Lab 15: BC’s Geology and Geologic Structures

Stuart MacKinnon, Fes de Scally, and Todd Redding

Geomorphology is the scientific study of the characteristics and origins of landforms. Landforms arise through the interplay between endogenic (fueled by Earth’s internal energy) and exogenic (ultimately fueled by the Sun) processes. Endogenic processes tend to be responsible for the rock types and geological structures found in any particular area. Where geological structure dominates the surface landforms, they are called structural landforms and are the main focus of this lab.

This lab will provide experience in identifying and analyzing rock samples of the three major classes, identifying tectonic plate boundaries, examining geologic maps and cross-sections, interpreting British Columbia’s geologic history and rocks from the various geologic belts that span the province, and analyzing local geology in the field.

Learning Objectives

After completion of this lab, you will be able to:

  • Distinguish between the three major classes of rocks;
  • Understand the basic terminology relating to geological structures;
  • Identify different types of tectonic plate boundaries;
  • Interpret geological maps and geologic cross-sections;
  • Analyze rocks from across the province of British Columbia, and predict which geologic belt the rock samples were obtained from;
  • Collect and interpret local rock samples; and
  • Provide an overview of your local geology related to your rock samples.

Pre-Readings

Classification of Rocks: Igneous, Sedimentary and Metamorphic

Rocks can be classified into three main categories.

1. Igneous Rocks

Igneous rocks form from the cooling and crystallisation of magma. Igneous rocks are divided depending on the environment in which the magma cooled:

Intrusive igneous or plutonic (named after Pluto, the god of the underworld in Greek mythology) rocks, form from magma that cooled deep underground. Because the magma in this case cools very slowly, intrusive igneous rocks usually contain relatively large mineral crystals. Common examples include granite, granodiorite, diorite and gabbro.

Extrusive igneous or volcanic rocks, formed from magma (called lava once it reaches Earth’s surface) in volcanic eruptions. Because in this case the magma cools quickly on Earth’s surface, the mineral crystals in the rock are either very small or non-existent. In situations where the lava cools very quickly, a volcanic glass called obsidian is produced. Common examples of extrusive igneous rock include basalt, dacite, andesite and rhyolite.

The precise type of intrusive or extrusive rock that is produced from cooling magma is determined by the magma’s chemical composition, especially the abundance of silica (SiO2). The silica content plays an important role in the physical characteristics of an igneous rock, including its resistance to weathering and erosion (see Table 2 on p. 95 of the textbook). It also plays an important role in the explosiveness of volcanic eruptions, because magma with higher silica contents is “stickier” and more likely to produce an explosive eruption.

2. Sedimentary Rocks

Sedimentary rocks are formed by the lithification (compaction, cementation and hardening) of weathering and erosion products which have accumulated in a fluvial, marine or lacustrine environment over long periods of time. These products can be of two basic types, which provides us with a sub-classification of sedimentary rocks:

Clastic sedimentary rocks are made from ground-down rock as well as surviving ancillary minerals and clay minerals. Common examples include sandstone, conglomerate, siltstone or mudstone, and shale. Clastic sedimentary rocks exhibit a huge variation in their resistance to weathering and erosion depending on the degree of lithification of the clastic sediments they’re composed of.

Carbonate or chemical sedimentary rocks are made from the precipitation of minerals, primarily calcium carbonate, dissolved in water. Common examples are limestone (CaCO3) and dolomite (CaMg(CO3)2).

 

3. Metamorphic Rocks

Metamorphic rocks are formed by the alteration (but not complete melting) of a sedimentary or igneous rock by heat, pressure and other forces below Earth’s surface. Common examples include gneiss, marble, slate, schist and quartzite.

The specific type of metamorphic rock is largely a function of the original (or “parent” rock). For example, shale (a sedimentary rock) typically metamorphoses into slate, and granite (an intrusive igneous rock) typically metamorphoses into gneiss.

The type of metamorphic rock also depends on the degree of “cooking” that has occurred during the process of metamorphism, resulting in foliated and non-foliated rocks depending on whether the constituent minerals in the parent rock have been realigned into “foliations” during metamorphism.

Metamorphic rocks are typically more compact compared to their parent rock. This means that they are harder and more resistant to weathering and erosion, with a consequent effect on the development of denudational landforms. Quartzite, for example, is much more resistant to weathering and erosion than its parent rock (sandstone).

Tectonic Plate Boundaries

The theory of plate tectonics provides the model that underlies our understanding of modern geology and the interactions between oceans and continents. Plate tectonics explains why the highest and lowest points on Earth occur where they do. Plate tectonics also explains why and where we can observe highly deformed rocks at or near the surface. These deformed rocks can be termed geologic structures, and include features such as folds and faults.

The type of plate boundary determines the types of deformation that may occur. Transform, or strike-slip, plate boundaries occur when two plates move along each other in a predominately horizontal motion (scenario a below). Divergent plate boundaries occur when two plates move away from each other (scenario b). Convergent plate boundaries occur when two plates move toward each other, or collide together (scenarios c and d).

 

Figure 15.1. Map of the main global tectonic plate boundaries and schematic illustration showing the main types of plate interactions. Source: D. McDonnell, CC BY-NC-SA 3.0. View source.

Geologic Structures

“Geological structure” can be defined as the arrangement and attitude of rocks in Earth’s crust or lithosphere. Structure results from tectonism, the deformation of Earth’s crust by endogenic forces. Tectonism includes both diastrophism, large-scale deformations of the crust producing mountain ranges, ocean basins, etc., and vulcanicity or volcanism, the creation of crustal material on a more localised scale through volcanic activity. Both sets of processes are a result of the mechanism of plate tectonics.

Geological structure can be relatively simple if tectonic forces have not deformed the crust to any great degree. Examples include the horizontal beds of young sedimentary rock which underlie much of the Prairie provinces of Canada. On the other hand, where deformation has been severe, the resulting geological structure can be extremely complicated. Examples include the intensely deformed sedimentary and metamorphic rocks of many of BC’s mountain ranges or the Himalaya of South Asia.

Tectonism can produce a wide variety of geological structures including folds (flexures or bends in the crustal rocks due to compressional forces), faults (ruptures or fractures in the crustal rocks due to strain), thrusts and joints. Note that folding rarely involves rupturing of the rock, but faulting does.

Folding and faulting impose two types of attitudes on the rock. Dip is the angle (measured in degrees) which the rock strata (layers) make with a horizontal plane, measured in a direction perpendicular to the strike of the rock strata (Figure 15.2). Strike can also be measured along faults. It is commonly expressed using the cardinal directions of the compass or as a full-circle azimuth.

Figure 15.2. Dip and strike in geological strata. Note that apparent dip is any dip angle not measured perpendicular to the strike; it underestimates the true dip. Source: Fes de Scally, CC BY-SA.

Geologic History of British Columbia

The geologic history of BC dates back to a time when there was actually no BC west of today’s Rocky Mountains: the western edge of the ancestral North American continent, composed of the ancient plutonic rocks of the Canadian Shield craton (continental core), was situated roughly where Calgary and Dawson Creek are located today (Figure 15.3). West of this ancient shoreline, the submarine continental shelf extended roughly to where the town of Golden is situated today (just east of Revelstoke in Figure 15.3).

Figure 15.3. Geological belts of British Columbia, which correspond roughly to the major physiographic regions of the province. Source: Fes de Scally, CC BY-SA.

The following are important intervals in BC’s geologic history:

1.7 Ga to 180 Ma

Over an immense time period of about 1.5 billion years, sediment eroded from the ancient Canadian Shield craton is deposited in a miogeocline (a part of the submarine continental shelf along a tectonically quiescent continental margin where sediment deposition occurs) just offshore of the western margin of ancestral North America.

Although this continental margin forms a part of different supercontinents at various times over this period, including Rodinia and Pangaea, it is tectonically quiescent. This allows uninterrupted sediment deposition in the miogeocline. Both clastic sediments (muds and sands) and carbonate sediments (from coralline organisms) are deposited, a fact which will have important implications for the formation of the Foreland Belt much later.

750 Ma

Earth’s Rodinia supercontinent breaks up and the Pangaea supercontinent begins to be assembled. The wedge of sediment in the miogecline offshore of ancestral North America continues to build and is eventually lithified into sedimentary rock. This includes the burial of marine organisms around 530 Ma which form today the world-famous Burgess Shale fossil beds in Yoho National Park of BC.

245 Ma

The Pangaea supercontinent begins to break up, and Earth’s tectonic plates begin their slow movement into their modern configuration.

200 Ma

Up to 2,000 km away in the ancestral Pacific Ocean, at an old plate boundary, the Intermontane Superterrane begins to be assembled when the volcanic island arcs of the Stikinia and Quesnellia terranes are amalgamated with the sea floor sediments of the Cache Creek and Slide Mountain terranes. This produces a mélange (mix) of sea-floor sedimentary rocks and volcanic rocks.

180-150 Ma

A change in the direction of plate movement causes the ancestral North American continent to collide with the Intermontane Superterrane. The tremendous heat and pressure of this slow-motion collision cause rocks of the miogeocline and the superterrane to be metamorphosed at a “weld” to form the Omineca Belt. The metamorphic rocks of the Omineca Belt today form the Columbia and Cassiar Mountains as well as Quesnel and Shuswap Highlands of BC.

Following the collision, the Intermontane Belt to the west consists mostly of the mélange rocks of this ancient superterrane, but with occasional deeply buried and very old plutonic rocks protruding through these much younger rocks; these today form high peaks such as Big White Mountain near Kelowna. The shoreline of ancestral North America is located roughly at the western boundary of the Intermontane Belt by 150 Ma (Figure 15.3).

120 Ma

The sedimentary rocks of the former miogeocline have by now been pushed eastward and upward by the tremendous force of the Intermontane Superterrane’s collision to form the western Rocky Mountains of the Foreland Belt. These forces not only produce significant folding of the rocks, but also thrust faults when layers of strong, resistant carbonate rock are broken and shoved eastward in thick thrust sheets. The force of the collision also creates a deep topographic depression east of the newly formed Rocky Mountains, which shortly begins to fill with eroded sediment to eventually form weak clastic rocks such as shale and mudstone.

100-60 Ma

Another superterrane—the Insular Superterraneassembled earlier far offshore from the volcanic island arcs of the Wrangellia and Alexander terranes, now collides with the western edge of the Intermontane Belt. The mélange of sea floor sedimentary rocks and volcanic rocks in the resulting Insular Belt today makes up the mountains of Vancouver Island and Haida Gwaii.

Following this collision, the west coast of BC looks much like it does today. The heat of this collision also creates the intrusive igneous (plutonic) rocks of the Coast Belt, which today make up the Coast and Cassiar Mountains and Okanagan Highlands of BC.

The force of this collision also continues the building of the eastern Rocky Mountains in the Foreland Belt, with thrust faulting continuing to push rocks as much as 250 km eastward. For example, the rocks of Mount Rundle in Banff located near the eastern edge of the Foreland Belt—were originally formed near where Revelstoke is situated today (Figure 15.3). During this thrust faulting, the weak shales and mudstones deposited after 120 Ma east of the ancestral Rocky Mountains were shoved in between layers of strong limestone to form the classic weak-strong-weak-strong layering in the geological structure of the Foreland Belt. Further east, the horizontal layers of weak post-120 Ma sedimentary rock in the Interior Plains belt (Figure 15.3) were unaffected by the force of the superterrane collisions, forming the modern flat Prairie landscape.

85 Ma

The motion of oceanic tectonic plates to the west of BC changes, and instead of moving northeastward toward the North American plate, the motion becomes more northerly. This stretches the continental crust and produces extensive strike-slip faulting in BC. The best example of this is the 750 km of lateral displacement along the northern section of the Rocky Mountain Trench (at the boundary between the Omineca and Foreland Belts in Figure 15.3). This stretching also creates the parallel-to-the-coast orientation of BC’s geologic belts (Figure 15.3) and the province’s many mountain ranges.

60-50 Ma

A standstill in plate movement allows the geologic belts thrust eastward by the superterranes’ collisions to slump back toward the west slightly in a process called gravity sliding. This creates roughly northwest-southeast oriented valleys in BC, including the southern portion of the Rocky Mountain Trench and the Okanagan Valley. This “relaxation” of the crust also allows the deeply buried Monashee gneiss—at 2 Ga, the oldest rocks in BC—to be exposed in the Okanagan Valley.

55-36 Ma

Further relaxation of the crust allows extensive volcanic lava flows to cover much of the BC Interior, especially in the Intermontane Belt. As a result, many of the original rocks of the Intermontane Belt are buried by basaltic lava flows. Further episodes of volcanic activity in this belt and in the Coast Belt occur at about 21-7 Ma, 15-2 Ma, and 3.5-0.01 Ma. In general, volcanic eruptions in the Intermontane Belt produce basaltic rocks of lower silica content, while more explosive eruptions of lava with higher silica contents in the Coast Belt produce rhyolitic rocks and breccias.

40-5 Ma

A non-orogenic period brings mountain building to a halt, and allows erosion by streams and rivers to begin to shape the modern drainage pattern of BC. An erosion surface of gentle hills forms west of the Foreland Belt, and by 10 Ma the mountains of the Coast Belt are so low that there is no longer a climatic ‘rainshadow’ on the leeward (east) side.

5-1 Ma

A final orogenic period occurs when the subduction zone under small tectonic plates west of the North American coastline steepens (see Fig. 12 on p. 135 of the textbook), resulting in a “reheating” of the crust in the Coast Belt. This reheating uplifts the 40-5 Ma erosion surface by about 2000 m, resulting in the modern mountains of the Coast Belt. The reheating also produces the volcanoes of today’s Cascade Volcanic Arc stretching from the southern Coast Mountains of BC to northern California, as well as uplifts and warps much of the plateau surface in the Intermontane Belt. The resulting undulating plateau surface is clearly visible from Pennask Summit along Hwy. 97C (the “Okanagan Connector”) west of Kelowna.

2.6-0.01 Ma

Glaciations during the Pleistocene epoch erode the modern landscape pattern of BC, including the rugged mountain ranges of the Foreland, Omineca, Coast and Insular Belts.

Lab Exercises

The following lab exercises provide some practice at applying the concepts discussed above. You will need a calculator, plus an internet connection to download a map and access Google Earth. Some of the exercises may be easier if you are able to print the relevant portion of the map. It is assumed that you have successfully completed Lab 12. It also assumed that you can convert between metres and feet. The exercises should take you 1½ to 3 hours to complete.

EX1: Classification of Rocks

  1. Classify each of samples 1A-1F in the slide deck below (Figures 15.4a–15.4f) as “igneous”, “sedimentary”, or “metamorphic,” and provide a one-sentence explanation of the reasoning for each of your choices. If the slide deck does not display below, click here to access it.

 

  1. Classify igneous rock samples 2A-2D in the slide deck below (Figures 15.5a–15.5d) as “intrusive” (also known as plutonic) or “extrusive” (also known as volcanic), and provide a one-sentence explanation detailing your logic for each choice you made. If the slide deck does not display below, click here to access it.

 

  1. Classify sedimentary rock samples 3A-3D in the slide deck below (Figures 15.6a–15.6d) as “clastic” or “carbonate or chemical,” and provide a one-sentence explanation detailing your logic for each choice you made. If the slide deck does not display below, click here to access it.

 

  1. Classify metamorphic rock samples 4A-4D in the slide deck below (Figures 15.7a–15.7d) as “foliated” or “non-foliated,” and provide a one-sentence explanation detailing your logic for each choice you made. If the slide deck does not display below, click here to access it.

EX2: Plate Tectonic Boundaries

  1. Figure 15.1 is a map showing the major plate boundaries found on Earth. The schematic cross-sections show 4 models of relative plate motions. Use this map to determine which of these cross-sections best represents the characteristics of the plate boundary at each of the numbered locations (1-9) found on the map. Explain your answers.
  2. At which of these numbered boundaries would you expect to find old rocks that have been folded and deformed? Explain your answer.
  3. At which of these numbered boundaries would you expect to find young undeformed rocks? Explain your answer.

EX3: Geologic Structure Basics

Figure 15.8 shows a geologic cross-section of the area near Banff, Alberta, Canada, that was created by the Geologic Survey of Canada (GSC). Click here to download a PDF version of Figure 15.8. Click here to download the map it corresponds to.

 

Figure 15.8. Geologic Survey of Canada: Banff East-Half Cross Section. Source: Geological Survey of Canada, Open Government License. View source.
  1. Using the GSC Banff geologic cross-section, match the following structural features with the corresponding location name found on the geologic cross-section map.
Structural Feature Location Name on Map
a. Gently dipping strata
b. Eroded asymmetrical anticline
c. Steeply dipping strata
d. Asymmetrical syncline overlain by recent glacial and fluvial deposits
Brewster Creek
Fatigue Creek
Fatigue Mountain
Sulfur Mountain Thrust
  1. Which location name has the oldest rocks at the surface? Explain your answer.
  2. The “Q” beds on the geological map are fundamentally different from all the other formations shown on the geologic cross section. In what way are they different?

EX4: Analysis of Rock Samples from British Columbia

  1. Examine the following 8 rock samples in the slide deck below (Figures 15.9a–15.9h), then name the geologic belt in Figure 15.3 from which each sample was obtained. Provide a two- sentence rationale for your choice of geologic belt for each sample. In answering this question, use information on rock type supplied in the description of BC’s geologic history, along with information from internet regarding the rock names. If the slide deck does not display below, click here to access it.

EX5: Examination of Local Rocks and Geology

  1. Now that you have learned about basic rock types and identification, and the rocks and geology of BC, it is your turn to be a geologist.

a. Take a walk around where you live, and locate and bring home 5 different rocks. Parks, beaches, etc. are great places to look.

b. Using what you’ve learned about the 3 major classes of rocks, identify each rock as igneous, sedimentary, or metamorphic. Can you get more specific for any of them? For example, if you have igneous rocks, are they intrusive or extrusive? Do any of your rocks look like the ones in Exercise 4?

c. Take photos of your 5 rocks and share them with your peers on a discussion board. Then, view what your peers shared.

  1. Research the local geology of where you live (if it seems limited in the immediate area, broaden your search a little bit), and write a paragraph describing the rock types and geologic history of the area. Your paragraph should be 6 to 10 sentences long, and it must be written in your own words. Your instructor will provide details on formatting and references.

Reflection Questions

  1. BC is quite unique in that is has mountain ranges dominated by sedimentary rocks (i.e., the Rocky Mountains), mountain ranges dominated by metamorphic rocks (i.e., the Monashee Mountains) and mountain ranges dominated by igneous rocks (i.e., the Coast Mountains). Assume you have never visited any of these mountain ranges and write a short 2-3 sentence explanation about which one you think would be most geologically interesting to you, and why.
  2. If you live in BC, which geologic belt do you live in, and what kinds of rocks dominant your area? Did the rocks you found in Exercise 5 fit what you were expecting? If you live outside of BC, do the rocks you found match the local geologic description and local rock types that you researched as part of Exercise 5?
  3. Did students from similar areas post similar looking rocks? Why or why not?
  4. Exercise 2 examined large global tectonic plate boundaries, but when you “zoom in” there is a lot more going on. Look closely at the western North American plate boundaries in the detailed map from Exercise 2, and examine the boundaries that exist between the Pacific Plate, the Juan de Fuca Plate, and the North American Plate. Answer the following questions, and explain your thinking.

a. What will eventually happen to the Juan de Fuca plate?

b. Why are there volcanoes in the Cascade and Coast mountains?

c. If you could see 20 million years into the future, where would you have to look for land west of the San Andreas Fault?

References

Cannings, R., & Cannings, S. (2004). British Columbia: A Natural History. Greystone Books: Vancouver, BC, Canada.

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