^{Former volcano Mount Konocti overlooks Clear Lake. Photo courtesy Stephen Nakatami of Flickr under Creative Commons license; all other photos by Andrew Alden.}

The Bay Area has been very familiar with earthquakes for at least 20 million years. Volcanoes might seem more remote, but their traces can be seen in many places. Lava flows and ashfall beds are widespread in our rocks, marking the progress of an ancient volcanic center through the area. And while the nearest active volcanoes are beyond the Sierra Nevada, they're close enough to keep an eye on.

California has three major sources of volcanism and one minor one. The minor one is what we have in the Bay Area, but let me mention the others first.

The classic type of California volcanism arises from subduction. This diagram shows how it looks for northernmost California today, with an oceanic plate traveling beneath North America. Water and sediment on top of the downgoing plate acts like a flux, promoting melting in the overlying plate. That's how volcanoes are produced all around the Pacific "ring of fire," and all of California used to look that way.

_{Cross-section from US Geological Survey Professional Paper 1515}

Today the Cascade Range volcanoes are produced by this mechanism. California members of the Cascades include Mount Lassen, which last erupted in 1917, and Mount Shasta, which may have erupted in the 1700s. South of that, this system was interrupted when the San Andreas fault system formed and began extending northward (now the boundary between the two tectonic regimes is a triple junction at Cape Mendocino). An example of Cascade volcanic deposits crops out south of Ocean Beach near San Francisco, a prominent ash bed in the seacliffs of the Merced Formation. It's known as the Rockland ash and came from an ancestor of Lassen volcano, called Mount Tehama, about 600,000 years ago.

The second major kind of California volcanism is beyond the Sierra, in the Mammoth Lakes area and points south. The last "supervolcano" eruption from that area was about 700,000 years ago, and ash from it (the Bishop Tuff) fell here although I don't know where to point you to it. That volcanism is related to stretching of the crust in the Basin and Range province, which basically includes all of Nevada and surrounding counties.

The third type of volcanism is related to Yellowstone, of all places. The geysers and lava flows of Yellowstone are the current location of an eruptive center that began about 16 million years ago in Oregon and slowly burned its way eastward across Idaho, leaving enormous plains of solid lava behind. Northeastern California has a lot of it. One of the first eruptions in that series came from California, sending a glowing flood of basalt lava our way. It flowed more than 200 kilometers as far as Vacaville, where it's mapped as the Putnam Peak Basalt.

_{Boulders of basalt akin to that in Yellowstone and Idaho lie west of Winters.}

You can inspect this rock at leisure on Route 128 west of Winters, about 2 miles west of Pleasants Valley Road. It's the location given at the top of this post.

OK, on to our own local volcanism. As the San Andreas fault system cut northward through our region, it cut off the preexisting subducting plate like a letter opener slicing across an envelope. The plate continued to descend, leaving behind it a traveling "slab window" that briefly allowed the hot underlying rocks of the Earth's mantle to send up magma. In the Bay Area, slab-window volcanic rocks came in three pulses. The first pulse dates from 11 to 8.5 million years ago. Its lavas have been dismembered by motion along the San Andreas and related faults, and now it occurs east of Hollister (the Quien Sabe Volcanics), in the East Bay Hills (well exposed in Sibley Volcanic Regional Preserve), at Burdell Mountain in Marin County, and the Tolay Volcanics between Petaluma and Santa Rosa.

_{Mineral-filled bubbles in lava beds at Sibley Volcanic Regional Preserve}

The next pulse of slab-window volcanism produced the large Sonoma Volcanics between 8 and 2.5 million years ago. A good place to see these is at the Petrified Forest park west of Calistoga, where whole redwood trunks have been fossilized in the silica-rich ash.

Since then, slab-window volcanism has migrated north to the Clear Lake/Geysers region. Mount Konocti, overlooking the lake, is a recent volcanic construction. The famous hot springs get their heat from this volcanism. And the huge geothermal power complex at The Geysers, based on natural steam heated by underlying magma, supplies electricity to the Bay Area, helping you read this story.

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One of California's most distinctive and mysterious bodies
of rock is well exposed at Shell Beach, north of Bodega Bay
in Sonoma County. All photos by Andrew Alden.

A big swatch of the Coast Range is a set of rocks that once baffled generations of California geologists. It's a dog's breakfast of different things, most of them familiar in the region, mixed together with no pattern that anyone could make sense of. The geologists who explored California were no slouches, but all they could do was to map these suites of rocks in a catch-all category called Franciscan melange.

Around 1970 the new theory of plate tectonics found just the place for Franciscan melange, and Shell Beach is just the place to ponder and admire it. I've made several visits there and don't recall any shells—maybe a better name for it is Melange Beach. And right nearby is another mystery from the ice ages. For anyone into geology, Shell Beach is a great workout.

Melange, we now know, is what happens to rocks in subduction zones, which is where one tectonic plate plunges beneath another. Before the San Andreas fault began carrying coastal California sideways to the north, the plate west of us was being subducted directly eastward against North America. (A remnant, called the Juan de Fuca plate, is still doing that off the Pacific Northwest.) Rocks and sediments caught between the plates were mixed and tumbled like snow in front of a snowplow. And that's what melange represents, and that's how the Franciscan got so scrambled. Shell Beach shows us the whole range of the Franciscan in one compact site.

First let's get oriented on the geologic map (from U.S. Geological Survey map MF-2402).

At the top, the Russian River enters the sea at Jenner. Shell Beach is due south of the "Qt" symbol, part of Sonoma Coast State Beach. You can also see it from offshore on the California Coastal Records Project site. "Qt" stands for Quaternary terraces, which I told you about down at Pebble Beach. Up here there are only two terraces mapped, but subtle signs indicate more of them higher up. "KJfs" stands for Cretaceous-Jurassic Franciscan sandstone, but it includes a large share of melange. The tiny orange dot represents Mammoth Rock, which we'll talk about later. Here's the view from the terrace looking south. That's Bodega Head in the farthest distance.

The cliffs are all melange. Most of it is a shale and sandstone matrix that has been thoroughly disrupted by tectonic mixing. The sea stacks out in the water are chunks of hard rock within the melange that have resisted erosion. Where these crop out of our rounded oak-dotted hillsides, the local geologists call them knockers. But resistant blocks occur in all sizes, both larger and smaller.

The stairs down to Shell Beach pass by a big greenish body of serpentinite, our state rock, in the gully. It's worth a detour to inspect them. This soft rock type doesn't form knockers. (I should remind you that all collecting or defacing of rocks is prohibited in this state park.) The beach is small, with dark sand and not much of it, and the coast is cool and breezy—not a place for surfing, picnics or volleyball. What's special about it is the range of rock colors in one place. I'll give you a small sample.

The palette does cluster around green and blue. Greenstone, shown below, is ancient seafloor lava that has been changed by time and pressure, but not enough to hide its original pillow shapes.

Chert is a flinty rock that acquires subtle translucent colors, setting off its waxy luster, during subduction.

What excites geologists, and may catch your eye, is that Shell Beach also exposes the soft matrix rocks that held and polished these boulders during subduction. Matrix is seldom seen elsewhere because it quickly turns to soil or washes away. In addition to all these is eye candy, things you just want to turn into background images or jigsaw puzzles.

If you have time, take the trail north from the beach toward these two ancient sea stacks. The first, Mammoth Rock, is in the center and the second behind it to the right is Sunset Rocks. Some 125,000 years ago, these stood among the waves and endured until the land rose and the sea fell away.

The stacks are a mixture of rock types that is largely blueschist, a tough stone formed by high-pressure metamorphism. At Shell Beach, chunks of it extend the palette all the way to indigo. The second stack has a real treat—polished spots that have been interpreted as marks left by ice age mammoths that used the rock as rubbing posts, just as cattle do today. KQED showed you these in its Ice Age Bay Area series in 2008.

I have presented more detail about the polish here and more about the rocks here.

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^{The tsunami of March 11 broke docks and damaged boats in Santa Cruz Harbor. Most earthquake-generated tsunamis in this part of California will cause similar levels of damage. Photo courtesy Sequoia Hughes of Flickr under Creative Commons license.}

Last week the Bay Area got a tiny taste of Japan's seismic disaster when tsunami waves stirred our waters, a giant agitating the San Francisco Bay and coast with a flick of its pinky. The waves we saw overwhelming the east coast of Honshu were attenuated to small surges here at the opposite side of the Pacific Ocean.

In central California, we will always have good warning of these big seismic tsunamis because they are not created nearby. Our tectonic setting is not conducive to Japan or Sumatra-style tsunamis. But I said seismic tsunamis, the ones that earthquakes cause. There are two other kinds that mean you should always practice tsunami awareness when you're at the beach. And National Tsunami Awareness Week, scheduled by purest coincidence for next week, is a good occasion to train yourself and your family.

Standard tsunami awareness is pretty simple, simple enough to put on a sign that says, "In case of earthquake, go to high ground or inland."

_{Photo courtesy Bruce Manchon, all rights reserved}

That refers to an earthquake that you feel, not one you hear about on the radio. I can be a little more specific. Don't worry about small earthquakes, namely the short, sharp shocks we feel often around here. Worry about a long-lasting earthquake, one with slow rhythms. If one of those happens while you're at the beach, look—you want to leave anyway, because a large earthquake like that may mean trouble at home. If the sea starts acting strange, do what the sign says, period. Otherwise, follow your usual earthquake protocol: Get away without dawdling, drive warily with your radio on, remember your family plan, use your phone no more than absolutely necessary.

The tsunamis that arrive from distant quakes, or teletsunamis, come with several hours of warning. The nearest earthquake faults that could send a damaging tsunami our way—subduction zones—are off northernmost California, part of the Cascadia seismic zone that stretches up the Oregon and Washington coast into Canada. A tsunami arising from a magnitude-8 or larger event up there would arrive here at least a couple hours later. Tsunamis from major earthquakes in Alaska, far eastern Russia, Japan and the Philippines will give us much longer warning times. There are enough people on a typical beach, with phones and text devices and radios, that you should be able to count on sufficient warning even for a Cascadia event. In addition, local emergency responders will be out in person to warn beachgoers. (If you're on the beach alone, be more alert.)

If you hear about an approaching tsunami, I must advise you: don't be irresponsible and rush to the beach. We're all intrigued by geological phenomena, and every red-blooded geologist has "witness a tsunami" on his or her geological bucket list. But remember the person taking pictures at Crescent City (a town also ravaged by a tsunami from the 1964 Alaska earthquake) who was washed out to sea. Think about the surfers who wandered around Santa Cruz Harbor, risking themselves and worrying others, as the waters rushed in and out.

However, if you choose to ignore my advice, then you should do as I wish I could have done, and proceed in a responsible manner to a safe place high above the water, obeying authorities, not congesting emergency escape routes, prepared for the worst. UC Santa Cruz geologist Christie Rowe did that and recorded the tsunami's arrival. She adds, "I would advise people not to panic, to check the West Coast Tsunami Warning Center website and select a vantage point well above the predicted wave height."

But not every tsunami is a seismic tsunami. Two other kinds of tsunamis, not monitored by dedicated networks, have a chance of happening somewhere in the world during the average lifetime: landslide and impact tsunamis. A landslide tsunami, caused by large mass movements into or beneath the sea, is quite plausible along our steep coasts and rugged offshore seafloors. Be wary of one even after a relatively small local quake. An impact tsunami, caused by an object from space crashing into the ocean, has no upper size limit and no preferred location. The odds are small but every beach in the world, ours included, faces the risk. So be like a sailor and always keep your weather eye out.

Learn more:
California tsunami information
National Tsunami Awareness Week
tsunami.gov
West Coast/Alaska Tsunami Warning Center

And check out QUEST's story "Scary Tsunamis":

QUEST on KQED Public Media.

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The Great Wave off Kanagawa is often mistakenly associated with the Tsunami.

"If a tree falls in a forest and no one is around to hear it, does it make a sound?"

The philosopher George Berkeley posed this philosophical question and a quick internet search found a somewhat scientific answer in an 1894 issue of Scientific American. There they wrote: "Sound is vibration, transmitted to our senses through the mechanism of the ear, and recognized as sound only at our nerve centers. The falling of the tree or any other disturbance will produce vibration of the air. If there be no ears to hear, there will be no sound."

Maybe sometimes vibrations are heard much later, only when the right person is listening.

On January 26, 1700, at about 9:00 p.m. Pacific Standard Time one of the largest earthquakes ever to strike the Pacific Northwest rumbled across the Cascadia Subduction Zone. This massive earthquake sent a giant 33 foot high tsunami crashing onto shore, inundating the quiet coastline. While there is no written account describing the earthquake, tsunami or consequential damage, the devastation was enormous.

So wait. If there was no written record, how can we know the exact time and date when the tsunami struck? How can we know how big it was or what kind of damage it did? It took some digging and an impressive bit of scientific detective work by geologist Brian Atwater. First scientists discovered an unusual layer of sand in a marsh area that left a clue that a wave had struck, taken sand from offshore and brought it far inland. The scientists were able to date this thin sand deposit to around 1700, plus or minus 25 to 50 years. Then through tree-ring dating they were able to narrow that down to within five or ten years. Further study of tree roots narrowed it down even further to winter, 1700. Then investigators went to Japan and checked for evidence of a tsunami during that time. They looked for one which did not have a known earthquake associated with it. These were known as “orphan tsunami." There, in the records from 1700, was a tsunami the struck Japan, a wave that had the right pattern, right size, and was generated at the same place, the Cascadia Subduction Zone all the way on the other side of the Pacific Ocean. January 26, 1700, 9:00 p.m.

Can it happen again. Yes. Are we listening?

Watch the Scary Tsunamis television story online.

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