June 30, 2011

Field photo Set #3

The newest pair of field photos are quite local to me, as they are a slice of glaciomarine layer cake left over from the last waxing of the Cordilleran ice sheet, specifically the Wisconsonian. The following pair show exposures of a glaciomarine clay unit sandwiched in-between granite bodies and Holocene soil & sediment, found within North Vancouver's uplands. What you'll see are portions of the Capilano Sediments member of the Sumas Drift group formed in the Quaternary.

The first photo (at right) is a classic exposure within the Capilano Canyon park & suspension bridge (49° 21.470'N 123° 06.698'W). You'll notice the beige coloring of clay that is packed into mud; this unit is quite distinct from surrounding Holocene soil, the greenery, and the granodiorite bedrock. It has a very smooth consistency, and upon scrutiny has micro-level faults from minor stresses placed on it. The layer dips a good 10-12º to the SE, with a NE-SW strike.

The foliage around the site is quite dense, even in the winter. Biomass detritus litters the photo, which was taken in the late winter months. This Pleistocene marine clay was formed at the front of valley glacier lobes stemming from the Cordilleran ice sheet. The continental slope at the front was relatively high, so deep sea marine deposition was not dozens of kilometers from the glacial terminus, but rather a few kilometers. Post-Pleistocene isostatic rebound was quick in the region, and it elevated this layer to at least 50m asl, and in some areas up to a few hundred meters. Post Eocene tectonic uplift of the coast mountains caused the northern edge of these types of sedimentary units to be gently warped upwards to the north and draped over the lower slopes of the older rocks of the north shore mountains.
This exposure of the same member is found along the Lynn Peak trail (49° 21.735'N 123° 01.519'W), about a quarter of the way up, roughly 300m asl. This portion is mantled by Holocene soil, and surrounded by a lot of till and colluvium. This particular photo shows the most distinct boundary surface between the clay and surrounding members. It is likely a lens of material formed during episodic activity (see link on Surficial and Bedrock geology). The continuity of the glaciomarine member is hard to follow along the north shore, as it is buried under steepland sediments, landslide debris, and thick vegetation. So spotting a slice of the cake is considered good luck for intrepid geological explorers.

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June 25, 2011

Accretionary Wedge #35: No Jive, it's Ogive

Gilkey glacier ogives, rimmed by medial moraines
(58° 49.280'N 134° 21.481'W)
Evelyn over at Georneys is hosting AW #35, and the bloggers of the geoblogosphere are submitting their favorite geology words. Coming up with a favorite geoscience word was tough. I thought of going for the comically crude, but none would have been my favorite. So I went from experience and what pops into my head first. On the rare occasion I've ventured across the top of a glacier, and when doing so my companions and I always played a game of 'spot the feature'. You had to get right the specific type of crevasse or moraine, and there were no points for pointing out firn or glacial ice. My greatest success at spotting a feature was straight off the chopper at the Gilkey trench in the Juneau Icefield. The Gilkey glacier had these strange alternating bands of light and dark crescents pointing westwards towards Berner's Bay. "Ogives!"

So what are these patterns on the surface of valley glaciers, and how do they form? Ogives are curved bands across the surface of a glacier, with convexity facing downhill. The bands are characterized by alternating dark and light groupings. The darker bands are devoid of ice-bubbles, are formed from melting & refreezing of ice in the summertime, and contain sediment accumulated at icefalls where open crevasses become a pit of deposition. The lighter bands are filled with snow & air bubbles from the non-summer months when precipitation is greatest, and a fresh snowpack acts as a layer of protection against weathering. Thus ogives are a seasonally created phenomenon. The crescent shape is due to velocity/friction differences between the lateral edges of a glacier where velocity is low & friction is high, and the center of a glacier where velocity is high & friction is low.

glacial flow lines relative to surrounding bedrock
Due to their darker color, the summer bands have greater conductivity to solar radiation, and thus are topographically lower due to increased melting. My experience traversing the Gilkey glacier was that the trough created is noticeable but pretty minimal, on the order of a 8-10 foot amplitude between a dark bands trough and a light bands crest. Interestingly, the combined width of one light + one dark band corresponds to the distance a glacier traveled in a year, thus it is a proxy element of glacial motion that can give a decent measurement of an advancing glaciers speed.
During summer, the glacier's surface melts and crevasses collect windblown particles, creating the dark band
During winter, the surface is covered with snow, protecting it from weathering and creating the light band
There you have it. Ogives! A wonderful pattern seen in some of natures freezers. The Vaughn icefall in the Juneau Icefield is as close to an idealized conveyor belt of the banded pattern you can get, but there are others. Soon I plan to visit Mt. Rainier, whose alpine glaciers are purported to have some of their own ogives.
A valley glacier replete with ogive banding, stemming from near Mont Blanc in the Graian Alps
(Credit goes to Sue Ferguson for the title of this post, an homage to her excellent guide book "Glaciers of North America: A field guide")

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June 20, 2011

Kata Tjuta - the forgotten sibling

Kata Tjuta, Anangu for "many heads"
Uluru is a sandstone gem of Australia's interior landscape. It rises off the plain as a formidable inselberg of rusty red solitude. It's what comes to mind about the Australian countryside for many non-Australians, whether as the Anangu name or the anglicized 'Ayers Rock'. Savvy geologists, physiographers, and locals, however, will know about the lesser known gem of conglomerate rock domes not 30 kilometers to the west of Uluru. The Olgas, known as Kata Tjuta in the Anangu language, are a collection of 36 conglomerate rock domes rising off the plain to various heights. The highest is Mount Olga, with a prominence of 546m.

Basic stratigraphic cross-section of
Uluru and Kata Tjuta
Kata Tjuta is part of the same stratigraphic formation as Uluru (see diagram at right), a unit of arkosic sandstone derived from Cambrian alluvial fan deposits of sediment that were buried and lithified over deep time. The difference in shape & topography between the two is due to the structurally weaker conglomerate of the Olgas (the Mount Currie Conglomerate) being exploited by folding, faulting, and subsequent hydrologic weathering & erosion. The conglomerate matrix is comprised of granodiorite, basalt, gneiss, and fine sand as the cementing material. Essentially, Uluru is the tougher end of the layer, though it does not lack features created from weathering such as pits and tafoni-based honeycomb surfaces (indicative of Mediterranean paleoclimates). Even though the conglomerate lithology of Kata Tjuta lacks the feldspar mineral content that Uluru has, fractured blocks were exploited, making the initial mass structurally weaker, even though it is mineralogically stronger. Both ends started out as a large singular massive block, but Cenozoic Australian climates have taken a greater toll on the Olgas for the reasons stated.

Back in the Neoproterozoic, the Petermann mountain range was more formidable than it is today, due in major part to the Petermann orogeny. However, the Neoproterozoic/early Paleozoic climate was more temperate, so its peaks were being denuded by precipitation at a greater rate than the more subdued Holocene landscape is by current aeolian forces. Alluvial fans were created along the flanks of the Petermann foothills, each differing slightly in lithology, but not in origin. These fans formed a piedmont range that was a major part of Petermann foreland basins, such as the Amadeus, Georgina, Ngalia, and Officer basins of then central Australia. The fan material developed sequentially into flysch once a eustatic change in sea level occurred in the late Cambrian, covering the region in a shallow sea. Continuing weight of added sediment + the weight of the increasingly deepening sea was enough pressure to lithify the alluvial fans/flysch into arkosic sandstone (Uluru) & conglomerate (Kata Tjuta), each portion representing a different fan thus a slightly different lithology, all connected together during the melding of adjacent sedimentary units. If we could remove the overburden of sand & schist members, we likely could find the area where the conglomerate and sandstone grade into each other.
Google Earth snapshot of Central Australian plain, with Kata Tjuta @ left and Uluru @ right (VE = 3x)
Coordinates for centerpoint = 25° 18.710'S 130° 53.730'E
Australia was quite geomorphically active during the Paleozoic; a contrast to the quiet old continent of today where relief and rates of denudation are comparatively low, and atmospheric hazards dominate. After the transgression of the early Paleozoic, the sea receded and orogenic activity took over in the Devonian, thrusting and folding and faulting Central Australia, to the point that the Mount Currie Conglomerate formation folded with surrounding units into a distinct syncline. The Late Paleozoic – Mesozoic – Tertiary periods began a slow march of weathering & erosion of overburden, until the exposure of the ends of the Mount Currie Conglomerate finally revealed a more resistant rock. Exposure to the atmosphere is estimated to have occurred during the Jurassic. A basin between Uluru and Kata Tjuta collected aeolian sands and dunes throughout the Quaternary, thus planing the region through depositional mechanisms. All that stood out in the plain were the tips of rocks derived from a resistant lithology. The rock domes of Kata Tjuta specifically dip 10-20º with a SW-NE strike.

The finer features of the Olgas have been primarily shaped by precipitation during more temperate paleoclimatic conditions. Freeze-thaw processes acted on joints in the rock, fracturing the surface. Rivulets of creeks and small waterfalls promoted the formation of potholes and gorges. Since granite is a primary ingredient in the Kata Tjuta Mount Currie conglomerate, spheroidal weathering was able to smooth and accentuate a rounded dome shape to the remaining 36 mini-bornhardts by working on the angles & corners. Of course, iron content exposed to the atmosphere colored the veneer of Kata Tjuta to that typical iron oxide rusty-red. Some visible structures noticeable when perusing the rocks include limited tafoni structures among the rock domes. Slickensides are also apparent, indicating displacement of large sheets of the conglomerate during times of acute mass wasting.

As an aside, I stumbled upon an interesting take on the formation of Kata Tjuta when google-searching: Tas Walker's Noachian interpretation of Kata Tjuta. It is a prime example of working backwards with the scientific method, where a proposed theory is the starting point, and evidence to support it is anecdotally surmised to fit that theory. Remember, if a null hypothesis cannot be rejected, and credibility cannot be established through peer review, it is not science, and certainly not geomorphology science. Current [accredited] geomorphology research reveals noticeable increase in relief amplitude of both Uluru and Kata Tjuta inselbergs throughout the Cenozoic, which is atypical of the ideal cycle of erosion (see additional links). Interesting mechanisms must be at play, and a deeper look into the asthenosphere, lithospheric flexure, and the mass dynamics between the overburden and the Mount Currie Conglomerate are called for.
Thanks to my brother for the above photo of the Olgas, circa 2007
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June 11, 2011

A new toy! Can I play?

I like volunteering. You meet compassionate people with similar interests & concerns in an informative setting. However, I live in the realities of a capitalist western country, so the [very] limited free time I have outside of university studies restricts my volunteerism, as it logically is the first thing to get cut during crunch time and the search for funds. Summertime is a bit different: classes are few or nonexistent, daylight is plentiful, and my general mood is happier & more energetic. Consequently, I tend to volunteer with local organizations that have environmental directives, and over the years I've found a few that have programs which are laid out in a simplistic fashion, and thus easy to volunteer portions of my free time for. One in particular I'm on my 4th year with involves canvassing residents of Langley on how to protect their dwindling groundwater supply through conservation and community action (~50% of the townships water supply is provided by unconfined or shallow-confined aquifers).
A sample of vesicular basalt as a control in an experiment. The sample was obtained from Mt. Rainier
In more recent years, with my growing interest in academic research, I've started asking university professors if they or their grad students need any field/lab assistance with their research. Any that have ongoing work always say yes, and when I come in with enthusiasm and interest in reading & discussing their work, they shoot back with an even greater level of enthusiasm. Probably the best aspect of volunteering in labs or the field is not the possibility of paid work, nor the time to pick the brains of current researchers, nor getting my foot in the door. Those are all excellent aspects, but the best has to be the exposure to the precision technology that I get to [cautiously] fiddle with and test out, and see its application towards specific facets of quantified research.

One recent new addition to the university had the petrologist professor quite giddy, and him and I got to play around with the new device for awhile, figuring out all its quirks and functions, and running some initial control tests to ensure proper functionality. The device was something I'd never heard of before, but the explanation of its logic and level of precision made perfect sense. I speak of a Helium-based pycnometer (pictured right).

It is hard to get an accurate measurement of density for vesicular rocks, such as vesicular basalt, pumice, scoria, etc... due to the irregular arrangement of void space in their matrix where gasses exsolved. Helium is a relatively inert gas, so functions better than a nitrogen/oxygen/argon mixture which could be adsorbed by silicic material. Helium is better at diffusing within rock samples of high surface area with the tiniest, micrometer-level pore spaces, ie. vesicular rocks. Thus the displacement of Helium between containers (one with the rocks and one without), and application of the ideal gas law, and we get the volume of the rock sample with deadly accuracy. We tested out the device using some vesicular basalt (pictured above) gathered from pyroclastic flows ejected from Cascade Arc volcanoes. Looking at the basalt petrographically was important as well, so I made thin sections for viewing under the microscope and we viewed the optical mineralogy of the basalt.

Mt Edziza stratovolcano, which has erupted felsic
magmas such as rhyodacite or trachyte/comendite.
Image courtesy Canadian Encyclopedia
The pycnometer is supposed to help the professor's research of the geochemistry of the Edziza volcanic complex within the NCVP. I hope to assist in as much of it until the concluding phases and journal publication, mainly because it allows further access and experience with new physical geography/geology toys. I've also recently got some fresh experience with a 15m long sediment transport flume, but that's another research tale I hope to tell after more time with the flume.

For other undergrads I strongly recommend volunteering your time & energy to your university profs and grad students. Trust me, they are likely to welcome your assistance, and you'll benefit from the experience and the contacts, especially if post-graduate studies is on your radar. It gets your foot in the door, and is thus invaluable.

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    June 6, 2011

    Geology at a Birthday Party

    My niece recently had her 10th birthday, and I felt it fitting to add to her burgeoning rock & mineral collection with some flint, chert, and serpentine. I mentioned the properties of serpentine to her, and its connection to asbestos, which is a carcinogen. She must have forgotten the part where I mentioned there is no chance of inhaling carcinogenic asbestos from the fist-sized rock I gave her unless she grinded it into a fine mist, nor does it have the mass of chrysotile needed to make her lungs even notice. But lo and behold a few hours later she had placed a paper on top of her rock box ..."Warning - Cancer rock inside".

    An opportunity to educate her on the properties of minerals, and she took to it. Kids get right away that rocks are essentially a collection of minerals in different ratios. I'm amazed at how easy it is for kids to quickly grasp many things geological; Sometimes I wish I had such a malleable mind when I'm engulfed in my university studies. Specific gravity/density is a hard one to explain, but streak powder, cleavage, fracture, hardness, were all easy to demonstrate given that visual demonstration of them is straightforward. If you try this, I would recommend not using the mirror example for hardness, as some kids might run off to try it on bathroom mirrors.

    Later on after her party, we sat down to watch some old Simpsons episodes on TV, and Treehouse of Horror V was what was showing. Homer invented a time machine by modifying a toaster, and it catapulted him back to what appeared to be a blend of the Paleozoic and Mesozoic eras, and even some Neogene (a Megatherium alongside a T-Rex??). His father gave him sage advice about not touching anything in the past, as doing so could alter the future in ways he couldn't possibly imagine. Homer was chased by a T-Rex, eventually escaping. Unfortunately, upon sitting down for a rest, he squashed what is considered the first land-walking animal. I speak of the Eusthenopteron:
    courtesy 20th Century Fox
    Homer killed the terrestrial evolutionary process! For some reason, that lead to the human race being 50-feet tall giants. Wouldn't it lead to us remaining aquatic animals?? Why am I trying to find evolutionary logic from a late-night cartoon? Meh, I was just pleased to see these archaic lifeforms I'm familiar with used for satire.