San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Geology Topics]
Geologic History in our Backyard:
Rhyolite Cobbles

Red rhyolite rocks eroded from the mountains of Sonora, Mexico and were carried down rivers, and rounded into cobbles along the way. These cobbles were eventually deposited near the ancient west coast of Mexico, 35-48 million years ago.

Much later, the fault bounding the Pacific and North American plates broke loose. A chunk of land, now known as southern California and the peninsula of Baja California slowly tore from the Mexican mainland. Over the past fifteen million years, this land mass has traveled 185 miles (300 km) northwest at an average rate of 2 inches (59 mm) per year. (Peninsular California is still moving.) Approximately 500,000 years ago, the rock layers containing the rhyolite cobbles were eroded from their ancient resting place years by local streams and rivers. Some cobbles were carried along the beach by offshore currents. Later these cobbles redeposited in the red sands that are now exposed in the Museum excavation. Recently uncovered by earth movers, a few red rhyolite cobbles sit on Museum desks, quiet storytellers of a rocky past.

Photo of rhyolite cobble broken open, by Tim Murray, 1999 Rhyolite cobble broken open.
Photo by Tim Murray, 1999