The Golden Gate

The Golden Gate. Deep Sea Geology Exposed. , The scene before you is the result of a tectonic collision that went on for millions of years, beginning in the Jurassic period – about 200 million years ago – and ending here about 15 million years ago. During that time the Pacific seafloor thrust beneath the Franciscan Complex along 500 miles of this coast. The 85-million-year-old sandstone rocks on which you are standing, resulted from that collision., In fact, the headlands on both sides of the Golden Gate are world-renowned for their deep ocean rocks now visible at the Earth’s surface: serpentine formed in the Earth’s mantle, pillow basalt from the ocean floor, and deep-sea sediments including chert from the open ocean, and the graywacke sandstone seen here from the continent’s margin. Graywacke and pillow basalt are the most common rocks at Lands End., As important as gold was to the modern history of California and San Francisco, the Golden Gate Bridge does not take its name from the precious metal, but from the dramatic geology it spans., Explorer John C. Fremont named the harbor entrance Chrysopylae, or Golden Gate, in 1848; inspired by Chrysoceras, or Golden Horn (also known as the Bosphorus Strait) in Istanbul, Turkey., “Between these points is the strait about 1 mile broad in the narrowest part, and 5 miles long from sea to bay. To this gate I gave the name Chrysopylae, or Golden Gate.” , John C. Fremont, Lieutenant , U.S. Corps of Topographical Engineers, 1848, Images, left to right: , The tectonic plate boundary at the western margin of North America makes this a perpetually dynamic landscape. The Golden Gate headlands and entire Franciscan Complex were forged millions of years ago where the Pacific crust moved (subducted) beneath the North American Plate. Today, tectonic activity continues along the plate boundary at the San Andreas Fault, where the still moving plates cause California’s notorious earthquakes., Graywacke sandstone beds surrounded by shale at Lands End were deposited at the continental margin by underwater landslides that poured into the subduction trench., Pillow basalt (altered to greenstone) was erupted from volcanoes at a mid-ocean ridge before being transported to the subduction zone, where it became attached to North America.