It's been a while since I last posted about my time in Bishop, California, back in September, when I attended a GSA field forum on the structural and neotectonic evolution of the volcanic tableland.

For reference, here's a list of the previous posts about that trip:
...Faults of the volcanic tableland
...The Bishop Tuff
...The flipping fault

So, picking up where I left off, I thought it would be worth a post to mention the gorgeous drainage channels one sees etched into the top "Ig2" welded layer of the Bishop Tuff. These channels are interpreted as being Pleistocene in age, when the area was wetter than it is now.

Here is a photograph of the most spectacular of these channels, as viewed from the rim:

We visited this vantage on our second day in the field. A hiking path at the bottom of the dry channel imparts a sense of scale.

Here's a Google Map of the area from the perspective of a hawk:

Where the road comes most closely tangential to the canyon is the point where we stopped to take a look at it, and where the above photograph was captured.

Further upstream along the channel, we find it broken by normal faulting. Check out the view across this graben (a graben is a normal-fault-bounded valley, downdropped relative to the highlands next to it). There, you see the distinctive crescent-shaped profile of the drainage channel, but offset along several fault scarps:

There are three scarps on the far side of the graben, and an additional one that Peter is standing on, on this side of the graben. Just behind Peter, you can see a broken relay ramp, too. View is to the northwest; those are the Sierras in the distance.

Here is a Google Map of the area, showing the drainage channel crossing the graben. This conclusively shows that the channel must be older than the faulting which produced the graben.

This Google Map shares its southeastern corner with the northwestern corner of the first one I showed. You can see this for yourself by dragging either one in the appropriate direction. They both share the white-knuckled place where the road goes straight down the fault scarp, rather than sensibly down a relay ramp. That wasn't my favorite thing to drive.

Here's another drainage channel, similarly bone dry, that we visited in our fourth day in the field. Perspective is to the east: those are the White Mountains in the distance:


The Google Map shows a more interesting relationship this time. Instead of the faulting cross-cutting the channel's orientation, this channel approaches the graben to the southeast, curves around (deflecting from its original downhill course) and drops down the relay ramp to the northeast, into the graben (breaking up into multiple channels en route). There, it resumes its original downhill trajectory to the southeast:

This suggests that at least some of these faults were rupturing the "Ig2" layer at the same time that water was flowing over the surface (i.e. before the Owens Valley's climate dried out, post-Pleistocene). The stream's course and the faulting were coeval.

So what was the source of these streams? Did they originate on the volcanic tableland, or were they derived from the Sierra Nevada, prior to incision by the Owens River (which makes a deep canyon a mile or two west of here)? Fred Phillips, of New Mexico Tech, holds up a piece of evidence:

That is not a rounded cobble of the Bishop Tuff. That's a rounded cobble of granite. While the majority of cobbles in these channels are locally-derived chunks of the Bishop Tuff, there are also clasts which originated elsewhere, beyond the volcanic tableland itself. This suggests a source area with a granitic outcrop. One candidate location is Casa Diablo Mountain, north of the (south-sloping) volcanic tableland. Another possibility is the Sierras, to the west.

Another possibility entirely is that the source of the cobbles could be anywhere, and they were brought to the volcanic tableland not by streams but by paleoindians, who used them as grain-grinding stones in their metates.

Labels: california, conferences, faults, granite, pleistocene, rivers, travel, volcano, weathering

On the day before the GSA meeting began, I participated in a field trip to the Boring Volcanic Field, a zone of anomalously-located volcanic vents around Portland, Oregon. The field is named for the Boring Hills, adjacent to the town of Boring, Oregon, which is named for a dude named "Boring." Kim Kastens noted this funny name on the Earth and Mind blog recently. The USGS maintains an information page on the field here.

Today, some photos...

Atop Rocky Butte, field trip leaders Rick Conrey (WSU) and Russ Evarts (USGS Menlo Park) orient the group with a map highlighting the various units comprising the Boring Volcanic Field:


Mount Hood hides its peak in the clouds:


At our first outcrop stop, the field trip participants get out and look at the Boring rocks:


Here, a Boring lava flow overlies Troutdale Formation fluvial gravels:


Annotated version for the untrained eye:


In places, a "baked" zone of contact metamorphism can be seen in the Troutdale as it got scorched by the lava that flowed on top of it (bright red), but the characteristic red color was missing underneath one spot, the central overhang in this photo:

Weird, huh? Maybe the metamorphosed sediments need a certain amount of rain-mediated chemical weathering before they "blush"?

Well-rounded clast from the Troutdale: vesicular basalt from the Columbia River Plateau:


Another nice Columbia River flood basalt boulder, this one with phenocrysts of plagioclase, and a concentric zonation of texture (massive in the center, vesicular towards the edges):


Plus, you can find cobbles derived from further afield: gneiss (from Idaho?), quartzite (Belt rock?), etc:


Between cobbles of the Troutdale, you can see hyaloclastic sand (immature sand with lots of hydrated basaltic glass fragments, apparently produced by interactions of magma and water in the source area, upstream):


More hyaloclastic sand:


Oooh! A "crack panel" on the side of some cooling columns at another stop! These horizontal slats are produced in individual fracture-propagation events, and each one concludes with a little ridge called an arrest line.


Mafic pyroclastics that underlie the lava flows at this second stop:


More mafic pyroclastics, on a cinder cone in Mount Tabor Park.


This is a pretty neat outcrop: you can see normal faults cutting these angle-of-repose inclined volcanic strata, presumably forming in slumping events.


Annotated version of this same photo, highlighting a marker layer and its offset along the fault:


The weather was pretty grim for this trip, so that was a bummer. But it's Portland, right? What did we really expect? Anyhow, I enjoyed being introduced to this suite of rocks -- boring out of context, but interesting given their location well west of the main axis of Cascade volcanism. Unfortunately, the field trip didn't really address why the Boring rocks are there. I was expecting some sort of detailed discussion of the possibilities: an evaluation of different models for their generation and passage to the surface... but that really didn't happen in any substantive way. So it wasn't the most amazing field trip I've ever gone on, but it was a nice day of checking out a cool suite of rocks.

Labels: meetings, oregon, pleistocene, pliocene, sediment, volcano

There's been a lot of hubbub in the geoblogosphere over the past couple of days about caldera-forming eruptions. The trigger for all the discussion was a report about a really old (Permian) caldera-forming eruption in Italy. This invokes discussion of our modern worry-inducing "supervolcanos," like Yellowstone. There's also a lesser-known place in California, where the USGS maintains a volcano observatory there just like they do with Yellowstone: Long Valley Caldera. This caldera formed ~760,000 years ago, and deposited a lot of ash, collectively called the Bishop Tuff. The Long Valley Caldera's true area is about 300 km², although central sagging has generated ring faults that give it a topographic area of ~350 km². The Bishop Tuff is thickest right around the caldera, but ash from this eruption can be found as far away as Nebraska.

I got to see the Bishop Tuff firsthand the week before last when I spent a week in the Owens Valley as part of a Geological Society of America Field Forum. I was lucky to be introduced to the tuff by Wes Hildreth, a volcanologist at the USGS's Menlo Park office. Wes "wrote the book" on the Bishop Tuff, and shared an immense amount of information and perspective with the Field Forum participants. I am indebted to him for all the information I'm sharing here. Maggie Mangan just took over the reins of the Long Valley Observatory, and she also participated in the Field Forum. I also really benefitted from talking to her about the eruption. (Any errors that you may find here, of course, are my own.)

The Bishop Tuff is the most striking of many volcanic eruptions along this same system. It's the only one that has produced a caldera. It was preceded by dacite and basalt eruptions at 3.5 to 2.5 Ma, and then by rhyolite and obsidian during the appropriately-named Glass Mountain Interval, from 2.1 to 0.8 Ma. (The Glass Mountain Interval is pretty cool in its own right: at least 60 eruptive units, each high-silica rhyolite!) The focus of both of these was further to the northeast. That area is also home to some post-Bishop eruptions, the youngest of which is at Mono Lake (only 250 years ago). In 1989, a dike came within a few km of the surface, and degassed a CO₂ "burp" which killed trees near Mammoth Mountain, which lies on the caldera boundary.

The Bishop Tuff is compositionally similar from bottom to top: it's all rhyolitic pyroclastics, whether it's welded (fused together) or not. Some went north from Long Valley Caldera under the Mono Lake area, while the bulk of it went south towards Bishop, forming the Volcanic Tableland. it has a density of about 1.5 g/cm³.

In this photo, Kim Bishop (yes, that's really his last name) and Peter Lovely (yes, that's really his last name) check out the first of the ashfall deposits, dumped atop lake sediments in a cool outcrop on the southern margin of the Volcanic Tableland, north of Bishop and the Owens River:


A close-up of this contact:

The ashfall portion of the Bishop Tuff has 9 subunits, and you can see the first (F1) and the base of the second (F2) here, overlying the silty lake sediments.

Here's another outcrop, in the Owens River Gorge, where you can see the welded ashflow "caprock" up top, and down below, and outcrop that showcases nonwelded ashfall and ashflow deposits. I've put a box around the area that I'll zoom into in the next photo:


The ashfall deposits are finely stratified and well-sorted, with no reworking. Overlying them, the first of the ashflow (ignimbrite) units shows characteristic poor sorting: big blobs of pumice mixed in with the finer pyroclastics. Most of the ashflow is pinkish in color, but you can see here that the first of it is white, same as the ashfall:

Why pink in the ashflow portion? It's hot when it gets deposited, and heat retention promotes oxidation. The earliest ashflows were dumped atop ashfall (which gets deposited cold), and so likely lost much of its heat downward; hence less oxidation. The entire eruptive seqence is preserved in the Volcanic Tableland north of Bishop. Here, at the southern rim of the Tableland, we're getting the latest flows. The earlier flows didn't make it this far south.

Here's a close-up of that basal ashflow, from the first outcrop. My field notebook is 18 cm "tall," for scale. Note the white color and all the large pumice clasts:

The iron to titanium ratio in these clasts suggests that they erupted at 770 to 800 degrees C. The temperature of the eruption increased as it progressed. This corresponds with an increase in the mafic content of the tephra over the course of the eruption. In the early layers, there's about 77.7% silica, but when you get towards the end of the eruption, you see that number drop to 74%, as well as a doubling of iron content, a quadrupling of the Ca content, and ten times as much magnesium as in the earliest strata.

Here's a close-up of some semi-welded material. This is float, so I don't know precisely where in the sequence it fits, but I would guess the "Ig1" layer, the lower of the two welded ashflows.


And another. One thing I noticed about a lot of the included pumice blobs is that their vesicles were all stretched out into cigar-shaped tubes (prolate), like an L-tectonite. Anyone have any idea what's up with that? I would expect oblate strain ellipsoids (pancake-shapes) here due to post-depositional compaction, but that's not what I noticed...


We made a trip to the lip of the Owens Gorge to look down on the upper ignimbrite (ashflow tuff) layers of the Bishop Tuff:


The first half of the eruptive sequence, dubbed "Ig1" (for "ignimbrite 1") is below the sharp line. In the upper half of the sequence, Ig2, you'll find rhyolite lithics that can be sourced to the earlier Glass Mountain Interval, as well as pyroxene-bearing pumice. You can see here some abortive cooling columns in Ig1:

Likely these don't extend very far down because as soon as they started forming, Ig1 was buried underneath piping hot Ig2 ashflow. This addition of heat disrupted the cooling front and truncated the fracturing process. Sorry I don't have a sense of scale in this photo: it's hard to do when you're photographing the opposite side of a deep gorge. I'd guess these columns are a meter or so across. In one spot, a little downstream (southeast) of here, you can actually see a little ashfall intercalated with these ashflows (it's the F9 ashfall subunit). This, Wes Hildreth told us, is most unusual and quite handy for interpreting the stratigraphy of the Bishop Tuff. The only other place he's seen such a thing is in the Valley of 10,000 Smokes in Katmai, Alaska.

Some close-ups of the Ig2 unit, which is classic "welded" tuff with nice pumice blobs and rhyolite lithics, as well as pyroxene-bearing pumice:


Rhyolite lithic clast in "Ig2" welded Bishop Tuff ashflow deposit:

That's likely from the earlier Glass Mountain Interval, through which the Bishop Tuff erupted.

The "Ig2" layer wasn't the last part of the Bishop Tuff eruptive sequence, but the stuff deposited on top of it was unwelded, and has since been eroded away. In order for a tuff to weld, it needs to be close to 600 degrees C when it stops (this temperature is for rhyolite: it's actually composition and H₂O dependent). But the welding process (essentially superhot glass fragments warp around one another and lock into place) has made for a resistant layer atop the modern Volcanic Tableland, and this layer preserves the weaker layers beneath, preventing them from being eroded (except, say, where a river incises downward through the caprock). It's a nice example of differential weathering. Cosmogenic ¹⁰Be measurements on the upper welded tuff suggest a modern weathering rate of 2 mm/1000 years.


Now here's the thing that I thought was most interesting about the Bishop Tuff: it's big, and it erupted quickly. There are about 200 km³ of ignimbrite (ashflow tuff), another 100 km³ of fallout (ashfall tuff) out to Utah and Nebraska, as well as 300 to 350 km³ of welded tuff that filled the downdropping caldera (2.5 km of subsidence). That's a lot of magma fluffed out and ejected onto the surface! But Wes calculated that this whole sequence, from the first puff of ash descending from above to the last of the sizzling nuee ardentes, lasted a mere 6 days: a single huge eruption! And, Wes added, "on the seventh day, it rested."

Further reading (I particularly recommend taking a look at Figure 5d!):
Hildreth, Wes, and Wilson, Colin, 2007. Compositional Zoning of the Bishop Tuff. Journal of Petrology 48 (5):951-999.

Labels: california, permian, pleistocene, volcano

Here we have some nice little glacial striations exposed in the Grinnell Glacier cirque in Glacier National Park, Montana. These grooves were carved by pebbles and other clasts within the glacial ice as it flowed over this outcrop of the Mesoproterozoic Helena Formation (part of the Belt Supergroup). Perhaps some of the same pebbles you see in this photo were responsible for acting as carving tools -- though the 'hand' that wielded them, Grinnell Glacier itself, melted away from this point sometime since 1939.

Also of interest to me in this photo is the lingering stain of water around the joint set in the upper right. I'm fascinated at the interplay between physical and chemical weathering, and seeing stuff like this emphasizes how even a simple hairline fracture can help funnel water, with all its destructive effects, deeper into the heart of an outcrop. Weathering is focused on these areas, and in another century this outcrop may look quite different.

Labels: glacial landforms, glaciation, joints, montana, national parks, pleistocene, proterozoic, sediment, snow, weathering

Paul Hoffman is speaking next Wednesday (April 22) at the Geological Society of Washington; I've just recieved word that his topic has changed from Snowball Earth to "The Pleistocene glacial controversy and the discovery of climate warming and crustal dynamics." I'm curious to see what Hoffman has to say about the 160-year old controversy as to whether there had been recent "Ice Ages," and how that relates to his currently-controversial ideas about the Snowball. Whatever he says, it's likely to be thought provoking.

Labels: geologists, glaciation, gsw, ice, pleistocene, science and society, snowball earth

Contrary to what you may have heard, it's not all basalt. Even the basalt is astonishingly varied: the extrusive rock of a thousand faces... Here I'll share some pictures I took of rocks in Hawai'i:

There's pahoehoe:


...and there's a'a:


Here's a pahoehoe flow oozing over my boot (just kidding; it was cold when I did this):


Pahoehoe lobes can drain out, leaving only the outer skin as rock, but with a hollow center. These are lava tubes (nickel for scale):


Another one (nickel for scale):


Cool texture on the inside of this lava tube (nickel for scale):

...and zooming in a bit closer (it looks like wrinkled cellophane!):


A stack of cross-sectioned pahoehoe flows, showing their tubular (totally tubular, dude) shape:


Some Hawai'i basalt is massive, like this cobble...


...or like this cobble of hawaiite, a dense form of basalt found atop Mauna Kea (where it apparently erupted beneath Pleistocene ice caps):


But the majority of Hawai'i's basalts are vesicular, meaning they contain "Swiss Cheese" type holes that result from gas bubbles. When the lava erupts, it experiences less pressure at the Earth's surface than it was subjected to at depth. As a result, many gases (steam, CO2, sulfur dioxide, chlorine, argon, others) exsolve from the lava solution and make bubbles. If these bubbles don't get a chance to pop before the lava sets up into igneous rock, then they are preserved as vesicles. Sometimes the vesicles are small:


...and sometimes they are big:


Sometimes, they are really big. Here's one I could fit my entire Nalgene water bottle into:


When vesicles later get filled in with mineral deposits, we call them amygdules. Here's some vesicles that have gotten a light coat of a white mineral on their interiors: the first step to converting a vesicle into an amygdule:


Some of the vesicles show strain (almost certainly due to late-stage flow in the increasingly-viscous lava, getting stretched out like air bubbles in pouring honey). Surface tension on the bubble wants to make it spherical, and the lower the lava's viscosity, the easier it will be to attain that perfect spherical shape, minimizing the surface-area-to-volume ratio. So when we find them in cigar-shapes or pancake-shapes instead, that's a clue that they've been deformed. Deformed not by tectonic forces (ductile flow at depth in an orogen), but ductile flow as a result of their formation, in a sluggishly oozing blob of lava:


Another example of stretched-out vesicles:


A lonely vesicle in an otherwise massive basalt:


Not sure what's going on here, but it looks cool (popped vesicles in sticky lava?):


Another thing you see a lot of in these Hawai'ian basalts are phenocrysts of certain minerals. Here, for instance, is a cobble showing nice olivine phenocrysts:


...and another:


Here's one I showed you last week when we discussed Green Sands Beach:


Here's an outcrop which shows phenocrysts of plagioclase feldspar instead:


And a river cobble (also vesicular) bearing a healthy population of feldspar phenocrysts:


Holy feldspar, Batman! This rock has a huge proportion of feldspars (you'll note that it's still vesicular, though: in spite of the overwhelming volume of macroscopic crystals, this is still an extrusive rock):


Here's something else caught up in a finer grained (and yes, vesicular) basaltic matrix: another piece of basalt!

This is a xenolith of slightly-older basalt showing flow banding in its own trains of vesicles, that after solidification got broken off and included in younger flows of basalt. I'll post some additional xenolith photos later this week.

It's not all basalt, though. Here's a breccia made of basaltic cobbles (penny for scale):


And a closer shot of the same outcrop (penny for scale):


Finally, a rock I was surprised to see: an intermediate-composition extrusive igneous rock called benmoreite (nickel for scale, and note the rock hammer impact marks):


Benmoreite is way more felsic that anything else on the island. According to my volcanic advisor Jess, it's the result of late-stage partial melting of basaltic source rocks in the island's oldest volcano, Kohala. In other words, it's a distillation of basalt: concentrating the most felsic components in this decidedly-lighter-complected rock (nickel for scale):

Labels: basalt, geology, hawaii, igneous, national parks, pleistocene, primary structures, volcano, xenoliths

A new study in Current Biology looks at mitochondrial DNA evidence from 160 woolly mammoth fossils on both sides of the Bering Land Bridge, and finds that the beasts trooped east from Asia into North America, and then marched back again 40,000 years ago, at which point the Asian mammoths slid into decline and extinction. The interpretation by the study's authors is that the North American prodigal mammoths returned to the mother country and possibly wiped out their Asian cousins.

The original article on the Current Biology* site. *Link wasn't working quite right this morning...
Scientific American's treatment of the story.
An article in the New York Times reviewing the study.

Labels: alaska, critters, fossils, mammals, pleistocene, russia