EGU Blogs

Erosion

It’s all about scales

It has often been said that geology is the study of scales. Time scales, large scales, small scales and many others. Indeed, one of the most crucial parts of any photo or map is the scale. Furthermore, geologic concepts can be applied from the planet scale to the atomic scale and every size in between. What confuses most people though is not only do we work in terms of huge size/magnitude variations but we also work with huge temporal variations. Processes that can take billions of years to nanoseconds all have their place in geology. Perhaps this is part of what makes it such a fascinating science?

Anyway, I thought it might be fun to share a few variations in scale that I have noticed recently.

A large hoodoo in Dinosaur Provincial Park, Alberta. I would say it is around 5m high and the rock on top is at least 5m across if not larger. (Photo: Matt Herod)

Some smaller hoodoos also located in Dinosaur Provincial Park, Alberta. I woudl say these ones are about 50cm high. By the way, the round object between the two hoodoos at the back is a dinosaur vertabrae. Likely a Hadrosaur. (Photo: Matt Herod)

Some really small hoodoos. I found these on a recent recon trip to a sand pit north of Ottawa near Cantley, Quebec. (Photo: Matt Herod)

Pretty cool eh?! People generally think of hoodoos as big spectacular structures that look like they are performing an incredible act of balance. Hoodoos form through erosive processes on the soft sediment underlying a large rock. They can be formed by wind, rain, or freeze-thaw cycles that erode the soft sediment below the boulder. However, eventually the sediment directly beneath the rock/pebble is protected from above preserving the hoodoo.

Here is another example of how scale has nothing to do with process. What I mean by this is that the same process that forms large features is also capable of forming small ones, like the hoodoos above.

A medium size rat-tail at Cantley Quarry, Quebec. It is probably around 8m long and the rock in the front is about 1m. That is my co-TA, Brett, for scale. (Photo: Matt Herod)

This monster rat-tail is over 50m long! You can see the xenolith in the front where the rusty discolouration is. There is also a smaller rat-tail in the foreground. (Photo: Matt Herod)

A “micro” rat-tail. This tiny one is only a few cm’s long but it still has the same basic structure: comet shaped with an obstacle in the front. Although, in this case the obstacle is a mineral grain and not a gneiss boulder.

Okay, so hoodoos are not the only erosional feature than can be formed at a variety of scales. However, I think these rat-tails are even more impressive than the hoodoos. Rat tails are glacial erosion features that form underneath a glacier by vast, fast flowing rivers of glacial meltwater actually eroding the rocks and forming these streamlined features. The dominant bedrock in the area is a soft marble that contains xenoliths of hard Grenville gneiss. The gneiss acts as a barrier to the meltwater forcing it to flow around it. This protects the marble directly behind the xenolith and forms the rat-tail.

Scales don’t just vary in erosion features though. There are lots of other great examples of scale variation throughout geology.

File:Cristales cueva de Naica.JPG

Massive selenite crystals in the famous “cave of crystals” in Naica, Chihuahua, Mexico with a person for scale. (Image: Wikipedia)

Gypsum (var. Selenite) Fuentes de Ebro, Zaragoza, Aragón, Spain Cristal 6 cm (Author: Enrique Llorens) (Photo: FMF Mineral Gallery – used with permission)

The gypsum crystals of the crystal cave were formed by heated groundwater that was evidently extremely saturated with respect to gypsum. I haven’t done the calculations about how insanely over saturated this water must have been to precipitate crystals this large, but maybe if I’m bored one day I’ll try it out. The chemical formula of gypsum is CaSO4 – 2 H2O. The water filled the cave and the crystals were able to precipitate around 500,000 years ago. The dating method used was U-Th disequilibrium dating.

I have a funny anecdote about the picture of the cave above. I remember when news of the cave hit the media. I was taking advanced mineralogy at the time and someone brought that picture in to show our prof and get his reaction. His first reaction, before doing any research, was that the picture was photoshopped and the cave did not exist. It just goes to show that a) this cave is incredible and b) it is possible to fool mineralogists sometimes.

Small fold in meta-sedimentary rock of the Pinnacle Formation in Sutton, Quebec. (Photo: Matt Herod)

Photo of yours truly demonstrating the curvature of this fold which is also in the Pinnacle Fm. in Sutton, Quebec (Photo: not Matt Herod, but with my camera)

Google Maps image of the Rideau Lakes area north of Kingston, Ontario. Notice the kilometre scale folding in the area and how the lakes conform to the structural geology of the area.

As you can see the study of geology is really all about scale. Every major geologic discovery is required to have context in terms of either the geologic time scale or size. I have given some examples of how size can vary regardless of process. If you have any examples  of scales in geology I would love to hear about it. Please post in the comments below. If your example is in picture form I would be happy to add them to this post.

Thanks for reading!

Matt

Geology Photo of the Week #7 – Oct 7-13

Sorry this post is a bit late…the Thanksgiving holiday was Monday, class this morning and then hockey! Anyway,  for the 7th edition of Photo of the Week we travel to the Pancake Rocks, in Punakaiki on the west coast of New Zealand’s South Island. I was there in 2011 for a conference in Wellington and went travelling around afterwards.  One of my stops was here. The Pancake Rocks are made of 30 million year old limestone that is very finely laminated. The daily pounding of the surf has led to the amazing convoluted structures in the picture. As to the formation of the Pancake Rocks themselves I cannot find a definitive answer…some sources claim that they are interbedded lime mud and coarser material. Others state that they contain stylolites. Unfortunately, I don’t remember from my own visit which is the case…although I feel like I would remember if it were stylolites so I am inclined to believe the bedding view. Plus the beds seem too regularly spaced for stylolites.  Does anyone else have a definitive answer?

Thanks for reading and let me know which hypothesis you support for the formation of the bedding.

Cheers,

Matt

Geology Photo of the Week #6 – Sept 30 – Oct 5

This is my first official post, besides the welcome post, at GeoSphere – EGU edition. It seems fitting to begin with a post that is part of a continuing series from my old home and is bridging the way to my new one.  The photo of the week, while still only six weeks old, is and will stay a regular fixture on my blog. The photo for this week is of some fantastic glacial striations in glacially polished marble located in Cantley, Quebec, which is about a 40 minute drive from Ottawa and is one of the most popular field trip sites for students from uOttawa.

Glacial striations are an interesting feature. They are pretty much ubiquitous across Canada, but rarely are they so defined as they are in Cantley. Striations form from the abrading action of rocks and sediment particles embedded in the base of a glacier. As the glacier slides across the bedrock it scrapes these sediments along the rock below making these striations. In order to form a striation the glacier must be sliding, which only occurs when there is a little water at the bed-glacier interface to lubricate things. Furthermore, if you look at striations under high magnification you can see that the sliding is discontinuous. In fact, it is a series of perfectly aligned slips of only a millimetre or two that are connected to form a perfectly straight groove up to several metres long. Another great feature about striations is that they can tell us the orientation of glacial movement. Indeed, the parallel lines in the rock below point the way to glacier was flowing. However, it is very difficult to tell the glacial flow direction from a striation since the glacier could have been flowing in either direction to make a straight line. In Ottawa we know from numerous other glacial features that the Laurentide Ice Sheet was flowing roughly from north to south. During the Pleistocene epoch, about 10,000 years ago, glaciers were covering pretty much all of Canada as well as most of Europe. The ice sheet at Cantley was ~2km thick.

Glacial striations at Cantley Quarry, Cantley, Quebec. Click for a larger image. (Photo: Matt Herod)

Thanks for reading!

Matt