Driving Through the Most Dangerous Plate Boundary in the World: The "Dr. Who" of Mountain Ranges

The Whitney Crest of the High Sierra. Mt. Whitney is just out of sight to the right.

I was torn over the title for this addition to the "Driving Through" blog series. I thought of taking the zombie approach and calling them the living dead mountains ("they keep coming at you"), or the crazed killer angle ("they're never truly dead"), but these seemed...um...overly negative for a mountain range that I love. Then it occurred to me that there is a well-loved media character who is regularly mortally wounded and is regenerated with a new body. And Doctor Who is certainly loved by millions of rabid fans. And that seemed a good analogy for the Sierra Nevada of California. The mountain range has been uplifted, deeply eroded, rejuvenated, eroded, and risen yet again, and the mountains have been different each time.

Tenaya Lake and Mount Conness in Yosemite National Park

The are at least three different mountains that occupied this part of California in the last 400 million years, and most of them were related in some way to convergence, the subduction of sea-floor lithosphere beneath the western edge of the continent. The most recent incarnation of the Sierra Nevada has an enigmatic origin, and many uncertainties remain about the specific mechanics of uplift. But rising they are, even to the present day.

Highly deformed calc-silicate metamorphic rocks in Kings Canyon near Boyden Cave.

The earliest mountains were related to the collision of exotic terranes with the western edge of North America. Some of the rocks had their origins in early Paleozoic time, and were severely deformed during the Devonian period roughly 400 million years ago. Some researchers have related the deformation to the Antler Orogeny that is best revealed in central Nevada. Had one been there at the time, the scene might have been reminiscent of some of the complicated island arcs in Indonesia.

Highly folded slate near Dial's Rock Shop near Mariposa, west of Yosemite National Park

By Triassic time, about 250 million years ago, subduction zones were directly active along the west coast of North America, and chunks and bits (the exotic terranes) became incorporated into the continent itself. Mountains were pushed up along the coast in much the same way as mountains have formed in southern Alaska, but without the glaciers. The edge of the continent was located closer to the equator than today.

Mt. Shasta, in northern California, as a stand-in for the Ancestral Sierra Nevada

By late Jurassic time, about 150 million to 85 million years ago, the so-called Ancestral Sierra Nevada mountains pierced the sky. Subduction was feeding vast batholiths of granitic rock deep in the crust, and large volcanoes and calderas caused mayhem at the surface. These mountains may very well have resembled today's Andes Mountains, and indeed, the margin at that time is referred to as the Andean-style plate margin. And then...the mountains died away again. A massive erosional unroofing began that removed 5-6 miles of overlying rocks, exposing the granitic rocks. Evidence strongly suggests that the mountains had been laid low, to mere hills (although contradictory research exists). Sedimentary rocks from 40-50 million years ago show a system of coastal marshes and estuaries, sandy beaches, and large rivers with sources in at least central Nevada, and possibly farther inland.

Much of the western Sierra Nevada is composed of gently sloping, deeply eroded metamorphic rock, covered here and there by volcanic mudflow deposists (lahars), and ash tuff.

It's only been in the last few millions or tens of millions of years that the present-day Sierra Nevada began rising to their present-day prominence. The mountains rose as a westward tilted block, outlined by major faults on the east side, and the Great Valley of California to the west. As the mountains rose, deep valleys were carved by the westward flowing rivers like the Kings, Merced and Tuolumne. In the last two million years, global cooling brought about a series of ice ages, and glaciers scoured the upper parts of the river valleys, giving rise to the spectacular gorges that we visit today, Yosemite Valley being the epic example.

Yosemite Valley from the vicinity of Turtleback Dome.

So, our drive through the most dangerous plate boundary in the world will continue, following pathways into the heart of this mountain range that is both ancient and youthful at the same time.

Northern Convergence: A Geological Journey Through Canada and the Pacific Northwest

Some of the world's most dramatic landscapes exist within the strip of the North American continent between the High Plains of Canada and Montana and the Pacific Coast. The geologists call it the Western Cordillera, part of the mountain system that extends from the tip of South America to Alaska.

This mountainous terrain exists in large part because of a subduction zone that is or was once active offshore in the Pacific Ocean. This convergent boundary provided the mostly compressional forces that lifted these mountains.

We've just completed a journey across this incredible landscape, with the first ever extended tour of Canada by our department. Our travels took us from the Olympic Peninsula of Washington State to Vancouver Island, on a ferry across the Strait of Georgia to the Coastal Belt mountains and then into the western interior. We passed through Yoho, Banff, and Jasper National Parks in the Rocky Mountains, and spent some time on the High Plains before heading back into the States at Glacier National Park.

The journey was a collaboration between the geology and anthropology programs at Modesto Junior College. We learned the geological history of the land as well as the human history. It is a tough landscape, but people have lived and thrived here for at least 12,000 years.

The land also preserves the story of the ice times. Canada and the northern tier of states were once covered by thousands of feet of glacial ice. The glaciers had a profound effect on the landscape, and many of them still persist in the high country.

The High Plains reveal many secrets of past life forms on Earth. We paid a visit to one of the finest paleontology museums anywhere, the Royal Tyrrell in Drumheller, Alberta.

On our way back through Montana and Washington, we observed the effects of one of the greatest flood events in world history, the Spokane Floods of 16,000-12,000 years ago.

Our journey ended at one of the most visible effects of convergence: active volcanism. We explored part of Mt. Rainier, one of the most spectacular volcanoes on the planet.

Our route took us through some fascinating geology, and this new blog series will share some of the incredible things we saw. Stay tuned!

The Other California: The Oldest Rocks (well, maybe...)

Part of the mystery question I asked the other day suggested that the oldest rocks in California lie within a few miles of the Santa Clarita Valley. I kind of knew I would be opening a can of worms by pointing this out, and I will try to explain why this idea might be ambiguous at best, and highly controversial at worst. In essence, yes, some of the oldest rocks found in the state are found there, but defining how 'old' a rock is can be a real conceptual problem at times.

It wouldn't be that much of a problem if the rocks in question were volcanic or plutonic. Such rocks are usually ideal candidates for radioactive age dating. The rocks form by cooling from hot magma, and crystallization is either instantaneous (in the case of volcanic rocks), or takes place over several thousand years (as in plutonic rocks cooling deep underground). As the rocks crystallize, they trap small amounts of uranium, rubidium and other radioactive elements in a solid atomic prison. Isolated from weathering and other chemical changes, the radioactive substances start to decay at extremely regular rates, and the original material begins a very slow transformation to daughter elements like lead and strontium, or argon (in the case of radioactive potassium). By analyzing the proportion of daughter element to parent element, the age of the rock can be precisely determined. Because several methods can be used to date a single sample, results can be cross-checked for accuracy

A side note: Carbon dating is another form of radioactive age dating that is recognized by many people, if not well understood. It is rarely used in geology because it only useful in materials that are less than 100,000 years old. Since it is carbon-based materials like bone or wood that can be dated, the method is favored by archaeologists who are working with artifacts.

The problem comes when such rocks undergo metamorphism, that is, when they are changed by heat and pressure into new kinds of rock like gneiss, schist and marble. The heating in essence allows daughter elements to migrate out of the crystals and the radioactive 'clock' is reset to zero. If you age date metamorphic rocks, your results tell you when the rock was metamorphosed, not when the rock originally formed. This kind of information is critical to understanding the tectonic history of a region, but it isn't the age of the "oldest rock".

There is an additional complication. There are a few minerals that are not affected by the heat of metamorphism, including a mineral called zircon. When zircon crystals are present in a rock undergoing metamorphism, their 'clock' is not reset. So depending on which minerals are dated, you get the age of the metamorphism, or the age of the older zircon crystals. It gets even more tricky: zircon is also tough enough to survive erosional processes. So even though you can figure out the age of crystallization, the date you get may have come from the original plutonic rock, but it tells you little about how many cycles of uplift and erosion that the zircon may have gone through.

So there you go...it turns out that the "oldest rocks" in California can be found over a rather wide area. There are rocks that date to 1.7 or 1.8 billion years ago in Death Valley National Park, in the eastern Mojave Desert, in Joshua Tree National Park, and the San Gabriel Mountains, which form the eastern edge of the Santa Clarita Valley. Most of the rocks were part of a vast mountain system that developed in a series of terrane collisions as the supercontinent that we call Rodinia came together (here is an excellent overview if you want the gory details of the story). As I walked through Placerita Canyon, I was very close to exposures of the Mendenhall gneiss, which is dated, like the others, at 1.8 billion years.

Before calling this oldest rocks "contest" a tie, we should point out that the ancient rocks in the Basin and Range and Mojave Desert are part of a Proterozoic terrane called Mojavia, which has zircon dates that extend back to 2.5 to 2.7 billion years, and as far as I know such results have not been found in the San Gabriel Mountains terrane (but as always, I am welcome to correction!).

I wandered through the terrain around Placerita Canyon taking a look at some of the metamorphic and plutonic rocks of the San Gabriel Terrane (yes, terrain and terrane mean two different things; the first referring to the shape of the surface of the land and the other to the underlying crust). I haven't lived in Southern California for nearly a quarter century, but I immediately felt at home. The dry chaparral slopes and the oak and sycamore filled canyons were the places that I grew up in, poison oak and all. If trails weren't present, access was problematic. I had a number of memorable trips where I was climbing slopes using the bushes as a ladder. The San Gabriel Mountains are statistically the steepest mountain range in the world and I find that easy to accept. It is a wonderful place to explore.

One really big change, though: in the 1970s, when I was a budding geologist, the rocks of the San Gabriel Mountainswere mapped as "metamorphic-undivided" and were poorly known. Today there is a richness of data that allows us to see the role of the rocks in reconstructing the American West of nearly 2 billion years ago.

The Other California is my continuing series highlighting the geology of the fascinating places in my fair state that don't often show up on the postcards that tourists buy. The state is a big place and I have yet to see it all, so if you have a favorite corner of the state and would like to put together a blog entry, I would gladly add it to the series.

The Other California: I've Seen These Mountains Somewhere Before: The Big Ripoff!

This is an ongoing exploration of the "Other California"; the wonderful geological places in our state that are rarely found on a postcard. After some geological distractions (like giant earthquakes in Chile and tsunamis in Hawaii), we are back on the road looking at some peaks that look strangely familiar. They're made of granitic rock, they've been subjected to exfoliation and jointing, forming the sheer cliffs and rounded peaks. The mountains here have been glaciated. Where would you think we were, if you are a dyed-in-the-wool Californian?

If you guessed the Sierra Nevada, you would be wrong. And right, too, in a sense.

We started a tour of the Klamath Mountains talking about their origin as bits and pieces of oceanic crust and continental fragments that were assembled into a complicated exotic terrane that was attached to the North American continent in Mesozoic time. We followed up by discussing the alleged presence of Sasquatch in the region (though the discussion didn't last long; the Chilean earthquake proved far more interesting as a scientific issue that week).

Today we note the striking similarities between the Klamath Mountains and the northern part of the Sierra Nevada (see the map above). Both mountain systems have rocks that are primarily metamorphic sequences that have been intruded by Mesozoic granitic rocks. The metamorphic rocks are of similar age and structure. They have been faulted in much the same way. They are, in few words, the same mountain range, at least in the origin of the rocks. Direct correlations of the complicated metamorphic sequences have been established in recent publications by the US Geological Survey.

The oddest part of the story seems to be the overall shape of the two provinces. They look as if they were bent and torn apart from each other and separated by a distance of around 60 miles. And they were, around 140-150 million years ago, in late Jurassic or early Cretaceous time. A shallow sea opened between the two landmasses, and several thousand feet of sediments were laid down on the rocks (the Hornbrook Formation). It was a huge rip-off!

By Pliocene time (4 or 5 million years ago), the region was a flat low plain, but it rose rapidly, as much as 6,000 feet, in the last few million years. Rivers coursing across the flat surface quickly incised the deep canyons that characterize the region today (small gravel remnants of the rivers can be found on some of the higher parts of the topography).

The implications of the similarity of the two provinces was not lost on the Forty-niners. Gold was mined from the metamorphic rocks of the Sierra Nevada Mother Lode, and the rocks in the Klamaths were clearly of the same origin. Gold was discovered in the Klamaths in 1848, and numerous towns sprang up soon after. The region was the second most important gold mining region in the state of California after the Mother Lode, with several million ounces of gold produced.

For some recent research on the uplift history of the Klamath Mountains, check out: Cretaceous Sedimentary Blanketing and Tectonic Rejuvenation in the Western Klamath Mountains: Insights from Thermochronology