Red rhyolite cobble

Eocene Epoch
During the middle Eocene approximately 45 million years ago, this rounded rhyolite cobble was being transported in a river channel when it was finally deposited to form a conglomerate stratum in a sandstone matrix with other rounded rocks.

Jurassic Period
Much earlier, between 155 to 150 million years ago, the original rhyolite rock formed from magma erutping out of volcanoes that made mountains of rhyolitic lava and ash.

Place
These rhyolite cobbles are common in ancient Eocene sandstone and conglomerate deposits of western San Diego County. But a mystery puzzled geologists—since they don't chemically match with rocks that form our local mountains, where is the rhyolite bedrock source for these cobbles?

Description
The origin of the rhyolite cobbles in San Diego County represents a geologic mystery that was eventually solved by field mapping, rock identification, radiometric dating, and hypothesis testing. The final explanation also relied on results of regional fault studies, large scale plate tectonic investigations, and our general understanding of geologic time.

Rhyolite cobbles are river-worn blocks of igneous volcanic rock. They are usually reddish-purple, and sometimes gray. They are very hard to break with a hammer and contain relatively large crystals of quartz and feldspar in a background of fine-grained material.

Although geologic investigations failed to find a source for the red rhyolite cobbles in the local mountains, this work did locate a bedrock source area over 180 miles away in the mountains of Sonora, Mexico. How did they end up here? In Sonora these purplish-red rocks were originally formed from the cooling magma of volcanoes. They are especially rich in silica.

Over millions of years, these rocks gradually eroded out of the mountains of Sonora and were washed downstream in rivers flowing to the west, becoming rounded into boulders, cobbles, and pebbles by the process. Along the way the rhyolite cobbles combined with rocks being eroded from downstream bedrock sources. Eventually the transported pebbles, cobbles, and boulders were deposited in the river channels themselves, or farther downstream where the river met the sea in a vast delta complex. Some of the rocks even made it offshore where they were deposited in submarine canyons eroded into the Eocene continental shelf.

Explore More To discover more about the theory of plate tectonics, see the Ocean Oasis teacher's guide on Plate Tectonics: How Baja California and the Sea of Cortés were formed

When the action of plate tectonics split Baja California and the southern California region away from the Mexican mainland, the bedrock of rhyolite that is the origin of these rocks remained in Sonora, while the deposits of Eocene congloemrates were moved along with Peninsular California to their current location in San Diego. Interestingly, the Eocene submarine canyon deposits were moved even farther north where they are exposed today on the northern Channel Islands.

Suggested Reading
Abbott, Patrick L. 1999. The Rise and Fall of San Diego: 150 Million Years of History Recorded in Sedimentary Rocks. San Diego: Sunbelt Publications.

Text: Margaret Dykens and Lynett Gillette
Photograph: François Gohier

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