperial County, driven by plate tectonic processes related to the East Pacific Rise.
Where can evidence of San Diego’s ancient volcanoes be seen today?
Small, eroded remnants of the early Cretaceous volcanic islands are preserved today in the resistant coastal peaks of the San Ysidro Mountains, Mother Miguel Mountain, San Miguel Mountain, Dictionary Hill, Black Mountain, Mount Jerusalem, and the higher hilltops on Camp Pendleton. Good exposures of these metavolcanic rocks can also be seen in the active rock quarry along the north side of Mission Gorge Road west of Mission Trails Regional Park.
Better preserved, but more localized in distribution are the Oligocene volcanic rocks exposed at Cerro de la Calavera in Carlsbad and Morro Hill in Oceanside. Remnant volcanic plugs, volcanic flows, and pyroclastic deposits at these sites are evidence of explosive volcanic eruptions. At Cerro de la Calavera, an abandoned rock quarry on the south side of Calavera Lake has been excavated into the solidified throat of a volcano that probably last erupted at least 29 million years ago. Good examples of columnar jointing can be seen at this site.
Probably the best preserved examples of San Diego’s ancient volcanoes occur in the Jacumba Valley area along Interstate 8 in the eastern portion of the County. Here, an eroded Miocene volcanic plug forms the prominent landform known as Round Mountain, easily visible on the south side of the freeway. To the northeast at Table Mountain, resistant mesa tops preserve tabular volcanic flows, pressure ridges, and pyroclastic lahar deposits. Many of the ancient lava flows have a lower baked zone of brick red rock which formed when the lavas flowed out across the Miocene terrain. An abandoned quarry near the base of Table Mountain exposes thick accumulations of pink and purple colored lapilli and ash that were deposited as a cinder cone.
The youngest example of local volcanic activity is represented by Dike Rock, a prominent basaltic dike that occurs along the beach north of Scripps Institution. The dike is intruded into Eocene-age marine sandstones and shales and roughly parallels the shoreline. Although covered by water at high tide, this late Miocene volcanic feature forms a distinctive landform at low tide. Its resistant black rocky mass and linear orientation contrasts sharply with the soft beach sands and splashing waves.
Thomas A. Deméré, Ph.D., is Curator of Paleontology at the San Diego Natural History Museum. Round Mountain photo by Michael Field.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, April 2008