1. California Rock Stories Linking tectonics to rock formation 2. Geologic History of California: Part II Ellen Metzger BAESI – July 2012 Source of photos: KQED

California’s Rocks California, including the Bay Area, has a greater variety of rocks than do other regions of the United States. This reflects the state’s complex tectonic/geologic history.

Bay Area Rocks Young sedimentary and volcanic rocks Mesozoic rocks  Franciscan Complex  Coast Range Ophiolite  Great Valley Group Salinian basement

The Rock Cycle Source: USGS

Bay Area Rocks Do your students bring you round, grungy, fine-grained, black and green rocks?

How do geologists describe rocks?

Rock Description Color Texture Weathering/resistance to erosion

Cartoon of the subduction zone present on the West Coast 100 million years ago showing position of the Franciscan accretionary complex. Source: National Park Service

Cross section of western North America at about 100 million years ago / California tectonics in the past: a subduction zone

Terranes in the Bay Area Terrane A “terrane” is distinguished from neighboring terranes by its different history, either in its formation or in its subsequent deformation and/or metamorphism. Terranes are separated by faults. An exotic terrane is one that has been transported into its present setting from some distance.”deformationmetamorphism USGS Source: “ Rocks and Geology in the SF Bay Region,” by Philip Stoffer. USGS Bulletin 2195

Unique Bay Area Rocks Metamorphic  Glaucophane schist (“blueschist”) formed under high P-low T in a subduction zone.  Serpentinite - hydrated mantle rocks Mantle rock = ultramafic (Si02-poor), dense, dark Serpentinite = rock (CA State Rock) Serpentine = mineral Should serpentinite be “demoted” as our state rock?

Formation of Blueschist in a Subduction Zone Unique conditions: High pressure combined with low temperature Note depressed isotherms. Due to slow heating of cold, down-going oceanic plate

Unique Bay Area Rocks Igneous  Sea floor basalt Pillow lavas Greenstone (altered basalt) Sedimentary  Graywacke (“dirty” sandstone)  Radiolarian Chert Radiolarians: Tiny ocean animals that make their skeletons of silica (SiO2)

Rocks we’ll see on the field trip Chert Sandstones Pillow basalt  Altered basalt: greenstone Volcanic rocks with:  Phenocrysts  Amygdules

California Tectonics: Present Source: USGS

California Tectonics: Past

Geologic History Video The following slides are from a presentation by Mark Williams, University of Colorado at Boulder

Cenozoic By about 50 million years ago the ancestral Nevadan Mountains had eroded down to relatively low-lying hills. Large rivers flowed through the region and deposited gravels rich in gold. The southern Sierra Nevada and Mojave region were elevated enough to allow for the erosion and deposition of thick layers of marine sedimentary rocks in what are now the southern Coast Ranges and western Transverse Ranges.

Cenozoic This deposition continued without interruption until around 40 million years ago. Over the next 10 million years the coast shifted back and forth, producing a patchwork of marine and non-marine sedimentary rocks in the Coast Ranges and western Transverse Ranges.

Cenozoic million years ago, the oceanic plate that had been subducting beneath the western edge of North America became completely overridden, starting in the south, and the North American and Pacific plates came into direct contact for the first time. Tangential motion and expansion replaced convergent motion as the North American plate began interacting with the Pacific plate. The San Andreas Fault system was formed. In far northern California and the Pacific Northwest, north of the Mendocino triple junction, convergent motion has continued right up to present times.

Cenozoic Volcanic activity, related to extension and thinning of the crust, became widespread in the Sierra Nevada and Mojave regions around 20 mya. Around mya a series of deep marine basins formed along the coast between Orange County and the San Francisco region. The appearance of the basins may have signaled the passage of what is now coastal southern and central California over the spreading zone. The rocks formed in these basins (Monterey Formation) are composed mainly of material derived from marine organisms, rather than terrestrial sediments.

Cenozoic About 5 million years ago mountain-building activity rapidly accelerated, and finally most of the modern mountain ranges were uplifted, including the Sierra Nevada and the large fault-block ranges to the east, the Coast Ranges, the Transverse Ranges, and the Peninsular Ranges. Subduction continued in the north forming the major volcanoes of the Cascades. Pleistocene glaciation in the Sierra Nevada and, to a minor extent, in the San Bernardino Mountains; recent volcanic eruptions in the Mojave and Great Basin regions; and the widespread volcanic activity that created the southern Cascade volcanoes (Mt. Shasta and Mt. Lassen) and the lava flows of the Modoc Plateau region.