Chapter 1 Introduction to Geology
In 1788, after many years of geological study, James Hutton, one of the great pioneers of geology, wrote the following about the age of Earth: The result, therefore, of our present enquiry is, that we find no vestige of a beginning — no prospect of an end. Of course he wasn’t exactly correct, there was a beginning and there will be an end to Earth, but what he was trying to express is that geological time is so vast that we humans, who typically live for less than a century, have no means of appreciating how much geological time there is. Hutton didn’t even try to assign an age to Earth, but we now know that it is approximately 4,570 million years old. Using the scientific notation for geological time, that is 4,570 (for mega annum or “millions of years”) or 4.57 (for giga annum or billions of years). More recent dates can be expressed in (kilo annum); for example, the last cycle of glaciation ended at approximately 11.7 ka or 11,700 years ago. This notation will be used for geological dates throughout this book.
To help you understand the scientific notation for geological time—which is used extensively in this book—write the following out in numbers (for example, 3.23 Ma = 3,230,000 years).
- 2.75 ka
- 0.93 Ga
- 14.2 Ma
We use this notation to describe geological events in the same way that we might say “they arrived at 2 pm.” For example, we can say “this rock formed at 45 Ma.” But this notation is not used to express elapsed time. We don’t say: “I studied for 4 pm for that test.” And we don’t say: “The dinosaurs lived for 160 Ma.” Instead, we could say: “The dinosaurs lived from 225 Ma to 65 Ma, which is 160 million years.”
See Appendix 3 for Exercise 1.3 answers.
Unfortunately, knowing how to express geological time doesn’t really help us understand or appreciate its extent. A version of the geological time scale is included as Figure 1.6.1. Unlike time scales you’ll see in other places, or even later in this book, this time scale is linear throughout its length, meaning that 50 Ma during the is the same thickness as 50 Ma during the —in each case about the height of the “M” in Ma. The Pleistocene glacial epoch began at about 2.6 Ma, which is equivalent to half the thickness of the thin grey line at the top of the yellow bar marked “Cenozoic.” Most other time scales have earlier parts of Earth’s history compressed so that more detail can be shown for the more recent parts. That makes it difficult to appreciate the extent of geological time.
To create some context, the Eon (the last 542 million years) is named for the time during which visible (phaneros) life (zoi) is present in the geological record. In fact, large organisms—those that leave fossils visible to the naked eye—have existed for a little longer than that, first appearing around 600 Ma, or a span of just over 13% of geological time. Animals have been on land for 360 million years, or 8% of geological time. Mammals have dominated since the demise of the dinosaurs around 65 Ma, or 1.5% of geological time, and the genus Homo has existed since approximately 2.8 Ma, or 0.06% (1/1,600th) of geological time.
Geologists (and geology students) need to understand geological time. That doesn’t mean memorizing the geological time scale; instead, it means getting your mind around the concept that although most geological processes are extremely slow, very large and important things can happen if such processes continue for enough time.
For example, the Atlantic Ocean between Nova Scotia and northwestern Africa has been getting wider at a rate of about 2.5 centimetres (cm) per year. Imagine yourself taking a journey at that rate—it would be impossibly and ridiculously slow. And yet, since it started to form at around 200 Ma (just 4% of geological time), the Atlantic Ocean has grown to a width of over 5,000 kilometres (km)!
A useful mechanism for understanding geological time is to scale it all down into one year. The origin of the solar system and Earth at 4.57 Ga would be represented by January 1, and the present year would be represented by the last tiny fraction of a second on New Year’s Eve. At this scale, each day of the year represents 12.5 million years; each hour represents about 500,000 years; each minute represents 8,694 years; and each second represents 145 years. Some significant events in Earth’s history, as expressed on this time scale, are summarized on Table 1.1.
|Event||Approximate Date||Calendar Equivalent|
|Formation of oceans and continents||4.5 to 4.4 Ga||January|
|Evolution of the first primitive life forms||3.8 Ga||early March|
|Formation of British Columbia’s oldest rocks||2.0 Ga||July|
|Evolution of the first multi-celled animals||0.6 Ga or 600 Ma||November 15|
|Animals first crawled onto land||360 Ma||December 1|
|Vancouver Island reached North America and the Rocky Mountains were formed||90 Ma||December 25|
|Extinction of the non-avian dinosaurs||65 Ma||December 26|
|Beginning of the Pleistocene ice age||2 Ma or 2000 ka||8 p.m., December 31|
|Retreat of the most recent glacial ice from southern Canada||14 ka||11:58 p.m., December 31|
|Arrival of the first people in British Columbia||10 ka||11:59 p.m., December 31|
|Arrival of the first Europeans on the west coast of what is now Canada||250 years ago||2 seconds before midnight, December 31|
We’re going on a road trip! Pack some snacks and grab some of your favourite music. We’ll start in Tofino on Vancouver Island and head for the Royal Tyrrell Museum just outside of Drumheller, Alberta, 1,500 km away. Along the way, we’ll talk about some important geological sites that we pass by, and we’ll use the distance as a way of visualizing the extent of geological time. Of course it’s just a “virtual” road trip, but it will be fun anyway. To join in, go to: Virtual Road Trip.
Once you’ve had a chance to do the road trip, answer these questions:
- We need oxygen to survive, and yet the first presence of free oxygen (O2 gas) in the atmosphere and the oceans was a “catastrophe” for some organisms. When did this happen and why was it a catastrophe?
- Approximately how much time elapsed between the colonization of land by plants and animals?
- Explain why the evolution of land plants was such a critical step in the evolution of life on Earth.
See Appendix 3 for Exercise 1.4 answers.
Figure 1.6.1 image description: The Hadean eon (3800 Ma to 4570 Ma), Archean eon (2500 Ma to 3800 Ma), and Proterozoic eon (542 Ma to 2500 Ma) make up 88% of geological time. The Phanerozoic eon makes up the last 12% of geological time. The Phanerozoic eon (0 Ma to 542 Ma) contains the Paleozoic, Mesozoic, and Cenozoic eras. [Return to Figure 1.6.1]
- Figure 1.6.1: © Steven Earle. CC BY.
- Hutton, J, 1788. Theory of the Earth; or an investigation of the laws observable in the composition, dissolution, and restoration of land upon the Globe. Transactions of the Royal Society of Edinburgh. ↵
Geologists take great pains to measure and record geological structures because they are critically important to understanding the geological history of a region. One of the key features to measure is the orientation, or attitude, of bedding. We know that sedimentary beds are deposited in horizontal layers, so if the layers are no longer horizontal, then we can infer that they have been affected by tectonic forces and have become either tilted, or folded. We can express the orientation of a bed (or any other planar feature) with two values: first, the compass orientation of a horizontal line on the surface—the strike—and second, the angle at which the surface dips below a horizontal plane, (perpendicular to the strike)—the dip (Figure 12.20).
It may help to imagine a vertical surface, such as a wall in your house. The strike is the compass orientation of the wall and the dip is 90˚ from horizontal. If you could push the wall so it’s leaning over, but still attached to the floor, the strike direction would be the same, but the dip angle would be less than 90˚. If you pushed the wall over completely so it was lying on the floor, it would no longer have a strike direction and its dip would be 0˚. When describing the dip it is important to include the direction. In other words. if the strike is 0˚ (i.e., north) and the dip is 30˚, it would be necessary to say “to the west” or “to the east.” Similarly if the strike is 45˚ (i.e., northeast) and the dip is 60˚, it would be necessary to say “to the northwest” or “to the southeast.”
Measurement of geological features is done with a special compass that has a built-in clinometer—a device for measuring vertical angles. An example of how this is done is shown on Figure 12.21.
Strike and dip are also used to describe any other planar features, including joints, faults, dykes, sills, and even the foliation planes in metamorphic rocks. Figure 12.22 shows an example of how we would depict the beds that make up an anticline on a map.
The beds on the west (left) side of the map are dipping at various angles to the west. The beds on the east side are dipping to the east. The middle bed (light grey) is horizontal; this is denoted by a cross within a circle. The dyke is dipping at 80˚ to the west. The hinge of the fold is denoted with a dashed line with two arrows that point away from it. If it was a synform, the arrows would point towards the line.
Exercise 12.3 Putting strike and dip on a map
This cross-section shows seven tilted sedimentary layers (a to g), a fault, and a steeply dipping dyke. Place strike and dip symbols on the map to indicate the orientations of the beds shown, the fault, and the dyke. Then answer the questions.
- What type of fault is this, and is this an extensional or compressional situation?
- What are the relative ages of the nine geological features shown here (seven beds, dyke, and fault)? Which are the youngest and oldest?
See Appendix 3 for Exercise 12.3 answers.
The topics covered in this chapter can be summarized as follows:
|12.1 Stress and Strain||Stress within rocks—which includes compression, extension and shearing—typically originates from plate-boundary processes. Rock that is stressed responds with either elastic or plastic strain, and may eventually break. The way a rock responds to stress depends on its composition and structure, the rate at which strain is applied, and also to the temperature of the rock body and the presence of water.|
|12.2 Folding||Folding is generally a plastic response to compressive stress, although some brittle behaviour can happen during folding. An upward fold is an antiform. A downward fold is a synform. The axis of a fold can be vertical, inclined, or even horizontal. If we know that the folded beds have not been overturned, then we can use the more specific terms: anticline and syncline.|
|12.3 Fracturing and Faulting||Fractures (joints) typically form during extension, but can also form during compression. Faulting, which involves the displacement of rock, can take place during compression or extension, as well as during shearing at transform boundaries. Thrust faulting is a special form of reverse faulting.|
|12.4 Measuring Geological Structures||It is important to be able to measure the strike and dip of planar surfaces, such as a bedding planes, fractures or faults. Special symbols are used to show the orientation of structural features on geological maps.|
Questions for Review
- What types of plate boundaries are most likely to contribute to the following?: a) compression, b) extension, and c) shearing.
- Explain the difference between elastic strain and plastic strain.
- List some of the factors that influence whether a rock will deform (in either an elastic or plastic manner) or break when placed under stress.
- Label the types of folds in this diagram, and label any of the important features of the folds.
- Explain why fractures are common in volcanic rocks.
- What is the difference between a normal fault and a reverse fault, and under what circumstances would you expect these to form?
- What type of fault would you expect to see near to a transform plate boundary?
- This diagram is a plan view (map) of the geology of a region. The coloured areas represent sedimentary beds.
i) Describe in words the general attitude (strike and dip) of these beds.
ii) Which of these beds is the oldest?
iii) What is “a” and what is its attitude?
iv) What is “b” and what is its attitude?
v) Which of these terms applies to “b”: “left lateral” or “right lateral”?
Answers to Review Questions can be found in Appendix 2.