Saturday, July 4, 2015

Driving Through the Most Dangerous Plate Boundary in the World: A Compilation of Fear(somely cool geology)

Without a doubt, subduction zones are the most dangerous plate boundaries on the planet. Divergent plate boundaries produce earthquakes and occasional volcanoes, but nothing on the fearsome scale of the calderas and stratovolcanoes and magnitude 9 earthquakes experienced at convergent boundaries. Transform boundaries produce earthquakes, but they are magnitudes smaller than those produced at convergent boundaries (despite what certain Hollywood movies have asserted recently). Hot spots, while not a plate boundary, can produce huge caldera complexes like Yellowstone, but such monsters have not had much of an effect on human history of the last few thousand years. It is the subduction zones of our planet that have caused the most human misery, in the form of massive earthquakes, tsunamis, and violent volcanic eruptions.
We have been driving through an example of one of the most dangerous plate boundaries in the world, but our particular example has been inactive for a very long time. Central California was a subduction zone complex for more than 150 million years, primarily during the Mesozoic era, but it changed into a transform boundary only a few tens of millions of years ago. The San Andreas fault is the resulting feature, and it is capable producing damaging earthquakes, but even the most destructive quakes, like 1906 in San Francisco (death toll 3,000), is but 1/30 of the energy of a magnitude 9 quake like that of Indonesia in 2004 (where the resulting tsunami killed 200,000 people).

It's taken a couple of months to work through our journey, so I've compiled all of the posts here so one can catch the continuity of the story. Here goes...

A New Blog Series
The introduction to the new series, a geological transect from the California Coast to Yosemite Valley, crossing an ancestral subduction zone that once caused geological havoc in a zone from Mexico to Canada (and still is in a few places).

Reconnaisance

An overview (in the most literal sense) of the lands we will traverse on our journey. We have a look at central California from above.

These Rocks are All Wrong!
Granite is exposed in the rocks of the Point Reyes Peninsula. But the arrangement of rock and sediment in subduction zones suggests that granite shouldn't be anywhere near here. It's the San Andreas fault. In California, it's always the San Andreas' fault.

Looking for the Big One
The peace and serenity of Tomales Bay belies a violent past. The San Andreas fault slices right through the bay, and produces large earthquakes with disturbing irregularity. The epicenter of the San Francisco quake in 1906 was not far from here.

Welcome to Geology's Junk Drawer
The Marin Headlands began as a giant collector of geological flotsam and jetsam from the crust of the Pacific Ocean. The jumble of rocks accumulated in an accretionary wedge, and were later lifted up into the mountains of the Marin Peninsula.

Geology's Junk Drawer on the Marin Headlands
Exploring the hidden corners of the Marin Headlands, we find Redwood forests, beautiful views of San Francisco, and disturbing reminders of World War II.

Terra Fatale on the Marin Headlands
Although the subduction zone that formed the rocks of the Marin has been extinct for a long time, there are still plenty of hazards remaining in the region, both geological and nautical. The Point Bonita Lighthouse has been present in one form or another for 160 years. It hasn't always worked, as there are upwards of 300 shipwrecks in the area. There are other hazards too.

The Alien Bursts Forth in the Diablo Range!
We head across San Francisco Bay and start an arduous journey through the Diablo Range. The range formed as an exotic (read "alien") terrane pushed upwards through the sediments of the Great Valley Group, piercing the surface and rising into the sky. John Hurt would no doubt approve...

Exploring the Belly of the Beast in the Diablo Range
We make the first part of a long drive through a rugged portion of the Diablo Range, one of the largest sub-ranges in the Coast Ranges of California. Along the way we cross through two exotic terranes of rocks that had been carried miles deep into the crust in the accretionary wedge of the subduction zone.

At the Portal of Hell in Diablo Range
Making our way down Del Puerto Canyon in the Diablo Range, we do the equivalent of traveling through the crust of the Earth all the way into the mantle, finding outcrops of peridotite and dunite, rocks containing the mineral olivine among others. For the mercury miners, this truly was a portal to Hell.

Exploring the Ocean Crust without Unobtanium
Del Puerto Canyon cuts a swath through the Coast Range Ophiolite, the remnants of the ocean crust that once lay at the bottom of the Pacific Ocean. Though mostly in private ownership, the canyon is one of the most scenic in the Coast Ranges, and a pair of county parks in the upper reaches invite exploration.

Into the Realm of the Drowning Dinosaurs
Sediments accumulated in a deep trough along the west coast of North America called a forearc basin for upwards of 100 million years. The layers reached a depth of 5 miles! In those waters swam mosasaurs (yes, like in the recent Jurassic World movie, but they forty feet long, not a hundred), plesiosaurs, ammonites, and occasionally a drowning dinosaur. The first discovery of a dinosaur in California happened here in 1936.

The Sea Floor that became the Greatest Agricultural Region on Earth
The American Serengeti is a vast plain 400 miles long and 50 miles or so wide that once was the sea floor. The grasslands of the Great Valley once supported millions of migratory birds and grazing animals. It still supports millions of organisms, but these days, those organisms are humans. 95% of the original prairie has been developed for agriculture and the region produces a quarter of the nation's produce. And all of the almonds and walnuts.

In the Pleistocene, a Different Kind of Danger
The Great Valley would have been a most dangerous region for a different reason in the Pleistocene. Among the great herds of grazing animals there were predators, and they were adapted to bringing down giant prey, not the small game we find today. Gigantic Short-faced Bears, Saber-tooth cats, American Lions, Jaguars, and Dire Wolves.

The Dr. Who of Mountain Ranges
At least three Sierra Nevada ranges have existed throughout time. They might have even once been higher than today. The distorted deformed metamorphic rocks tell the story of the earlier ranges.

A Gentle Landscape Belies a Fiery Past
The Valley Springs formation, exposed throughout much of the Sierra Nevada foothills region, forms gentle grass and oak covered slopes, but the rock is made of volcanic ash that originated in monumental explosions millions of years ago. The ash came from gigantic calderas, some of which were hundreds of miles away in central Nevada.

A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon

A lot of volcanoes in the world are made of mud. Lots and lots of mud. But this mud formed in violence. The Mehrten formation of the Sierra Nevada has other surprises too: gigantic tortoises, and six-foot long salmon...with fangs...

A Tale of Two Subduction Zones
There have actually been at least two subduction zones in California. Remains of the older one still make up the rocks of the Sierra Nevada Mother Lode, and those rocks were the source of gold in the Gold Rush.

Exploring the Underside of the Volcano

We wrap up the series in the heart of the ancient magmatic arc: Yosemite Valley. Walking among the towering cliffs, we are reminded that the rocks were actually formed within the magma chambers of volcanic systems, perhaps similar to Lassen Peak, Mt. Shasta, or even at times, Yellowstone.

Driving Through the Most Dangerous Plate Boundary in the World: Exploring the Underside of the Volcano

Quick, name the nation's oldest national park. I'll even give you a clue, it starts with 'y'. Did you guess Yellowstone? You are right. Did you guess Yosemite? You are also right, in a sense. Have you ever said "Yosemite" when you meant "Yellowstone"? I've done that plenty of times.
Castle and Beehive Geysers at Yellowstone National Park, Wyoming
Yellowstone technically is the nation's oldest national park, having been established in 1876 by an act of Congress, but Yosemite gains credit as the first "idea" of a permanently protected area when the valley was ceded to the state of California in 1864 by Abraham Lincoln.
Yosemite Falls from Swinging Bridge in Yosemite Valley

It's hard to think of two parks more different from each other. Yellowstone, with geysers, hot springs, bison, and moose. Yosemite, with huge vertical cliffs of granite and high alpine peaks. In Yosemite, the bears break into your car. In Yellowstone, the bears occasionally eat you.
Sunset in Yosemite from the Gateway View
The parks in one sense are very similar, and the similarity can be summed up in one word: "felsic". The word combines the mineral "feldspar", and the chemical "silica". These are the principle components of the plutonic rock granite, and the volcanic rock rhyolite. The rocks are identical in chemistry, but differ in origin: granite is silica-rich magma that cooled slowly deep in the crust, while rhyolite is a volcanic rock that often forms during extremely violent eruptions. The rocks of Yellowstone are mainly rhyolite and related rocks that formed in caldera eruptions, while those of Yosemite are principally granitic.
El Capitan in Yosemite Valley
We've reached the culmination of our journey across the most dangerous plate boundary in the world. As I've said before, it's not as dangerous today as it was in the Cretaceous period, but we can use these rocks to better understand subduction zones elsewhere in the world (it's a lot easier to study rocks at the surface than it is to drill miles into the crust, after all). We started the journey on the coast at Point Reyes National Seashore, worked our way along the Marin Peninsula, crossed into the Coast Ranges at Lick Observatory and Del Puerto Canyon, crossed the Great Valley, and worked our way up into the Sierra Nevada. And now we stand in Yosemite Valley, one of the most spectacular exposures of granitic rock on the planet.
Lassen Peak and Brokeoff Mountain, an active volcanic center in the southern Cascades Range

The subducting slab of oceanic crust in Cretaceous time was heated, and water that was released changed the melting point of the rocks at the base of the continental crust. The masses of molten rock worked their way up into the upper crust. Some of the rocks erupted at the surface as large Andean-style volcanoes, or as rhyolitic caldera eruptions. The chain of volcanic features is called a magmatic or volcanic arc. The surface may at times have even resembled Yellowstone, although present day Lassen Volcanic National Park or Mt. Shasta provides the best analog. But deep in the crust the rock was cooling slowly, developing into large crystals of quartz, feldspar and mica, then principle minerals of granite and related plutonic rocks.
El Capitan Granite, with gray-looking quartz crystals, white feldspar, and black biotite mica

Yosemite Valley provides a 3-D view of seven or eight different intrusions of granitic rock (for the purists, these include granite, granodiorite, tonalite, and diorite). Some of the rocks, principally the granite, stand out as bold cliffs like El Capitan and Cathedral Rocks. Other rocks were more easily eroded and formed recesses. The combination gives Yosemite the unique appearance that sets it apart from so many other glacially carved canyons.

When you visit Yosemite, take a moment to realize you are exploring the underside of massive volcanoes, quite literally Yellowstone or Lassen or Shasta from the inside out. It never ceases to amaze me that while the 3,000 foot cliffs are spectacular, they once were buried 4 or 5 miles deep in the crust, and that erosion has removed the missing rock (and filled our Great Valley in the process).
Half Dome from Washburn Point above Yosemite Valley

We've completed our exploration across the most dangerous boundary, and we are still alive! It's been several months with lots of interruptions, so you can expect a compilation of all the posts soon so you can get the whole story in one place. I hope you've enjoyed the journey!

Thursday, July 2, 2015

Driving Through the Most Dangerous Plate Boundary in the World: A Tale of Two Subduction Zones

To many people, the Sierra Nevada means granite. The Sierra Crest, with towering peaks of granite carved by glaciers. Yosemite and Hetch Hetchy valleys, with vertical walls of granitic rock. Lake Tahoe, surrounded by granite peaks. Those who visit the mountains find "granite" to be three-quarters true. A quarter of the mountain range is composed of other rocks. Some slopes are covered by the volcanic rocks mentioned in the previous post. But in the Mother Lode, that of California's legendary Gold Rush, the slopes are composed of metamorphic rocks. And the story told by the rocks is fascinating.
It seems that one subduction zone wasn't enough for California. There have been at least two. The metamorphic rocks of the Sierra Nevada Mother Lode are the remains of the earlier zone, which was active during the Mesozoic era. Sometime in the Cretaceous period the subduction skipped west to become the Franciscan Subduction Zone that we have been exploring throughout the first part of this series on the most dangerous boundary in the world.
One might expect the rocks of the Mother Lode, having formed in the same manner as those of the Coast Ranges, would look the same. But they are a world apart. Why the difference? The older rocks have undergone some serious metamorphism over the long ages. Metamorphic rocks are those which have been changed by heat and pressure into a harder, denser form than the original rock. Shale (which is a soft rock that forms easily eroded slopes in the Coast Ranges) is transformed into slate or phyllite, hard rocks that can resist erosion, forming prominent "tombstone rocks" in the Sierra foothills. Volcanic rocks like basalt or andesite will change to an equally tough rock called greenstone, because the rock is, well, often green in color. Serpentine is commonly found in relation to the fault zones.
One of the most spectacular rocks formed when coral reefs or limestone terranes were added into the subduction zone chaos. The rock metamorphoses into marble, and being made of the mineral calcite, is easily dissolved when immersed in slightly acidic groundwater. Caverns are the result and there are hundreds of them in the Mother Lode. Natural Bridges (below) are accessible by a short trail near New Melones Reservoir. A small tributary of the Stanislaus River, Coyote Creek, flows through two caverns, partly composed of spring deposits.
The site is free and is popular in summer, especially for inner-tubers who float through the caves. At other times of the year, the caves are quiet and contemplative. Getting there requires hiking a relatively gentle mile-long trail.
One can also explore the commercial caves, of which Moaning Cavern is probably the most famous (the fact that a former student is a guide there is probably clouding my objective opinion). It has a huge vertical room with a unique spiral staircase constructed from the scrap iron of a World War One battleship. The other commercial caves are also spectacular and worth a visit: Black Chasm, California, Mercer, Boyden (in Kings Canyon), and Crystal (Sequoia National Park, and run by the natural history association).

The metamorphic rocks of the Mother Lode consist of three (actually more, but let's keep things a bit uncomplicated) terranes which become progressively older to the east. They include the Foothills terrane (with mostly Mesozoic rocks), the Calaveras Complex (late Paleozoic and early Mesozoic rocks), and the Shoofly Complex (the oldest, containing Paleozoic rocks). One of the best ways to see them is to follow Hornitos Road to Highway 140 through Mariposa, following the Merced River to Yosemite. The town of Mariposa and the Melones Fault are the boundary between the Foothills rocks and the Calaveras Complex. The boundary with the Shoofly is less obvious, but lies a short distance upstream from the "Geological Exhibit" near Savage's Trading Post on the Merced River.
Follow the route at the right time in the spring and you will be treated to a flower show that will astound. Imagine slopes a thousand feet or more covered by poppies and lupines. It didn't happen to any great extent this last spring though; the ongoing drought affects many aspects of life.
At the geological exhibit, one can view some spectacularly folded chert deposits in the river channel. The chert began as one-celled creatures in the open seas of the Pacific Ocean. The thick powdery rock (diatomite) was compressed and altered to the much denser and harder chert. It was swept into the subduction zone where it was deformed. These rocks bear a strong resemblance to the chert of the Marin Headlands

Up next: the culmination of our journey through the most dangerous plate boundary in the world!

Wednesday, July 1, 2015

Driving Through the Most Dangerous Plate Boundary in the World: A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon


Mt. Shasta is a big mountain. A really big mountain. Reaching an elevation of 14,179 ft (4,322 m), and rising nearly 10,000 ft. (~3,000 m) above the surrounding terrain, it has a volume of around 100 cubic miles. As such, it is the biggest volcano in the Cascades volcanic arc, and probably is on the short list of largest stratovolcanoes in the world (shield volcanoes like those of the Hawaiian Islands have a different origin and are much, much larger). The volcano is the surface manifestation of the sinking of the Pacific lithospheric plate beneath the North American continent along the Cascadia Subduction Zone. As the plate sinks deeper, superheated water is liberated, which lowers the melting point of the deep rocks, forming magma chambers that sometimes erupt at the surface.

The thing is, one would think that this massive edifice would be made of lava and volcanic ash, but a large percentage of the volcano is actually composed of...mud. Simple mud. Well, more properly, mudflow deposits, or lahars, as they are called by geologists. This isn't the kind of mud you find on the bottom of a lake or the sea. It was mud born of violence. The mud includes massive boulders, showing that the formation of the lahars involved the rapid melting of snow and ice during violent eruptions. The mixing of water, ash and rock fragments forms a slurry that moves quickly down the flanks of the volcano. To humans, they are dangerous. A lahar in Colombia in 1985 killed around 23,000 people in a matter of minutes.

On our journey through the most dangerous plate boundary in the world, we've explored the accretionary wedge and forearc basin of the massive subduction zone that once existed off the California coast, and now we've reached the base of the most visible part of the system: the magmatic (or volcanic) arc . The arc is extinct today in this part of California, but one is still active in the Cascades of Northern California and the Pacific Northwest.

Driving through the foothills of the Sierra Nevada in places like Knight's Ferry or LaGrange, just north of the Yosemite National Park region, it's hard to visualize the violent past of this landscape. The gentle slopes belie the intensity of the floods and flows that carried the boulders for forty or fifty miles from the volcanic centers that once existed near the crest around Sonora Pass and the Dardanelles. The rocks are called the Mehrten Formation, and they were deposited around 5 to 7 million years ago, when the subduction zone was still active at this latitude. Similar mudflow deposits cover much of the northern Sierra Nevada.

Of course, the volcanoes weren't always erupting. Decades or centuries may have passed between eruptions, and the tattered ecosystem would recover. Petrified wood has been found in the formation, and fossils of horses, camels, and other animals that lived on the river floodplains emerging from the mountains above. A few years ago I became aware of some of the most unique fossil species to be discovered in the formation, gigantic tortoises, and six-foot-long salmon...with fangs! The Sabertooth Salmon (Oncorhynchus rastrosus) once swam the ancestral Stanislaus and Tuolumne Rivers, right here in my own back yard. That's almost as cool as living in a place that once had Mosasaurs (oh yeah, they lived here too).

A former student of Modesto Junior College (and current geology major at CSU Stanislaus), Jake Biewer, is working this summer on a new exhibit at the Great Valley Museum highlighting his research on the Sabertooth Salmon and the large tortoises of the Mehrten Formation. I'm looking forward to seeing the results!


In our next post, we'll discover what it's like to be living beneath the volcano. In the meantime, check out this video of a lahar at Mt. Shasta.




Monday, June 29, 2015

Look West, Young Man (and Everyone Else): It's a Conjunction!

There are interesting things happening in the western skies this week. All month, there have been two bright objects in the evening sky, getting closer and closer. They aren't really getting physically closer, as Jupiter is far beyond Venus, but their line of sight is making them appear close together. Such "near-misses" are called conjunctions. They will appear closest on June 30 and July 1, so be sure to check it out!

It's been a long time since I've been able to photograph two planets in one frame at a magnification high enough to catch Jupiter's moons. Maybe tomorrow I'll even try a tripod (tonight I was just leaning on the hood of the car in the front yard).
I've been watching the dance of the planets for two weeks now. At Natural Bridges National Monument on the Summer Solstice, I caught the new moon adjacent to the planets. The Ancestral Puebloans were keen astronomers, designing their cities in the desert to align with astronomical and seasonal phenomena. Something like this in 1150 AD would most certainly have caught their attention.
It was a real privilege to be in a series of parks that are so remote from human light sources. The night skies were brilliant. The nights were nearly as interesting as the days at times. Not only is there the sky with the stars, planets, and Milky Way, but the night sounds and odors as well.

Get more information about the conjunction from EarthSky at http://earthsky.org/tonight/watch-the-great-race-of-venus-and-jupiter-in-june-2015.

Sunday, June 28, 2015

Home From the Back of Beyond: Some Images of Strange and Wonderful Places

Thunderstorm near Bandelier National Monument
No, it's not a volcanic eruption, but with the light show that followed that evening, it might as well have been. We were in the high desert of New Mexico at Bandelier National Monument, and the monsoons had arrived early. The lightning flashed every second or two for hours that night. It was magical.
Joshua Trees outside of Rainbow Basin, Mojave Desert of California
I'm back from a long, but epic journey with my students through one of the most intriguing landscapes in North America, the Colorado Plateau. We crossed the California deserts to Arizona and New Mexico, swinging north through southwestern Colorado and southern Utah. Our last few days brought us through the Basin and Range Province of western Utah and Nevada.
Banshee Canyon in Hole in the Wall, Mojave National Preserve
As with all trips, I try to do new things whenever I can, but the real purpose was to open a different world to our students. This class was a hybrid course that explored the geology and archaeology of the region. It's a marvelous region for doing such a class, as humans have impacted the landscape, and the landscape has left its mark on humans.
The Citadel ruin and San Francisco Peaks from Wupatki National Monument, Arizona
The land has seen a parade of cultures over the centuries. Understanding why they abandoned the region in the past has a lot to do with understanding the limits of life there today. Sometimes the problems are the same. The Ancestral Puebloans may have left because of 25 year drought. We are in the midst of a 15 year drought today, although we experienced a very short reprieve from the dryness. A lot of ran fell in the weeks before our arrival. The desert was unusually green for this time of year.
Meteor Crater was impressive as always, a reminder that sometimes situations on our planet change in a hurry.
The Crystal Forest at Petrified Forest National Park, Arizona
Some of our stops were familiar to most people, such as Petrified Forest National Park. People may have heard of it, but many haven't visited. It's a bizarre landscape of badlands topography and horizontal forests of trees that wouldn't be out of place in the Redwood forests of California, yet are more than 200 million years old.
The slot canyon at Kasha Katuwe Tent Rocks National Monument
Other places are exceedingly obscure, although they don't deserve to be. Kasha Katuwe Tent Rocks National Monument is one of those places. Unless you are from Cappadocia, Turkey, it is one of the most unusual landscapes you'll ever see. The slot canyon is one of my favorite hikes anywhere.
The greenery was really pretty stunning after fifteen years of crippling drought. The drought is not broken by any means, but the rainy conditions over the last few weeks allowed us to imagine this landscape under a different climate regime. At Chaco Culture National Historical Park, the rangers described the plant growth as the most intense they had ever seen. The elks wandering near the campground were looking downright fat (of course the doe was probably about to drop a calf).
Chaco Canyon represented the height of the political power of the Ancestral Puebloans, in the period around 1150 AD. By the 1200s they were building the fortresses of the cliff-dwellings in Mesa Verde. It wasn't the culmination of their society, but the prelude to abandonment. Whatever the reason, the canyons fell silent around 1285. The people had migrated south and east into the Hopi mesas area and the riverlands of the Rio Grande in New Mexico.
Cliff Palace in Mesa Verde National Park, one of the largest of the cliff cities.
One of our stops was especially eerie. Castle Rock was one of the last of the Ancestral Pueblo dwellings to be constructed. It came to an end after only a decade or two with a massacre. Archaeologists discovered more than three dozen slaughtered people in the excavations.
Castle Rock, in Canyons of the Ancients National Monument
The last part of our trip went back to the geological aspects of the plateau country. Despite having visited the Natural Bridges National Monument nearly two dozen times over the years, this was the first time I was able to hike under Sipapu Bridge, probably the second largest natural bridge in the world. The opening is more than 200 feet high, and from the bottom, it is immense. Those are mature cottonwood trees in the picture below!
Sipapu Bridge in Natural Bridges National Monument
We made the very hot hike to Horseshoe Bend near Glen Canyon Dam on the Colorado River. An entrenched meander, the loop formed when the land rose, trapping a river in its floodplain pattern. The stunning cliffs are composed of the Navajo Sandstone, the remnants of a Jurassic-aged sand dune "sea" that once covered many western states.
Horseshoe Bend, near Glen Canyon Dam. The Colorado River runs clear because the dam has captured the silt that once gave the river its name. Algae is able to thrive in the clear water.
The culmination of the Navajo Sandstone is found at Zion National Park in southern Utah. The Virgin River has carved an incredibly deep slot canyon at the Narrows. In many places the river fills the entire valley floor, meaning a hike is a wet affair. I didn't hesitate!
So, I am home for a couple of days, and will try to pick up the pace with a few long-delayed blog entries. You can certainly expect more information from our last couple of trips in the plateau country as well! It's good to be home for a spell.