The Geology of Muir Woods: Seafloor Rocks Beneath the Redwoods

A ranger holds serpentine, one of the rocks of the Franciscan Complex that underlies the redwoods' soil. NPS / Jace Ritchey.

A story that began at sea

The geologic history of Muir Woods began long ago and far out at sea, and it involves all three kinds of rock:

• igneous — formed as lava and magma cooled;
• sedimentary — tiny particles slowly cemented together;
• metamorphic — one kind of rock transformed into another by heat, pressure or both.

The Franciscan Formation

Muir Woods lies in California's Coast Range, 12 miles north of the Golden Gate Bridge, in the shadow of Mount Tamalpais (2,571 feet). Its rocks belong to the Franciscan Formation, a chaotic mix of rocks that began as crumpled seafloor, laid down about 150 to 80 million years ago. It is named for San Francisco, where it was first studied, but runs from southern California to Oregon. Fossils in it are rare.

The Franciscan rocks make ideal soil for coast redwoods, which grow only where the rocks are of Franciscan age. In Marin County, redwoods are limited to Muir Woods and the Mount Tamalpais area — not to the north, where younger sediment covers the Franciscan rocks, and not west of the San Andreas fault, on the Point Reyes peninsula, which belongs to the oceanic plate. The redwoods' range is bound up with the region's geology.

Basalt

The area's igneous rock is basalt, formed by underwater volcanoes far out in the Pacific — little more than cracks in the seafloor that slowly spewed lava over centuries. Basalt is dark, and full of tiny holes left by gas bubbles. Here it is pillow basalt: erupting into seawater, it cooled quickly into pillow-shaped blobs about a yard long.

Radiolarian chert

Plate tectonics carried these rocks toward North America until the plates collided. The heavier, denser oceanic plate was pushed beneath the continent, and rocks scraped onto the continental plate began to build the Coast Ranges.

While still underwater, the basalt collected sediment made mostly of radiolaria — single-celled plankton with elaborate skeletons of silica, the same mineral as glass and quartz. Over many years it hardened into chert.

• Look: red-brown in Muir Woods, though the color varies with the minerals in it, in bands no more than 2 or 3 inches thick, separated by thin layers of shale made from silt blown from the continent. In Marin, its wavy layers set it apart.
• Where: in the road cuts on the drive into Muir Woods.
• Hardness: the hardest rock in the area. The Coast Miwok, indigenous to Marin, traditionally use it for spearheads and stone knives.

Rocks of the Franciscan Complex on the Marin coast, including chert with its wavy layers. Image from the NPS's page.

Serpentine

As the plates kept colliding, the rocks were pushed above the sea, raising Mount Tamalpais. The enormous pressure folded and deformed the land so irregularly that the region's peaks and valleys follow no pattern — the twisted rocks in the road cuts above Muir Woods show it — and erosion later carved valleys such as Muir Woods' Redwood Canyon.

As the oceanic plate was driven deeper, it reached the mantle, and mantle rock was forced up through cracks above, transformed on the way. The most common result is serpentine, a metamorphic rock that forms when peridotite — most of the mantle's material — reacts with water, usually beneath the crust. California's state rock, it is bright green, blue or gray, and crops out along several trails in Muir Woods.

Serpentine, a gray-green rock common in the Franciscan rocks. Image from the NPS's page.

Muir Woods today

The rock cycle goes on in Muir Woods. Rock building, erosion and change continue too slowly to see, and over millennia will reshape the canyon in ways no one can predict. One thing is certain: thanks to their long lives and Muir Woods' protection, the redwoods will still be there to see it.

More: the Park Service's pages on geologic activity, serpentinite, chert and the basalt at Point Bonita, and the Muir Woods Geodiversity Atlas.

Sources

Based on "Geologic Overview of Muir Woods," Muir Woods National Monument, National Park Service; a work of the United States government in the public domain. A caption on the source dates the Franciscan rocks to 80–200 million years ago; this page follows its text. A map from the Save the Redwoods League and a photograph credited only by name are not reproduced.