Where Terranes Collide: The Geology of Western Canada

this is the Canadian portal arrow the name given to the mountain systems of Western Canada for almost 150 years many of the men and women of the Geological Survey of Canada have devoted their careers to discovering how and when these mountains came to be the research tells a fascinating story of moving continents the creation of mountain ranges and colliding terrains this story began about 1700 million years ago continues today hello my name is Chris Yaris I'm a geologist and I work for the Geological Survey of Canada the chordal era includes all of the mountain systems valleys and plateaus of western Alberta all of British Columbia Yukon and much of the Northwest Territories and from here at Pacific Rim National Park on Canada's west coast the Cornell era continues some 80 kilometers offshore we're at the toe of the continental slope the edge of the North American continent meets the deep ocean floor out there the Earth's crust is proved plates are moving created and destroyed by the processes of seafloor spreading and plate tectonics along the Juan de Fuca Ridge new seafloor is continually being created through injections of molten magma into the central rift of the ridge there it cools to become oceanic crust with each succeeding injection the newly created crust moves away from the ridge forming the Pacific plate to the west and the Juan de Fuca plate to the east at present the Juan de Fuca plate is converging toward us at about 4 centimeters per year or at around the same rate by which your fingernails grow where it meets the North American plate the crust of the seafloor is consumed or subducted beneath the continent it is through these and related processes acting over hundreds of millions of years that our continents and oceans have been created that our mountains have been formed part of a geologists problem in explaining these ideas has to do with time enormous intervals of time so to make this problem more palatable let me compare the history of the earth to the face of a 24-hour clock at the top is time zero midnight four and a half billion years ago when the earth began to form the first twenty seven hundred and fifty million years or until about 2:30 in the afternoon was taken up with the formation of our continents oceans and atmosphere the beginnings of the first forms of primitive life and the Assembly of the globe's continents into one giant supercontinent the remaining seventeen hundred million years for nine and a half hours witnessed the rifting a part of the giant supercontinent the growth of Western Canada and the extraordinary increase in numbers and diversification of life that included the appearance of our earliest ancestors some 3.5 million years ago or on our sand clock at about eight minutes to midnight now imagine that you're standing on a beach like this sometime between 1700 min and 750 million years ago before the giant supercontinent broke apart behind you our high mountain ranges that are eroding away the gravels sands and muds being carried by streams and winds westward to the sea out there in the gray mist but not far is Australia and next to it the continent of Antarctica these also are mountainous regions from where eroded sediments are transported to this landlocked sea over some 950 million years these sediments accumulate to form a sequence for sedimentary strata over ten kilometres thick here at Waterton Lakes National Park in southern Alberta we can see some of these sedimentary rock strata forming the cliffs of the surrounding mountains the rocks that form these mountains are over a billion years old among the oldest in the Canadian chordal era and yet the mountain structures themselves were formed comparatively recently it's like this fine old hotel when it was constructed in 1927 it was built of wood that was already a few hundred years old mountains are like that constructed out of materials that formed at one time and elevated into mountains at another time but for the moment we are looking only at the rocks the mountains will come later up there on the side of Vimy Peak variations in the rock layers record the changing conditions during the time the sediments were deposited these changes in color sediment type and other characteristics reveal varying rates of erosion from the mountainous source areas changes in climate and environment and a host of other variables for a billion years these sediments eroded from the mountains of North America Australia and possibly from the continent of Antarctica that is until about seven hundred and fifty million years ago giant supercontinent broke apart North America separated from Australia and Antarctica along a line not far from the locations of the modern communities of Penticton Salmon Arm Prince George and points north this put the edge of the continent several hundred kilometers east of its present location with the shoreline somewhere in western Alberta for the next half billion years sediment eroded from the ancient North American mountain systems was carried westward to cover the torn edge of the continent during that time the Western margin of North America often resembled the modern Bahamas worn clear shallow seas would spread inland from an ancient Pacific across the gently sloping continental shelf on the floors of these seas coral reefs flourished which ultimately were buried by the constant rain of sediment in late December of 1946 about a mile beneath the grain fields of LaDuke Alberta these same reefs were found to contain oil and gas the coral reefs one of which is exposed here at the surface above the town of Kenmore are like giant sponges made of calcium carbonate and full of interconnected holes deep below the surface of the prairies these reefs are completely surrounded by black organic shales from which petroleum created from decomposed marine animals migrated and became entrapped in the interconnected pores of the reefs the mountains of the eastern Cordillera largely consist of carbonate strata formed mainly from the fossil shells of ancient marine organisms such as those of these pale grey cliffs of Cascade Mountain on the outskirts of the town of Banff but as we drive further west the rocks changed to the dark somber tones of mud now converted to shale these were formed in the deeper quieter water of the western shelf closer to the edge of the continent where the fine debris eroded from the interior continued to rain down upon the seafloor as we reach the community of Salmon Arm in British Columbia's interior we are close to where the Continental edge of North America was probably located up until about 180 million years ago west of here was a vast ocean a forerunner to our modern Pacific out there other things were happening terrains were being formed pieces of crust that eventually collided with the edge of the continent terrains are pieces of the Earth's crust which preserve unique geological histories each of which is different from that of neighboring terrains the word is spelled te RR a and E as opposed to te RR AI n which refers to the form or character of the ground surface the western part of the Cordillera is made up of several terrains each of which formed in places far removed from where they are today an example of one such terrain is rangel iya a name reserved for the large fragment of crust that makes up most of Vancouver Island the Queen Charlotte Islands and parts of southeastern Alaska from the campground here at horn lake on Vancouver Island we can see some of the characteristic rock formations of Ren Galia up on the cliffs of Mount Mark the prominent pale grey and dark banded cliffs of the mountain are composed of limestone that accumulated upon the eroded cones of volcanoes the limestone is made of the broken shells of marine organisms such as corals brachiopods and crinoids that flourished in tropical waters were this terrain originated above the limestone the dark looking rocks are marine lavas that poured across the limestone banks and built a volcanic pile in excess of six kilometers thick covering all of rangel iya the other terrains that make up much of British Columbia and the Yukon have similar histories each is composed of rocks that formed in specific environments at different times and at different places it was the assembly of these terrains and their final collision with the ancient edge of the continent that resulted in the formation of the mountains of western Canada the collision story begins about 200 million years ago when several terrains names dicky Nia Qin Elia slide mountain and Cache Creek amalgamated into a giant super terrain called the inter Montaigne super terrain two of these terrains sticky Nia and Qin Elia consisted of chains of volcanoes and a tall reefs that had been forming for many millions of years in southern latitudes of the ancient Pacific Ocean the Cache Creek and slide mountain terrains consisted of ancient seafloor namely sediments and basalt volcanics which had formed at seafloor spreading ridges as they amalgamated some of the rocks of the Cache Creek terrain were caught between the colliding sticky Nia and Qin Elia the Cache Creek ocean closed and it's sea floor was subducted beneath quinella in a sense the Cache Creek terrain acts as a kind of glue between sticky Nia and Cornelia between Western and Eastern British Columbia the people of this community of Cache Creek live on that glue a friend of mine with the Geological Survey in Vancouver just loves this stuff this was once the floor of the ancient Cache Creek ocean now it's a jumbled up mess of blocks of limestone and chert basalt and mud and God knows what all mushed together into what geologists call a melange melange is a term describing the intense disruption that occurs to rocks of the seafloor at subduction zones where one piece of crust is consumed beneath another following the amalgamation of the four terrains into the inter montane super terrain and after millions of years of movement through plate tectonics and seafloor spreading the whole shabang rammed into and was thrust up onto the ancient edge of North America the effect of this on the rocks forming the edge of the continent was traumatic to say the least over a period of a few million years the intense heat and pressure arising from the collision caused the rocks at the colliding edges to be compressed intensely deformed and even melted the effect of all of this was the formation of a great wealth of traumatized Rock extending the length of the chordal era called the Oh Monica belt these metamorphic rocks here near Revelstoke our part of the Shuswap metamorphic complex of the all mineka belt these rocks probably were once part of the thick blanket of sedimentary strata that accumulated across the rifted edge of the continent these and younger strata were squeezed and heated to such an extent that they reacted like putty to the enormous forces of collision in the process the rocks were completely changed or metamorphosed into what geologists call paradise if you look closely you can see the metamorphic layering has been deformed into small folds under the enormous influence of heat and pressure so now we have a great collage of previously amalgamated terrains thrust onto and welded to North America the edge of the continent was now considerably to the west of here probably in the vicinity of the coast mountains then between about 160 million and a hundred million years ago North America ran into another huge chunk of crust this we call the insular super terrain it includes the rocks of Vancouver Island the Queen Charlotte Islands and parts of Western Yukon and southeastern Alaska rangel 'ya which i talked about earlier is part of the insular super terrain the effects of this second collision were no less dramatic than those of the first the result was the formation of the coast mountains which like the rocks of the armonica melt composed of igneous and metamorphic rocks resulting in part from high temperatures and pressures generated during the collision impressive though the effects of these two collisions are to me they're overshadowed by the main effect the formation of the rocky mountains and foothills the story of how our Rockies and foothills came to be has to do with thrust faults faults are marvelous things people have them so do mountains in fact we're it not for faults our Rockies I think most people would be pretty dull from here on the slopes of Mount Markway we can see across the bowl river valley and the town of Banff to the magnificent edifice of mount rundle which for me represents the quintessence of thrust faults the poet Earl Burnie must have had mount rundle in mine when for his tragic poem David he wrote the peak was up thrust like a fist in a frozen ocean of rock that swirled into valleys the moon could be rolled in let me try to give you a simple explanation of a thrust fault when the terrains collided with North America the sedimentary rock that had formed from layers of mud and coral reefs broke loose from the ancient basement rock on which it had settled these strata were shoved eastwards by the colliding terrain in some cases as much as 250 kilometers in the process these sheets of strata were thrust one up onto the other the older rocks which were originally at the bottom of the pile now sit on top of younger rocks add to this the effects of a few million years of erosion and we have what we see today in the Rockies we have now moved out into the foothills in front of the Rockies from here we can observe another effect of the collision you will recall that following the splitting apart of the giant supercontinent all of the fine muds and sands accumulating on the western shelf came from the east from the interior of the continent when the collisions occurred all of that changed suddenly there were new sources of sediment now from the West in the rising thrust sheets of the Rockies and the uplifted armonica belt as these mountains rose the forces of erosion attacked the rocks streams carried the debris eastward placing it in front of the rising thrust sheets the strata of the Rocky Mountain foothills which formed through the massive accumulation of this debris is a tribute to the powers of erosion erosion has dramatically changed the look of the Rockies slowly wearing them away over tens of millions of years when the last ice age began some 80,000 years ago the rate of erosion increased enormously the massive ice sheets had the power to move huge rocks such as this one which is now situated near Okotoks Alberta not far from the eastern edge of the foothills this rock appropriately named the big rock is known as an erratic a term given to something that doesn't originate where you find it in this case the big rock was brought to this location upon a conveyor belt of ice that moved out across the foothills from the mountains beyond out here on the Prairie the ice sheets from the mountains met the continental ice sheet along a line extending the length of Western Alberta a big rock is just one of several or attics which gathered along this join when the ice melted and began to recede about ten thousand years ago but transported blocks were let down on the surface of the Western Prairie their distribution marking the line of contact between the two ice sheets by no means are the effects of the ice age only apparent in Iraqis the fjords and mountains of Western British Columbia the mountains of the armonica belt along with mount rundle cascade mountain and all of the mountains we have seen oh their shapes to the phenomenal sculpting power of ice what is happening today is the chordal era still growing are the forces of mountain buildings still with us our collisions still occurring the answers are yes yes and yes again to illustrate this we've returned to the west coast to near the head of how sound north of Vancouver the impressive mountain in front of us is Mount Garibaldi a volcano this mountain together with Mount Baker Mount Rainier Mount st. Helens and several others belong to a chain of volcanoes which are built from melted sea floor the sea floor of the Juan de Fuca plate remember earlier we saw off the west coast of Vancouver Island the Juan de Fuca plate was converging eastward and being subducted beneath the western edge of North America as it descends below the continents it reaches a depth of about a hundred and fifty kilometers directly below this chain of volcanoes at this depth the temperature is sufficient to melt the already hot plate the molten material then Rises upward through the continent to appear at the surface in the form of volcanic lava and ash from which the volcanoes are constructed events such as earthquakes and the eruption of these volcanoes are dramatic reminders that plates are still moving that terrains are moving mountains are being built and eroded as the millenia pass our planet will continue to change future geologists will watch the Pacific Ocean disappear they will witness the collision of North America and Asia new mountain ranges will form old ones will be eroded away they will become lost in time they will travel to where terrains collide Oh

Channel: Geology

Views: 162,319

Rating: 4.8243804 out of 5

Keywords: Terrane, Geology (Field Of Study), British Columbia, Western Canada, Plate Tectonics, Educational, Chris Yorath, Wrangellia, Insular Belt, Coast Belt, Intermontane Belt, Omineca Belt, Foreland Belt, Geological Survey of Canada, Tectonics, Accretion

Id: jG0w6BgLWUE

Length: 25min 30sec (1530 seconds)

Published: Tue Apr 28 2015