Megan DiMarcantonio PHYSICAL GEOLOGY 2016 |

Canadian Rockies ABSTRACT

The Canadian Rockies were formed by tectonic plates shifts between oceanic-continent collision

during the Laramide Orogeny. The shallower angled subduction slab was more towards the

Southern end of the Rockies, which is why the mountain belt is a little more inland. This is because it

takes longer for adiabatic heating to take place. There are also foothills present on the eastern side

the Rockies due to faulting. The glaciation period where erosion happened created cirques, arêtes,

moraines, tills, horns, distinguished U-shaped valleys, and hanging valleys within the mountain

chain, and the rocks are made up of shales, siltstones, and limestones which were created during

the early to middle Triassic time period. There are also metamorphic rocks present, they were

created by having the sedimentary rocks closest to the mantle heated up by contact metamorphism

and turned into granite and gneiss rocks, which in turn were moving upward and carrying the

overlying sedimentary rocks with them. The Canadian Rockies creates more diversity then average

flat land due to elevation and rain shadows.

The Canadian Rockies were formed during the Laramide Orogeny by

oceanic-continental convergent plate boundaries. This created a volcanic

mountain chain, and over time those volcanoes became dormant and

igneous intrusions forced up the overlaying sedimentary rocks which were

present before the mountain belt was created. There was then a period of

glaciation which helped formed the overall slopes which we see today, and

was also a lot of erosion since they were first formed. The mountains also

play a major role in the climate present within that area, as well as species

tolerance due to the unique habitat the mountains provide.

Plate tectonics are largely responsible for how the Earth was formed

and the way we see it today. It all starts with divergent plate boundaries,

typically occurring as mid-ocean ridges (MOR). At these mid-ocean ridges

the oceanic crust is being pulled apart and the remaining space is being filled

in with magma, which cools and hardens into oceanic crust. At the same

time this is happening, another tectonic boundary is being pushed

below/against the surface. Those are called convergent boundaries and there

can be either subduction island arcs, subduction continental arcs, or

continent-continent collisions. Both of the subduction zones have oceanic

crust being submerged underneath another crust, either oceanic or

continental. Subduction zones are determined by density differences, since

oceanic crust is denser (3.0g) then continental crust (2.8g) then it is most

likely to be the subducting slab. Both of the subduction zones produce

volcanic belts due to adiabatic melting of the crust, which means when the

subducting oceanic slab goes beneath the crust, once it reaches a certain

depth it starts to melt and magma starts to rise (Figure 1). Since oceanic

crust is mafic (due to the MOR), when the subduction slab heats up it, the

hydrous minerals get carried under as well. At around 80-100 km deep the

minerals are no longer stable and start to dehydrate, the water then gets

driven off due to the increase in temperature. Once that water goes into

mantle rocks, which is mostly peridotite, it starts to melt and rise upwards.

Some proportion will make it to surface and produce igneous or

metamorphic type of rocks as well as volcanoes (Figure 2). Sometimes that

magma isn’t able to make it all the way to the top crust because it is too

dense, therefore it becomes an igneous intrusion and it pushes the rocks

above it upwards creating mountains (Figure 3).

Most mountains are made up igneous, metamorphic and sedimentary

rocks. Igneous rocks can be formed by basaltic lava flows, which are thin

and extensive flows due to the low viscosity. They can also be formed by

Andesite or Rhyolite lava flows which have moderate to high viscosity. Once

the volcanic arc forms from a subduction zone, and the igneous intrusions

force the sedimentary or metamorphic rocks above it upwards, the mountain

belt will change its overall shape due to the explosion of different types of

volcanoes and magma content. When a massive Plinian super volcano

eruption happens, it can create glaciation type of patterns due to the

amount of ash and aerosols in the air which decreases the earth

temperature. This glaciation period affected the way the mountains look due

to the movement of glaciers. There is also usually a change in the protolithic

component of the sedimentary rocks after time has passed because of the

change in temperature and pressure, again due to the subduction zones.

This converts some of the rocks into metamorphic rocks with little change to

their chemical composition. Metamorphic reactions happen to rocks that

have changed form while still in the solid state in response to their

environment and can either go through prograde metamorphism or

retrograde metamorphism without gaining or loosing chemical components

except water.

The Canadian Rockies in particular were uplifted originally by tectonic

activity in a convergent plate boundary. The oceanic-continent collision

between different plate boundaries all had a role in the way the original

volcanic arc was formed. The angle at which the subducting slab was

entering the earth is crucial for when the crust melting happened. The

shallower the angle, the longer it would take for adiabatic heating to take

place and the more inland the volcanic belt/ mountain chain would form.

This is why the mountains all form around roughly the same distance inland

from the shore and create a ‘chain’ instead of being scattered in different

areas (Figure 4). There are also foothills present between the Canadian

Rocky Mountains and the plains, kind of as a transition zone. Topography

typically does not go from mountains directly to flat plains, there is usually

some sort of foothills belt that contain the inner foothills which are a bit

larger and the outer foothills which get smaller until they flatten out to the

plains (Figure 5) (Osborn, Emergence). The foothills are created when a

batholith is present, or when a thrust fault happens. When a collision

happens between two plates and causes huge masses of rock to crack and

slide over its neighbors is instrumental in the formation of the Rockies as

well as the foothills (Ward).

The formation of the Rockies happened during the Laramide orogeny,

which was a period of mountain building in the Western part of North

America. But what is there presently isn’t what was originally formed, of

course because there is about 55-60 million years’ worth of erosion that has

happened since the Laramide orogeny. About 2km have been lost on the

mountain, and has left the more resistant rocks at higher elevations and at

the lower elevations, where less weathering has happened, the less-resistant

rocks still remain. There are also the major rivers to consider for the

draining/erosion on the eastern side of the Rockies that cuts across the

structural and topographic grain of the mountain because the overall slope

of the landscape is East North East (Osborn, Emergence).

The Rockies display classic indicators of glacial erosion which include

cirques, arêtes, moraines, tills, horns, distinguished U-shaped valleys, and

hanging valleys which were thought to have been created during the

Holocene epoch. There seems to be an overwhelming amount of evidence

that there was more than one glacier present due to the differences in debris

as well as the size of each valley. The thicker glaciers left a more well

developed moraine whilst the smaller glaciers left a less developed type of

lateral moraine. The trees and vegetation can also tell when the glaciers

were present by looking at the glacier trim lines. Cross dating the well-

preserved trees can also give an idea as to when that specific glacier passed

through the area as well as others later on by looking at the tills of different

ages. Unfortunately, cross-dating can only go back a certain amount of time,

but looking at the fossils present in the rocks and radiometric dating can also

be a useful technique for looking at the age of these rocks (Osborn,

Holocene).

Fossils are typically found within the rocks and index fossils are key for

knowing when that rock was formed. The rocks that make up the Rocky

Mountains were formed way before the mountain belt was created, and they

know this based off of both fossils and radiometric dating. Western Canada,

which are where the Rockies are located, are made up of shales, siltstones,

and limestones and were created during the early to middle Triassic time

period. They can tell this due to the amount of Triassic fishes preserved in

the sedimentary rocks (Neuman) (Figure 6). These rocks were originally

composed of sand silt and clay sized sediment and over time and they

decreased porosity and lithification happened which turned those

unconsolidated sediments into sedimentary rocks. The dating of zircons also

reveal the ages of the intrusive associated igneous rocks and the timing of

the Precambrian metamorphic events. It reveals that there were multiple

periods of magmatic and metamorphic growth in the Northern Rockies

(Doughty).

This happened because of the subduction zone, meaning that while the

igneous rocks (basalt from the MOR) were being heated up by the magmatic

activity and creating a regional type of metamorphism, so was the heating

up of the other sedimentary rocks around that area creating a contact type

of metamorphism. Since rocks want to be in equilibrium with their

surroundings when those sedimentary rocks undergo any changes, such as

pressure or heat, they change their mineralogy. When the mountain belt was

being created, the sedimentary rocks that were closer to the mantle were

being heated up by contact metamorphism and turned into granite and

gneiss rocks, which in turn were moving upward and carrying the overlying

sedimentary rocks with them. At the same time the igneous rocks were

being heated up and turned into magma and were changing their overall

composition as well (Gadd).

The difference in mountain shapes/dips within the entirety of the

Canadian Rockies were determined by different glacial and gravitational

driven processes. There were multiple glaciers in different spots all over the

Rockies, which means each slope had a different shaping development. For

example, the Canadian Rocky Mountains all have different slope angles, the

Castellate and the Matterhorn slopes are generally 35-65 degrees (Figure 7),

versus the Cuestas slopes are about 90 degrees (Figure 8). This involves

also the erosions process, and the type of composition the rocks are made

of. Even though the mountains undergo the same development, the rock

compositions are all slightly different because the amount of the starting

material was not exactly the same. The protolith was of the same type of

material but there were still different amounts of minerals present in each

specific rock, therefore changing the overall composition of each rock type

(Cruden).

There are many reasons on why the Canadian Rockies are important,

one being diversity. Meaning, that at higher elevations, there are different

species present because the higher elevation makes it harder for species to

thrive since it is a harsher climate. The higher up the mountain goes, the

less area is available so species must be able to compete for space as well

as have a higher tolerance to sun or shade as well as rainfall. This creates

more biodiversity and in turn, a higher species richness within that area.

Without those conditions, those species would not be able to thrive in that

location (Rosenzweig). It is also important because this process forms lakes,

which is another hot spot for species interactions and provided the animals

there with an alternative water source. The Rocky Mountains also creates a

rain shadow, which is geographically important because as wet air comes in

off the ocean, and rises over the mountains, it precipitates from increase in

pressure. The other side of the maintain now has a warmer and drier air,

again changing the climate drastically and the ability for some species to

tolerate these vastly different environments (Figure 9) and allowing different

organisms to live in that area that maybe before they would not have been

able to (Whiteman).

Overall, the Rockies create all different types of habitats for animals

due to their shape and structure. It is also a large tourist attraction because

of their massive peaks and for how long the chain goes on (through two

countries). The creation of the Rockies took a long time as well as the

formation of the rocks that make up the mountain chain. It is amazing to

see what Earth can really do, not just on the regional scale, but mountains

are all over the globe, and each one is created differently and contain

different materials.

Sources:

Cruden D. 2000. Some forms of mountain peaks in the Canadian Rockies

controlled by their rock structure. Quaternary International. 68-71:

59-65.

Doughty, P., and K. Chamberlain. 2008. Protolith age and timing of

Precambrian magmatic and metamorphic events in the Priest River

complex, northern Rockies. Canadian journal of earth sciences. 45:

99-116.

Gadd, Ben. 2008.Geology of the Rocky Mountains and Columbia’s. 1-3.

Neuman, A. 2015. Fishes from the Lower Triassic portion of the Sulphur

Mountain Formation in Alberta, Canada. Canadian journal of earth

sciences. 52: 557-567.

Osborn, G., G. Stockmal, and R. Haspel. 2006. Emergence of the Canadian

Rockies and adjacent plains. Geomorphology. 75: 450-477.

Osborn, G., B. Robinson, and B. Luckman. 2001. Holocene and latest

Pleistocene fluctuations of Stutfield Glacier, Canadian Rockies.

Canadian journal of earth sciences. 38: 1141-1155

Rosenzweig, M.L. 1992. Species diversity gradients: we know more and

less than we thought. Journal of Mammalogy. 73.4: 715-730

Ward Cameron. 2005. Formation of the Rocky Mountains.

MountainNature.com.

Whiteman, C. David 2000. Mountain Meteorology: Fundamentals and

Applications. Oxford University Press.

Figures:

Figure 1: Oceanic-continental convergent plate boundaries. Showing the formation of volcanic arc due to the subduction of oceanic crust. Source: Matthew Severs PowerPoint for Physical Geology 2016.

Figure 2: Hydrous minerals dehydrate off the water due to the increase in temperature and start to melt and rise upwards. Shows how adiabatic melting works. Source: Matthew Severs PowerPoint for Physical Geology 2016.

Figure 3: igneous intrusion of magma unable to make it to surface and sit below the lithosphere as batholiths or sills, which push the overlaying sedimentary rocks upwards. Source: Diagram by Roy Sites at Radford University.

Figure 4: Mountains are formed in chains and the mountain belt is showing that the subduction angle is more shallow near the Southern end of the Rockies because the chain is more inland then the Northern Rockies. Source: Encyclopedia Britannica, Inc.

Figure 5: Topography of the Canadian Rockies showing how it goes from steep mountains, to less steep foothills then to flat plains. Source: Wikipedia images.

Figure 6: Fish fossils found in the Canadian Rockies which goes to show the sedimentary rocks were formed during the Triassic time period. Source: Neuman journal article, “Fishes from the Lower Triassic portion of the Sulphur Mountain Formation in Alberta, Canada.” 2015.

Figure 7: Castle mountain slopes. Source: photography by Parihav found in Wikipedia.

Figure 8: Cuestas slope at Mt. Louis Canadian Rockies. Source: from the Collection of the Whyte Museum of the Canadian Rockies

Figure 9: Rain shadow diagram. Source: English Dictionary Education website.