Geology About San Mateo, CA

San Mateo is a city in San Mateo County, California located on the San Francisco Peninsula. With a population of over 100,000 people, San Mateo is known for its mild climate, picturesque setting and historic downtown. The geology of San Mateo and the surrounding area reveals a complex and fascinating history spanning millions of years.

In this comprehensive guide, we will explore the key geological features, formations and events that have shaped the landscape of San Mateo. From ancient seabeds to active fault zones, this post covers everything you need to know about the geology underpinning this Californian city.

Regional Geologic Setting

San Mateo is situated on the San Francisco Peninsula which separates the Pacific Ocean from San Francisco Bay. The peninsula itself is an extension of the California Coast Ranges geomorphic province.

This province consists of numerous roughly parallel northwest-trending mountain ranges, valleys and faults that extend over 600 miles from the Transverse Ranges in Southern California to the Klamath Mountains near the Oregon border.

The coastal mountains of the San Francisco Peninsula were uplifted due to the subduction of the Farallon tectonic plate under the North American plate.

This process of terrane accretion added new material to the western edge of North America and caused crustal shortening and uplift. The peninsula is bounded by the San Andreas Fault zone to the west and the Hayward Fault zone to the east.

Local Bedrock Geology

The bedrock geology of the San Mateo area consists primarily of marine sedimentary rocks of the Franciscan Complex. Formed between 140 and 80 million years ago during the Late Jurassic to Late Cretaceous periods, these rocks were originally deposited as sediments on the ancient Farallon seafloor. They were then accreted onto the North American Plate during subsequent tectonic activity along the subduction zone.

The most common rock types of the Franciscan Complex found in San Mateo are greywacke, shale, chert, greenstone and basalt. Greywacke is a hard sandstone made of angular rock fragments in a fine-grained clay matrix.

Shale is a fissile mudstone that splits easily into thin layers. Chert is a very hard, dense sedimentary rock form of microcrystalline quartz. Greenstone and basalt are volcanic in origin, formed by ancient submarine lava flows.

Within San Mateo, Franciscan Complex rocks are mostly found in the eastern hills while younger alluvial sediments make up the flat western portions of the city. The regional northwest structural grain of the California Coast Ranges is evident in the parallel ridges and valleys of the area.

Local Faults

The two most significant active fault zones near San Mateo are the San Andreas Fault and the Hayward Fault.

The San Andreas Fault passes less than 2 miles west of San Mateo at its closest point. This famous right-lateral strike-slip fault forms the tectonic boundary between the Pacific Plate and the North American Plate.

The 1906 San Francisco Earthquake, estimated Magnitude 7.9, ruptured the northern segment of the San Andreas Fault and caused major damage in San Mateo.

The Hayward Fault lies about 6 miles east of downtown San Mateo. This fault similarly displays right-lateral strike-slip offset. While not as long as the San Andreas, the Hayward Fault is considered one of the most dangerous in California because of its past activity and probability of future ruptures. The 1868 Hayward Earthquake (estimated Magnitude 6.8) was the last major event.

Other nearby potentially active faults include the Seal Cove Fault, San Gregorio Fault and the Pilarcitos Fault. While these faults are considered lower risk, they demonstrate that San Mateo lies within a broad zone of faulting and seismic hazards.

Natural Resources

Historically, the natural resources that drove early settlement and development of San Mateo were timber and water. The forested slopes of the Santa Cruz Mountains to the west provided vast amounts of coast redwood and Douglas fir lumber. More importantly, the mountain rainfall fed numerous freshwater creeks that still traverse San Mateo today.

Some of the major creeks are Laurel Creek, Borel Creek, San Mateo Creek and Beresford Creek. These streams provided a vital source of water for the early agricultural development of the San Francisco Peninsula.

Limestone deposits are also present east of San Mateo, near the unincorporated community of San Mateo Highlands. Limestone and dolomite were mined in this area in the late 1800s to supply rock for construction and cement production in San Francisco. While inactive today, these quarries provided important building stone for the region.

Modern Developments

The expansion and development of San Mateo over the past century has both utilized and impacted the local geology. Here are some of the major ways that humans have interacted with the geological landscape:

• Land reclamation – Large areas west of Downtown San Mateo towards the Bay were historically wetlands and tidal flats. Through the use of dredged sediments, these areas were filled in and reclaimed, allowing for urban growth and development. The Marina Lagoon is an example of one such area that was cut off from the Bay by transportation infrastructure.
• Foundation engineering – Constructing buildings and infrastructure on the variable geology and soils of San Mateo has required advanced engineering techniques. Methods like pilings, drilled piers, and improving subsurface conditions allow construction to overcome issues like soft soils and liquefaction. The new Bay Meadows Development utilized many such techniques.
• Groundwater management – Extensive pumping of the groundwater basin underlying San Mateo has led to overdraft conditions in the past. The construction of reservoirs like Crystal Springs Reservoir helps manage recharge and store surface water to reduce reliance on stressed aquifers. Monitoring programs track groundwater levels and quality.
• Slope stabilization – The hills and valleys characteristic of San Mateo necessitate stabilization measures like retaining walls, vegetation planting and grading controls. This minimizes landslide hazards, especially important in residential developments. The city’s Hillside Design Guidelines regulate construction on steeper slopes.

Conclusion

In summary, San Mateo’s scenic landscape owes much to its underlying geology. The uplifted Franciscan Complex bedrock, extensive fault zones, water-bearing creeks and historic resources all tell the story of this region’s formation.

While seismic hazards exist, proper engineering helps mitigate these risks. Understanding the dynamic geology remains key for sustainably developing the Greater San Francisco Bay region.

References

• USGS: San Francisco Bay Region Geology and Geologic Hazards
• USGS: Geologic History of San Mateo County
• California Geological Survey: Geologic Map of the Palo Alto 30′ x 60′ Quadrangle
• San Mateo County Geology and Building Laws
• Peninsula Geological Society – San Mateo County Geology