When Rhyolite Breccia makes more sense than Granite

Breccia? You betcha! Looking for Detachment has a fondness for breccias, and mentions a few others who think likewise. My field trip season never seemed to end this year (that's not a complaint), as I had two lab field trips to see the Del Puerto Canyon ophiolite this week, and a Saturday field trip to Pinnacles National Monument in the California Coast Ranges. It was a great trip for breccia bloviating.

Pinnacles is a marvelous, if widely unrecognized little park about 30 miles south of the town of Hollister. It was established as a national monument in 1908 by Teddy Roosevelt for the tower-like peaks of rhyolite lava flows and breccias that are quite out of character with the normally rounded hills of the Coast Ranges. Those who sponsored the park for protection were unaware of the deeper geological significance of the park.

The great San Francisco Earthquake of 1906 brought worldwide attention to the San Andreas fault. Extending from the Imperial Valley of southern California to Mendocino on the northern California coast, it is one of the more extensive fault systems on the globe, and it has generated two major historical quakes (1906 and 1857), and a more recent "moderate" quake (Loma Prieta in 1989). Paleoseismicity studies have revealed dozens of major earthquakes during the last few thousand years.

Prior to the advent of plate tectonics theory in the 1960's, large scale lateral movements were not considered possible on faults like the San Andreas, though it was clear that the most recent movements had indeed been lateral. A seminal paper in 1953 by Hill and Dibblee argued that the fault had shifted hundreds of miles since Cretaceous time, a highly controversial assertion for the time. In the decades that followed, the San Andreas came to be understood as a continental transform boundary between the North American and Pacific Plates. Still, the fault researchers hoped to find an unequivocal piercing point that would allow a direct measure of the exact amount of movement on the fault.

The Pinnacles volcano was a rhyolite composite cone that happened to erupt on the San Andreas fault about 22 million years ago. The San Andreas ripped the volcano apart, and the two halves began moving apart, 10 or 15 feet at a time during large earthquakes. In 1976, V. Matthews demonstrated that the Pinnacles rocks were virtually identical to the Neenach Volcanics in the Transverse Ranges of Southern California. The two halves were separated by 195 miles!

The park has a lot of offer visitors. An extensive trail system bisects the park, including two sections that explore underground caves formed by boulders piling into the narrow slot canyons carved into the rhyolite. The vertical rock faces are a climber's paradise, as shown in the photo above. The rhyolite breccia (formed from volcanic mudflows, also known as lahars) makes for lots and lots of toe-holds!

I once was privileged to introduce the famous rock-climber Royal Robbins when he spoke to a group of geology teachers in Modesto. I made a point that geologists know a great deal about the properties of rocks, but how many of us stake our lives on our understanding of those rocks? Rock climbers do that every day...

Hill, M.L. and Dibblee, T.W. Jr., 1953, San Andreas, Garlock, and Big Pine faults, California - A Study of the character, history and tectonic significance of their displacements: Geological Society of America Bulletin, volume 64, pp. 443-458.

Matthews, V., 1976, Correlation of Pinnacles and Neenach volcanic formations and their bearing on San Andreas Fault problems AAPG Bulletin 60: 2128-2141

Rocks fall in Yosemite: Park Service closes 1/3 of Curry Village

Via the Yosemite Blog, the National Park Service has decided to permanently close down about a third of the tent cabins and other facilities in the Curry Village area of Yosemite Valley. The decision follows a series of rock falls over the last few years that have killed or injured several people. Investigations are seeking an explanation for an increase in incidents below the cliffs of the Glacier Point area.

The photo of the day shows some of the trees at Happy Isles that were snapped off by a huge rock fall in 1996 that killed one person and damaged the Nature Center.

Rocks fall in Yosemite? I had no idea that could happen!

I had one of those nice "convergence" moments today...I was discussing mass wasting in my physical geology classes today, and this story showed up on the front page of my local paper concerning the danger of rock falls in Yosemite Valley.

There was a rather nasty mass wasting event in the valley last month, and it has been noted that there have been four times as many rockfalls since 1996 than in the previous 139 (although I seriously wonder if this is because more attention is being paid to such events in recent years). If this is a true increase in the number of incidents, then the speculation that human interference is a causitive factor needs to be investigated. Most of the attention centers on water use at Glacier Point, the spectacular overlook on the rim of the valley. Some of the recent events have occurred in summer when visitation (and therefore water use) at the point is highest. Other researchers, including the park geologist, disagree with the contention that water plays a part in the recent events.

The other controversy concerns the placement of cabin units in Curry Village where many of the incidents have taken place. Curry Village took a hit when flooding in 1997 destroyed dozens of cabins, and proposals to remove more from the base area of the cliff would decrease housing units even more.

It was a great teaching moment...except that only four of my students had looked at a newspaper this morning.

The Real Jurassic Parks: Where are the Dinosaurs?

A great many parks on the Colorado Plateau draw their scenery from Jurassic-aged sediments, and have been reviewed on a fair number of posts in the last month or two (see here for a listing). I haven't said much about the dinosaurs themselves because I was waiting to talk about the ultimate dinosaur park: Dinosaur National Monument (a most creative name, don't you think?)

The park draws its fame for an extraordinary bone bed that was discovered in 1909 by Earl Douglass. Although thousands of bones were quarried and sent to distant museums, there was enough foresight among the powers-that-were to preserve hundreds of bones in place where they could be viewed in situ by the public. By 1915, 80 acres around the quarry were set aside as a monument by President Woodrow Wilson. The park was expanded in 1938 to include the spectacular gorges of the Yampa and Green Rivers, an area of some 200,000 acres. The park was the setting of one of the seminal battles in the nascent environmental movement in the 1950's when an effort to construct a huge reservoir in Echo Park in the heart of the monument was turned back (some details here).

The source of most of the dinosaur fossils in the monument is the Morrison Formation, one of the most widespread sedimentary units in the western United States. It is exposed over a region extending from Canada to New Mexico and from the Great Plains to Arizona and Utah, forming a unique badlands topography in shades of purple, red, green and yellow. The rocks reveal a vast river floodplain system that was occasionally mantled with volcanic ash from eruptions of stratovolcanoes to the south and west. A shallow sea existed to the north.

Rivers are wonderful settings for the preservation of fossils. Animals are drawn, of course, to river (fluvial) environments, and occasional floods will trap many of them. As rivers meander back and forth across the plains, point bars (inside loop of a meander) become collection points for animal carcasses. They can be quickly buried and preserved in such situations (the bone bed at Dinosaur NM is an example of this). The muds of the floodplains likewise can be a tomb for mired animals. In other situations, the muds served as nesting grounds for some species, and nests, egg shells and juvenile dinosaurs are counted among of the significant finds in the Morrison.

A phenomenal number of fossils have been found in the Morrison Formation, including dozens of species of dinosaurs, as well as smaller reptiles, pterosaurs, crocodiles, primitive mammals, fish, amphibians, and invertebrates. Many of the most best-known dinosaurs were discovered in the formation: stegosaurs, sauropods ("long-necks" to you parents out there), and allosaurs. Twelve species have been discovered in Dinosaur National Monument alone, including the Camarasaurus in the third photo above.

The clay-rich nature of the Morrison Formation has been the undoing of the most famous aspect of the park. The bone bed was exposed in a steep face that came to be protected under a three-story-high shelter. Over the years the soils have settled and the structure was seriously compromised, with cracks and fissures in the building that represented a danger to the staff and public. As a result, the building has been closed since 2006. Unfortunately, park visitation plummeted, and the budget situation at the park has deteriorated.

Because of the budget shortfalls, some political controversies have arisen in recent months over the management of Dinosaur National Monument and the role of paleontologists in the work force at the park. Rebecca at Dinochick Blogs has been on top of the issue, with the most recent flurry of letters here.

If you have the opportunity to visit the park, do not miss the 200,000 acres of the park that are not the fossil exposures! A vast plunging anticline dominates the park, and the two rivers that cross the anticline have carved deep gorges that are extremely popular with river rafters. A road climbs from the south boundary of the park to spectacular overlooks at Harpers Corner and above Echo Park.

Geologic Haiku

Tropical limestone ledges
High on Sierra peaks
A long journey completed

A new geology meme! Rules here. The picture is of Boyden Cave Roof Pendant deep in Kings Canyon in California's Sierra Nevada.

Wild Animals in the Field!

I love a good meme! Especially an interesting one that morphs a bit so one can come back and pick out some new pictures. Wild animals in the field are so infinitely interesting especially when we see something new. I have previously blogged the bear I saw last month, and the bighorn sheep I stumbled across at Capitol Reef National Park a few years ago. And of course, the cows that are taking over the planet. I appreciated C.V.'s contribution at Cryology and Co. regarding cows and the creation of the world.

Today's choices include a weasel I saw on the trail to the summit of Lassen Peak in northern California last year. It looked like a ground squirrel popping in and out of the rocks, but I realized it was probably looking for squirrels. I had never seen one of these before, so I kinda forgot about climbing Lassen (been there, done that; oh, for shame!), and sat down and waited. After about 30 minutes I got the shot I was hoping for. I wasn't that close, but I have a 12x zoom to work with. (Correction 12-11-09: it's actually an American Pine Marten; I don't know these furry animals as well as I should)

The second photo is a gopher snake I ran across during an eastern Sierra Nevada field trip in 2006. Reptiles seem so alien, especially seen up close. I can relate to thinking like a weasel, but reptilian brains are beyond me.

It recalled one of my other snake stories...one time I was driving a road on the outskirts of town and saw a juvenile snake in the roadway in eminent danger of being run over. I decided to do the nice thing and pick it up and put it in the nearby field. I looked at it very carefully (I live in rattler country, after all). No vee-shaped head; check. No rattles on the tail; check. Can't be a rattler, so I pick it up.

Its tail started rattling, and the head was magically vee-shaped, and it was trying to bite me!

I threw the snake into the field as fast as I could and went home and went on the internet. I knew gopher snakes often rattle their tails, and that they have a vicious disposition when picked up, but it was news to me that they can actually flatten their skulls to look like a rattlesnake!

It would have been an embarrassing story to tell in the emergency room...

The third is an owl I saw deep in the forest at Muir Woods National Monument, north of San Francisco. It was hard to photograph in the dark shadows, and I wasted a lot of digital space trying to get it. I really wondered if I had finally seen the elusive Spotted Owl. As it turns out, it was probably a Barred Owl, which is a somewhat invasive species that takes over from Spotted Owls in second-growth forests. Just the same it was a beautiful bird.

The Real Jurassic Parks: Arches National Park, continued. And an Eminent Threat.

I didn't really mean to leave everyone hanging in the Jurassic during our months-long tour of the geology of the Colorado Plateau, but I am easily distracted by things like work and politics. I want to at least make it out of the Mesozoic era before Thanksgiving, so I am picking up the thread where we left off on Oct. 17 with a visit to Arches National Park.

The stratigraphy of the park is relatively straightforward: the visible scenery mainly includes the Navajo Sandstone, the Dewey Bridge member of the Entrada Sandstone, and the Slickrock member of the Entrada. The Navajo Sandstone has a huge role in the scenery of other parks on the Plateau (see discussions here), but is mostly muted here. It is familiar to park tourists as the "petrified sand dunes" that can seen along the road in the south part of the park, and the very dramatic white rock along the park entrance road. The Dewey Bridge member is a "crinkly" layer of sand, silt and gypsum at the base of many of the arches. The reason for the deformed layers is unclear. One intriguing proposal is that a nearby asteroid impact distorted the layers (Upheaval Dome in Canyonlands National Park is the proposed impact site). Most of the arches occur in the Slickrock member, a cliff-forming layer that is obvious throughout the park.

The origin of the park's famous arches (there are hundreds) is tied to the salts of the Paradox Basin discussed earlier in this post. The salt beds are thousands of feet thick, and when placed under pressure, can slowly flow upwards through the overlying rock, forming domes and anticlines (upward pointing folds). Over millions of years, erosion removed thousands of feet of overlying sediment, and the salt was close enough to the surface to be affected by groundwater: it was dissolved away. The tops of the folds collapsed inwards, and the Entrada Sandstone was fractured (jointing) into a series of parallel fins. A large northwest trending anticline crosses the park; most of the arches occur along the flanks of the anticline.

The top of the fins are exposed to the arid desert climate, where the rock dries quickly following the infrequent storms. At the base of the fins, where the rock is buried by soil and loose sand, the rock may stay moist much longer. This leads to the solution of the cement holding the sandstone together, and the fins start to weather from below. Eventually a small window may open up, and falling slabs of rock enlarge the opening. The largest arch, Landscape, has an opening the length of a football field. Unfortunately, it is so thin and fractured that it may not last our lifetimes (it is in the third picture of the day). A large chunk fell about fifteen years ago. Wall Arch, another arch in the same area, just collapsed last summer.

Today's first photograph shows a small arch in the vicinity of Delicate Arch (a corner of Delicate is visible on the left side). The foreground includes tilted layers of the Entrada where they collapsed into the salt anticline. The La Sal Mountains can be seen in the distance.

The second photo shows my nomination for one of the most dramatic trails in existence, the one that leads to Delicate Arch. This is definitely a case where the end of the trail is not the main attraction; the whole trail is a treat, from one end to the other.

Unfortunately, politics and politicians don't stop politicking, elections or not. Arches, like the Grand Canyon is being threatened by the possibility of gas drilling just outside the park boundaries. The outgoing administration seems to be trying to run a fire sale on leases before they leave office in January. It is especially irritating that the Bureau of Land Management seems to be trying to fly under the public radar in their announcements. More information on the issue (and maps of the proposed leases) can be found here.