# What is this rock formation?

Hi guys

So I went on a field trip with my class and this is one of the things we saw:

Can anyone tell me what that is from? I think maybe it's from a glacier or something of that sort that caused it...but I really have no idea and why it formed!

Thanks,

Elliott

Last edited by a moderator:

## Answers and Replies

davenn
Gold Member

Hi Elliot

That is a Dyke, and a really nice example of one.

They are formed in 2 ways...
1... the intrusion of magna into surrounding rock fissures forming a section like you see there of very different in composition to the surrounding material.

2... can also be formed by the intrusion/injection of highly mineralised and very hot water this can form large veins and dykes of say quartz, from which gold is commonly found and mined.

Dikes are always away from the horizontal. If it was horizontal it would be called as sill.

cheers

PS there are less common sedimentary dykes...
quote from Wiki"
"Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock layers. They can form in two ways:
When a shallow unconsolidated sediment is composed of alternating coarse grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks through overlying layers and forms a dike.
When a soil is under permafrost conditions the pore water is totally frozen. When cracks are formed in such rocks, they may fill up with sediments that fall in from above. The result is a vertical body of sediment that cuts through horizontal layers: a dike."End quote

dike or dyke .... depends on which country you come from ;)
Dave

Last edited:
davenn
Gold Member
I wonder if you took a rock sample from there ?

did you look at it closely?
magnmatic dykes tend to have fine crystals as they cool very quickly. This is compared to a very large magmatic intrusion that produces a batholith. they cool very slowly maybe over millions of years and the crystals will be larger.

In your field trips and personal wanderings look out for large granite outcrops, these are batholiths. look at the difference in crystal size and so you can determine which ones took longer to cool :)

You will also find that the crystals are smaller near the intrusion boundaries and larger as you look deeper into the intrusion.

is this a university geology trip you were on ? or ....?

cheers
Dave

Thanks guys.

But I thought that dikes were more or less striaght up and down? I've never seen one sideways like that? What would cause hot magma to rise at an angle like that instead of straight up and down?

And no I did not take a sample of the rock home but we did look at it closely and it's made up of limestone I believe (fine grained as you say)...there were some fossils of crinoids in the rocks aswell.

Also yes it is what you would call a "university" geology trip...although I'm not in university yet...more college type (I'm in Canada).

davenn
Gold Member
Dykes can be at any angle, right up to near vertical, if its vertical its usually called a pipe, if horizontal a sill. Dykes will come off a sill or a pipe at an angle.

wow Canada cool. you guys have the biggest dyke swarm that has been found anywhere in the world !!!
It is the Mackenzie dyke swarm in the Northwest Territories, Canada.

hope you enjoy your geology. I did a BSc in Geology some years back, absolutely loved it. An awesome topic learning about the world around you.

cheers
Dave

Last edited:
Evo
Mentor
davenn
Gold Member
Hi Evo

thats nice !! good swarm. I love the big crystals in a pegmatite too, always look great in a rock and mineral display

Thanks

cheers
Dave

Dykes can be at any angle, right up to near vertical, if its vertical its usually called a pipe, if horizontal a sill. Dykes will come off a sill or a pipe at an angle.

wow Canada cool. you guys have the biggest dyke swarm that has been found anywhere in the world !!!
It is the Mackenzie dyke swarm in the Northwest Territories, Canada.

hope you enjoy your geology. I did a BSc in Geology some years back, absolutely loved it. An awesome topic learning about the world around you.

cheers
Dave
Yup! Although I live in Montreal, Quebec so haven't seen those ones!

On our field trip we went to a mountain in Montreal close to my school (Mount Royal)

Halfway up the mountain there was a large formation that obviously also had a dike in it, but my teacher also said something about glaciers being present and having something to do with the formation of the mountain...but I totally forget because it was a couple weeks ago! :(

Here's some shots of some rock formations that were above the rocks I mentioned before in this post:

I have no idea what to say about these rocks as were on stairs pretty high up and couldn't even get close to them (my camera was zoomed in). All my teacher wants to know is what kinds of rocks these are, but I don't' even have an idea where to start! I mean are they igneous, metamorphic, or sedimentary?

And finally at the top of the mountain we saw these rock formations:

I'm pretty sure these are mafic igneous rocks because of their dark color, and they were also fine-grained (potentially Grabbro?). They were the highest rock we obvserved and also thus the youngest. They were also fairly weak.

Not sure what to say about these either lol these lab reports also leave me hanging on what to write for the "depositional environment" :(

Can read more about Mount Royal here: http://en.wikipedia.org/wiki/Mount_Royal Which I'm doing right now!

Last edited by a moderator:
Evo
Mentor
I love rocks. I'll have to take some pictures of some rock formations in my neighbor's yard. Everything here was underwater 440 million years ago, but these are different from the others.

davenn
Gold Member
Those first 2 pics look like sedimentary rocks, fine grained maybe mudstones etc maybe slightly metamorphosed

those last 4 quite interesting definately look igneous... those greenish look clumps look like olivine
I got some really nice samples of that from the basalts in Hawaii back in 1999.

just a little hard identifying rocks from photo's ;)

OK rules for a budding geologist....

1.... Rock hammer
2.... chisel ~ 2cm wide blade
3.... magnifier eyepiece x10 magnification is the norm.
4.... plastic clipseal bags for samples ( go to your local supermarket and buy a box of them. you get 100 for just a few $$4.... permanent ink marker pen ( texter -- or what ever you call them over there) to mark the bags with And do mark them... you will never remember where and when you got the samples if you dont mark them at the time of collecting REALLY REALLY important. Then when you get home to write up reports you have your samples to refer to and study. And they will be the start of your own rock and mineral collection :) probably some other things, but they skip my mind at present :) cheers Dave davenn Science Advisor Gold Member They were the highest rock we obvserved and also thus the youngest. They were also fairly weak. NOT necessarily ! they may have been exposed through erosion and may be quite old determining their age compared to the surrounding rocks would require some serious mapping to determine what is most recently layed down ( for sedimentary and eruptive volcanics) and what may have been exposed through erosion, tectonic activity ( faulting, folding etc) below is part of a huge granite batholith in the western USA.... Half Dome, a granite monolith in Yosemite National Park Now its on top, but its is very very old... all the younger rock has been eroded away over countless millions of years cheers Dave #### Attachments • 115.9 KB Views: 577 Evo Mentor OK rules for a budding geologist.... 1.... Rock hammer 2.... chisel ~ 2cm wide blade 3.... magnifier eyepiece x10 magnification is the norm. 4.... plastic clipseal bags for samples ( go to your local supermarket and buy a box of them. you get 100 for just a few$$
4.... permanent ink marker pen ( texter -- or what ever you call them over there) to mark the bags with

And do mark them... you will never remember where and when you got the samples if you dont mark them at the time of collecting REALLY REALLY important.

Then when you get home to write up reports you have your samples to refer to and study. And they will be the start of your own rock and mineral collection :)

probably some other things, but they skip my mind at present :)

cheers
Dave
Camera?

True, but I'm going on the basis that the Mount Royal area had some igneous activity in the past. Also the highest rock is also igneous...

Am I going on the wrong path here or what? I need to write what the depositional environment was at the time but I'm quite lost!

davenn
Gold Member
I love rocks. I'll have to take some pictures of some rock formations in my neighbor's yard. Everything here was underwater 440 million years ago, but these are different from the others.
yes please :) where is here ? roughly. dont need your street name or number hahaha

Tho I now live in Sydney Australia, my home is Dunedin in south eastern South Island of New Zealand. A very volcanically active area ~ 8 ma.

The oldest accessable NZ rocks, that I know of, are in the NW corner of the South Island
they date back ~ 500ma and are full of Trilobite remains.

I still collect rock and minerals but my main interests lie in seismology and volcanics.
I never worked in geology professionally, just did the university thing out of pure enjoyment :)
I run a digital seismograph system from home and am part of a world wide network called the PSN (Public Seismic Network).

Yes Camera ... thanks Evo :)

cheers
Dave

davenn
Gold Member
OK Elliott

that wiki page confirms that the green structures in your pics is most likely olivine :)

what has happened in your area there is very similar to what happened in the Yosemite
NP. The volcanics didnt become eruptive. They just intruded into the overlying sedinmentary rocks. Over the millions of years a lot of that sedimentary rock was eroded away leaving the gabbro stocks ( stocks are small versions of batholiths). in some places as in your very first pic the sedimentary rock still remains and shows evidence of the intrusions with the dykes.
Dont know how much time you have to do your report ? if a few days more I wouold be doing some science library searching at the university for some geological history of the area. Have a good read. Dont quote word for word the reports but put it into your own words. AND ALWAYS !!!! give your references as to where the info came from :)

Keep in touch ... let us know how you get on

cheers
Dave

PS... the depositional enviroment ... things to consider....
well that depends on the type of sedimentary rocks there .... are they seawater or fresh water marine ?
did the intrusions happen whilst the area was still covered in water ( sea, lakes) or did it happen after the water receeded.
some of that geological history reading will help you there.
Find out when the area was covered in water, and for how long.
find out if possible when the intrusions were thought to have taken place.
see if there is any info on what caused the significant erosion in the region, just high rainfall weather ?
rivers? something else like glaciers scraping off the top layers during the last ice age.

I would suspect the glaciation of the last ice age being the main culprit. get some dates on that event :)

cheers
Dave

Last edited:
davenn
Gold Member
OK

a timeline something along these lines ....

1.. deposition of sediments on a lake or sea floor
2.. igneous intrusions during or after water inundation
3.. iceage glaciation stripping off much of the soft sediments
4.. retreat of the glaciers exposing the remains of the sediments and the intrusions poking through

all you need to do is to confirm that possibility and put some dates to that time line :)

cheers
Dave

Great thanks so I'm pretty much done my report I just have one more question. This is the 2nd stop we saw after seeing the picture in my FIRST post. I haven't posted this before but I now have questions baout it and it could help you with understanding the depositional environment:

So obviously there was a change in the depositional environment for the rocks to be layered light-dark-light like this...but what would have changed? Could this have been caused by the sediments in the water (as the water was drying up?) settled at the bottom and kept getting compressed?

I thought of that, but it seems unlikely that it would occur like that so many times to give you the same light-dark-light pattern.

One of my friends in the class said it was seasonal deposition, where in the winter one layer was deposited, and then in the summer a different layer...but what would cause this? I can't really find any info about it on Google. Like why would there be different layers forming at different times of the year?

Thanks,
Elliott

Last edited by a moderator:
Evo
Mentor
Your rocks are killing me. I'm a rock junkie, have been since I was a baby. My neighbors think I'm nuts, sitting outside with a magnifying lens (all of our rocks are fossil rocks).

davenn
Gold Member
Hi Elliott

That first pic with all the obvious banding is common with seasonal layers of silt on a lake.
Think about what happens there evey year.... You have a winter time with little water flow, followed by a big thaw in spring time. Those raging torrents of thaw water, in many streams and rivers, transport huge amounts of sediment from the surrounding land and into the lake building up a layer through spring and summer and till everything freezes up again for another winter.
Now I'm saying beyond all doubt that thats what happened in your area, but its a pretty good likelyhood.

That second pic does look like an iron band. A possibility was that at some time during those thaw flows a river/s cut through some iron deposits and erroded out a lot of the material and deposited it in the lake.
if its the only really noticeable band, the river may have cut through that deposit on one go (in one season) and that was it. or maybe the river changed course and didnt go through the deposit again.

All sorts of possibilities like that. Thats what I like about doing geology. You have to do a bit of time travel and go back and imagine what things were like when a land form was produced. Then you have to move forward in time and try to imagine/figure out what happened to that land form to produce what you see today. :)

Dave

Last edited:
@Elliot,

If you look at the sedimentary rock into which the dyke is intruded you will see that there are fractures running parallel to the dyke and about 60 deg. to those planes. Magma intrudes along lines of weakness and these are related to the stress field imposed on the rock. (A sub-set of structural geology deals with measuring and interpreting such fracture patterns, fault planes, folding and other deformation.)

There does not seem to be any major thermal metamorphism in the country rock, on the margins of the dyke. The one indication I can see is vertically above the arrow you have added. You can see a small fragment of dyke rock adhering to the country rock, where the rest of the dyke has fallen away. That suggests a strong bond between the two related to thermal alteration and probably minor chemical infiltration of the country rock.

The absence of major thermal metamorphism will certainly be related in part to the small size of the dyke, but perhaps also to its low temperature. The colour is typical of fine grained granitic igneous rocks in which orthoclase feldspar is a major component. Your dyke is probably a rhyolite or similar.

If you look to the far right of your picture you will see there is part of an even thinner dyke. It has the same orientation.

The country rock is a blocky shale and is well indurated, so it has been buried to a substantial depth. I am very rusty on the quantitative relationship between overburden load and degree of compaction, but I would guess it has had no less than 8,000' of sediment atop it.

It doesn't have the 'feel' of a lake sediment. I would think the distal portions of a delta, or deep sea. There are some high concentrations of iron oxide, as evident from the red staining on the shale.

The new photos you've posted contain a wider variety of rock types. The lighter materials look as if they may have ahigh carbonate content. Dave, I'm sorry, but I don't buy the lake hypothesis. It's not impossible, but the variety of lithologies present and the coarseness of the layers argue against it. If this is a close part of the sequence showed earlier then I'm leaning more to the delta hypothesis.

There have been some remarks about glaciation and it has almost been implied that the glaciation and the deposition were not all that far removed in time. Forget that. My gut feel - and this could be amusing because I may be competely wrong - is that these are Lower Palaeozoic sediments that have been intruded much later (probably during a Late Palaeozoic orogieny) by the igneous material Elliot has shown us. Though I'm then left wondering why the strata are currently horizontal. I'm off to do some literature searching on wherever it is Elliot said these are from, then will return, shamefaced, when I discover how wrong my gut feel was.

Last edited:
turbo
Gold Member
Pegmatite dikes are my favorite places to hunt for minerals. Many years ago, feldspar was mined in western Maine for the porcelain industry, and related deposits were mined for large sheets of mica - used for heat-resistant windows. During WWII some of these same locales were mined for beryllium, which had previously been more commonly sourced from Africa. Years later (if you can get permission) it is possible to find gemmy beryl (aquamarine), morganite, and tourmaline at these old mines. The most productive method of mining is to dig through the tailings (dump) instead of hard-rock mining, because depending what the miners were looking for years back, they regarded almost anything else as waste.

davenn
Gold Member
The most productive method of mining is to dig through the tailings (dump) instead of hard-rock mining, because depending what the miners were looking for years back, they regarded almost anything else as waste.
Or often back then They only took what was easy to get at. In New Zealand and Australia, it has been common with modern mining companies to go back through old tailings with better recovery techniques and get almost as much gold etc as what the original mining recovered.

Dave

davenn
Gold Member
It doesn't have the 'feel' of a lake sediment. I would think the distal portions of a delta, or deep sea. There are some high concentrations of iron oxide, as evident from the red staining on the shale.
thats true, and if you noticed I had also mentioned seafloor :) where ever you see lake just read lake/sea :) You are going to get significant amounts of carbonate materials in both situations.
I like the lake, seafloor/delta ideas equally they are all plausable. I guess without having been to the region and to see the material for myself or its extent its a bit difficult to definatively determine. .... Was it a very large lake ? or was it the costal shore of the sea that covered much of the southern half of the continent ? The delta idea fits into both of those scenarios.
I guess the deal clincher would be the observation of microfossils that were endemic to either fresh or salt water.

But there is no doubt that glaciatiation is responsible for the massive denuding of much of the top layers of upper sediments across large portions of the North American Continent. Which resulted in the underlying rocks being exposed as shown in photo of the region around Yosemite NP.
Elliott has also commented, as does other info, on his region about the same style of exposure of stocks.

cheers
Dave

Last edited:
The first pic is not metamorphic indeed, the edges are too well defined and there is little or no co-mingle with the surrounding matrix. My first gut feel was ancient alluvial runnoff layer (I know someone said "river delta" and I am leaning that way myself).

The layers of iron can be from Glaciation if we look at your area. There are huge iron deposits not all that far north in Labrador.

You live in one of the best places in the world geologically speaking. http://www.mcgill.ca/gault/sainthilaire/" [Broken] is world famous for having rare and unusual specias of stone (over 370 species discovered there and 50(!!!) new ones to science with many more unidentified yet). If you ever get the chance to do a trip there (it is heavilly restricted) go!

Last edited by a moderator: