Strange Volcanic Sisters on California's Central Coast

A geo gift for Christmas!

One of my relatives has an eye for fun and fitting gifts. Most recently she gave me a tote bag featuring the Nine Sisters of northern San Luis Obispo County—rocky peaks all in a line, reaching from the town of San Luis Obispo to the sea. The northwesternmost, Morro Rock, is the one everybody knows.

Morro Rock, "the most striking scenic feature on the coast of California" (Fairbanks 1904).

The other Sisters are not nearly as famous, though thousands of people race past them daily. Nor is their number agreed upon. When I was a kid on the Central California Coast long ago, there were seven. Now the more commonly used number is nine. But as geologists know, there are many of these very strange peaks—all of the same rock, all very steep, and all neatly aligned. And they wonder: How did this happen? Why are they here?

The 23 Sisters or Morros (Spanish for "hills") are also known as the Morro-Islay Volcanic Complex.

The Nine Sisters are labeled, with others in between; from northwest (top) to southeast (bottom). See full panorama by SLOhiker.

In October of 1860 William H. Brewer, a recent hire with the brand new California Geological Survey, boarded a steamer in New York City. After a week at sea, he arrived in Panama, crossed the isthmus by railroad in a day, boarded a steamer headed north, and ten days later arrived in San Francisco, where he learned of Lincoln's election. The evening was spent observing celebrations—"fireworks, processions, etc."

After supplies had been secured, Brewer and three others traveled back south by steamer to San Pedro, the port of Los Angeles. In early February, they headed north by horse and mule-drawn wagon, prepared to survey as far as Monterey.

California Geological Survey field party, 1864 (not the 1861 crew); Brewer in chair (Brewer 1930).

As they worked their way north, Brewer wrote letters describing the surrounding country and their adventures in great detail. He mailed them to his brother (whenever postal service was available), who had been directed to share them with family and friends, and then hold for Brewer's return. Amazingly, all but two or three letters were delivered, and in 1930 they were published (source of quotes here).

In early April they entered San Luis Obispo County on terrible roads—"no road in fact, but a mere trail, like a cow path, hardly marked by the track of wheels, and often very obscure." A bad wreck in the Arroyo Grande delayed them for a day as they reassembled and reloaded the wagon. They continued north on the so-called 'better' road to San Luis Obispo, arriving a few days later.

Brewer found San Luis Obispo to be "a small miserable town" in a lovely setting:

"San Luis Obispo town lies in a beautiful, green, grassy valley, about nine miles from the sea. ... This valley is more like a plain, from four to six miles wide and fifteen or twenty long, running northwest to the ocean."

They camped near a butte that was "beautifully rounded, about eight or nine hundred feet high and perfectly green."

Brewer's party likely camped near today's Cerro San Luis (Leif Arne Storset photo).

Though beautiful, the butte was quite odd in the way it rose so suddenly from the plain. Even more strange, there were many more like it.

"These buttes are a peculiar feature, their sharp, rugged outlines standing so clear against the sky, their sides sloping from thirty to fifty degrees! ... A string of these buttes, more than twenty in number, some almost as sharp as a steeple, extend in a line northwest to the sea, about twenty miles distant, one standing in the sea, the Morro Rock, rising like a pyramid from the waters."

"Through this plain rise many sharp peaks or 'buttes'—rocky, conical, very steep hills" (hakkun photo).

An unnamed butte, one of many (Ronn Koeppel photo).

As for their geology, Brewer noted only that the buttes were "mostly of volcanic origin, directly or indirectly". If he thought it odd to find volcanos here, he didn't say (1).

Brewer is often credited with today's name: "these buttes are in a line, nine in number, and I propose to call them the Nine Sisters." But in reading his letters I found no such statement. He never called them the Nine Sisters and counted at least twenty. Claude AI found this false quote in multiple places, "copied uncritically from source to source".

Before leaving San Luis Obispo County, Brewer and a companion climbed and measured the Santa Lucia Mountains. It was a lovely day—cool and clear—and views from the crest were worthy of contemplation.

"the breakers on the shore were perfectly distinct twenty miles distant! [italics his] To the southwest and west lay all the lovely plain of San Luis Obispo, the buttes rising through it—over twenty were visible—brown pyramids on the emerald plain. We sat and contemplated the scene for over an hour before leaving."

"brown pyramids on the emerald plain" (Basar photo).

About forty years later, Harold W. Fairbanks of the US Geological Survey surveyed, mapped and described the geology of the San Luis Quadrangle (west-central San Luis Obispo County). He too was struck by the curious line of rocky peaks and ridges, which he called the "San Luis buttes".

"South of the town of San Luis Obispo there begins a line of peaks and ridges which extends northwestward for about 16 miles. It terminates in Morro Rock, lying in the ocean off Morro Bay. This series of buttes constitutes the most striking topographic feature of the quadrangle. There are about 12 ... Many of them are almost completely isolated and rise from the open valleys with bold and frequently precipitous rock faces." (Fairbanks 1904, source of quotes here).

San Luis Obispo c. 1903, with two of the San Luis buttes behind.

"Hollister Peak rises from a base but little above tidewater to a height of over 1400 feet" (Ronn Koeppel photo).

The San Luis buttes were clearly volcanic, the rocks made that clear. Fairbanks identified dacite and andesite in roughly equal abundance; dacite is now considered the dominant type.

Dacite contains visible crystals set in a fine-grained gray groundmass—classic porphyritic texture. This led Fairbanks to call the rock dacite-granophyre ("phyre" from porphyry), a term no longer in standard use. But he was correct about the porphyritic texture, and that led him to another conclusion, also correct. The San Luis buttes are volcanic plugs formed at shallow depths, not extruded magma. They were exposed later when erosion removed the softer surrounding rocks.

Dacite: plagioclase feldspar (large whitish crystals), biotite and quartz in a gray groundmass (Johnston 2021).

In the Geologic History section of his report, Fairbanks tried to place the volcanos in the greater scheme of things, but though their age "could not be definitely ascertained from any observations made." It appeared that their intrusion did not deform adjacent Cretaceous rock, and therefore they must be older. He assigned them to the early Cretaceous Period, between 140 and 100 million years ago (2). We now know they are much younger, emplaced 27 million years ago (Beck & Johnston 2011).

Like Brewer before him, Fairbanks did not try to explain why these volcanos had erupted here. It was an understandable omission. Geology was still a young science; sixty years would pass before geologists came up with widely-accepted explanations for volcanism.

Excerpt from Fairbanks's geologic map; San Luis buttes are the orange blobs from upper left (Morro Rock) to lower right (Islay Hill). Click on image to view.

The great progress geologists have made in deciphering the hows and whys of landscapes is due largely to the theory of plate tectonics. In brief, the Earth's surface consists of giant plates—on the order of a dozen large ones and many smaller. Though massive, they are not stationary. They shift, jostle, collide, rise and sink, expand and contract, and deform each other in various ways. Their movement is much too slow for us to sense, just 2 to 10 cm per year, like the growth of a fingernail. In contrast, the results are spectacular—for example mountain ranges, ocean basins, earthquakes and volcanos.

But in spite of our understanding of plate tectonics, the volcanic buttes between San Luis Obispo and the sea remain puzzling. The problem is their location. Volcanos can't erupt just anywhere; there must be a source of magma. But magma doesn't occur just anywhere. It forms with melting of the mantle, the immense mass of solid but soft rock that lies well below the Earth's surface.

Earth's internal structure (IsadoraofIbiza). The voluminous mantle is the source of volcanic magma, but only under the right conditions.

Though the mantle underlies all of Earth's crust and forms 84% of its volume, it only melts sufficiently for volcanism in special situations. The common ones are: (1) mid-ocean ridges, where two plates are moving away from each other; (2) hotspots perhaps created by rising plumes of anomalously hot mantle (they're controversial); and (3) subduction zones where one tectonic plate dives under another deeply enough to melt (Nelson 2015). The Sisters fit none of these scenarios.

California's Central Coast 40 million years ago, expected areas of volcanism circled in white. But he Sisters erupted into a thick stack of sedimentary and metamorphic rocks (from Johnston 2021; annotations mine).

The Morro-Islay volcanos all erupted into a thick stack of sedimentary rocks (3), well away from the usual tectonic settings. And the amount of magma was far too little to have been produced by a hot spot (think about all the volcanic rock in the Hawaiian islands!). So why did these volcanos erupt here? Because 27 million years ago there was a window of opportunity—specifically a slab window.

If we were to visit the coast of North America 27 million years ago and look west, we would see ocean to the horizon. But something very interesting was going on below the surface. Not far away, the seafloor was spreading along a mid-ocean ridge, with mantle rock welling up and melting, and volcanos erupting (yes, underwater!).

Mid-ocean ridge in action; orange upwelling is melted mantle (USGS).

That mid-ocean ridge was the boundary between two tectonic plates—the Pacific to the west and the Farallon to the east. The entire system was moving eastward, forcing the Farallon Plate to dive under the North American plate. This was a straightforward example of subduction until the mid-ocean ridge arrived. When it reached the subduction zone, the Farallon Plate continued sinking eastward while the Pacific Plate moved northwest. No longer connected, they opened a slab window where mantle could rise, melt, and produce the magma that formed the Sisters.

And there would be more drama—not just volcanos but earthquakes as well. With the Farallon Plate gone, the Pacific Plate continued moving northwest, but now along the coast of North America. Subduction was replaced with a transform fault moving in slips and jerks, periodically wreaking havoc (earthquakes). This slab window turned out to be a major tectonic event—giving birth to the San Andreas fault as well as the Sisters!

Creation of the San Andreas transform fault (parallel but opposite arrows) with the arrival of a mid-ocean ridge (dark pink band) (USGS, highly modified).

To end this story, let's return to its beginning—to Morro Rock and the words of William H. Fairbanks. In Economic Geology, the final section of his report, he wrote:

"The buttes extending from San Luis Obispo northwestward to Morro Rock furnish excellent and durable stone for building purposes. A quarry has been opened on Morro Rock for the purpose of supplying material for the Port Harford breakwater, and blocks of any size can be obtained. It is to be hoped, however, that the grandeur and symmetrical proportions of this mass will not be marred, as equally good material can be obtained from the other buttes."

Morro Rock was quarried off and on from 1889 to 1963. It now belongs to the State of California, and has been designated both a state and historical landmark (more here). And fortunately, its "grandeur and symmetrical proportions " are still with us.

Notes

(1) Brewer's very brief discussion of the origins of the buttes isn't surprising. He was a surveyor, not a geologist. In fact his title was Principal Assistant in charge of Botanical Department. But he was an astute observer, shown by his tally of the buttes for example.

(2) Fairbanks was not convinced that the San Luis buttes were Cretaceous in age. In his Geologic History section he noted "There were at least two epochs of igneous activity during the Cretaceous, and three if the formation of the San Luis buttes be included."

(3) The sedimentary rocks intruded by the Morro-Islay volcanos are part of the Franciscan Complex— a diverse assemblage of sedimentary and metamorphosed rocks accreted to the North American plate during subduction—an accretionary wedge.

Sources

Beck, MD, Johnston, SM. 2011. U-Pb geochronology and geochemistry of the Morro-Islay volcanic complex, southern California. Abstract.

Brewer, WH. 1930. Up and down California in 1860–1864 (introduction by Francis P. Farquhar). Oxford University Press. Available at Hathitrust.

Fairbanks, HW. 1904. Description of the San Luis Obispo Quadrangle, California: Geologic Atlas. San Luis Folio 101, USGS. 7 PDFs

Johnston, SM. 2021. The Morro-Islay Volcanic Chain and what's a slab window anyway? Video lecture. Highly recommended.

Morro Bay National Estuary Program. 2024. A Geologic History of Morro Rock (includes the geology of the Sisters, with simple diagrams).

Nelson, SA. 2015. Structure of the earth and origins of magma. Lecture outlines; very clear!

Sierra Club, Santa Lucia Chapter. The Nine Sisters of San Luis Obispo County. Web Archive.

Wikipedia's Morro Rock article includes the geology of the entire Morro Rock-Islay Hill Complex.