1.5 Three Big Ideas: Geological Time, Uniformitarianism, and Plate Tectonics
In geology there are three big ideas that are fundamental to the way we think about how Earth works. The ideas are like the sound track to a movie- sometimes we might not even notice them, but at the same time they affect our perception of what is happening. In the rest of this book these ideas may be mentioned explicitly in some cases, but in other cases it will be helpful for you to realize that they are relevant, even if they are not being discussed by name.
Geological Time (Deep Time)
Earth is approximately 4.57 billion years old (4,570,000,000 years), which is a long time for geological events to unfold and changes to happen. The changes themselves might be tiny. For example, over a year, a chemical reaction might eat away a few layers of atoms at the surface of a rock. But over time the changes accumulate and have a great impact. Over hundreds of millions of years the chemical reaction could cause a mountain range to crumble into grains of sand, and be swept away by rivers.
For geologists who study very, very slow processes, 10 million years might be a short time, and 1 million years might be trivial. For these geologists, intervals of 1 million years aren’t even useful to consider, because the changes over that time are too small to see in the rocks that accumulated.
As you read through this book, keep in mind that the well of geologic time is indeed deep, and “ancient” is defined in a whole new way.
Expressing Geological Time in Numbers
Special notation is used for geological time because, as you might imagine, writing all those zeroes can become tiresome. Table 1.1 shows common abbreviations you will see throughout this book.
|Table 1.1 Abbreviations Used to Describe Geological Time|
or billions of years
|Earth is 4.57 Ga old.|
or millions of years
|Earth is 4,570 Ma old.|
|ka||kilo annum or thousands of years||The last glacial cycle ended 11,700 years ago, or 11.7 ka.|
Expressing Geological Time Using the Geological Time Scale
The geological time scale (Figure 1.6) is a way of breaking down geological time according to important events in Earth’s history. Time is divided into eons, eras, periods, and epochs, and these intervals are referred to by names rather than by years. Giving intervals of geologic time names rather than using numbers makes sense because we won’t always know the age in years (the absolute age) of a rock or fossil, but we can place it in context based on our knowledge of the geological record. We can describe its relative age by saying that it is older than or younger than another rock or fossil.
The tricky thing about the geologic time scale is that the boundaries are always changing. As our knowledge of the absolute age of an event improves with new discoveries, it might be necessary to nudge a boundary earlier or later. Sometimes the original reason for defining a boundary no longer holds, but we agree to use it anyway. For example, the Phanerozoic Eon (the last 542 million years) is named for the time during which visible (phaneros) life (zoi) is present in the geological record, and its start was meant to mark the first appearance of these organisms. In fact, we now have evidence that large organisms — those that leave fossils visible to the naked eye — have existed longer than that, first appearing by 600 Ma at the latest.
An Early Definition of the Proterozoic
Notice that in Figure 1.6 the Proterozoic Eon precedes the Phanerozoic Eon. This was not always the case. Figure 1.7 shows an excerpt from a periodical published in 1879, in which the Proterozoic is defined as covering the Cambrian through Silurian. The author refers to “the most extreme adherents of the Murchisonian party in geology,” a reference to the contentious assertion by Scottish geologist Roderick Murchison (1792-1871) that the Silurian Period should encompass the Cambrian and Ordovician periods as well.
A Way To Think About Geological Time
A useful mechanism for understanding geological time is to scale it 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 in Table 1.2.
|Table 1.2 Some Important Dates Expressed As If All of Geological Time Were Condensed Into One Year|
|Event||Approximate Date||Calendar Equivalent|
|Formation of oceans and continents||4.5 – 4.4 Ga||first week of January|
|Evolution of the first primitive life forms||3.8 Ga||end of February|
|Formation of Saskatchewan’s oldest rocks||3.4 Ga||end of March|
|Evolution of the first multi-celled animals||600 Ma||beginning of November|
|Animals first crawled onto land||360 Ma||end of November|
|Vancouver Island reached North America and the Rocky Mountains were formed||90 Ma||December 16|
|Extinction of the non-avian dinosaurs||65 Ma||December 18|
|Beginning of the Pleistocene ice age||2 Ma||10:10 p.m., December 31|
|Oldest radiocarbon date from people living in Canada (British Columbia)||13.8 ka||11:58 p.m., December 31|
|Earliest evidence of human activity in Saskatchewan||11.5 ka||48 seconds before midnight, December 31|
|The last of the glacial ice retreats from Saskatchewan||6 ka||41 seconds before midnight, December 31|
|Hudson’s Bay Company establishes a permanent settlement at Cumberland House in northern Saskatchewan||243 years ago||2 seconds before midnight, December 31|
|Source: Karla Panchuk (2017) CC BY 4.0, modified after Steven Earle (2015) CC BY 4.0 view original
Uniformitarianism is the notion that the geological processes occurring on Earth today are the same ones that occurred in the past. This is an important idea because it means that observations we make today about geological processes can be used to interpret and understand the rock record. While this idea might not seem remarkable today, it was ground breaking and even controversial for its time. Many people who heard about it for the first time thought about the age of the Earth in thousands of years, but uniformitarianism required them to think on timescales almost too vast to comprehend. For some, this implied questioning their most deeply held religious beliefs.
The Scottish geologist James Hutton initially presented the idea in 1785. Charles Lyell, also a Scottish geologist, paraphrased this idea as “the present is the key to the past” in his book Principles of Geology. This is how it is often described today.
To be clear, “the present is the key to the past” can be viewed as an oversimplification. Not all geological processes occurring today occurred at all times in the geological past. For example, some important chemical reactions that happened on Earth’s surface today require abundant oxygen in the atmosphere, and could not have occurred prior to Earth developing an oxygen-rich atmosphere. Conversely, there was a time in Earth’s history when continents as we know them hadn’t yet developed. Some events, such as devastating impacts by objects from space, have never been witnessed on the same scale by humans. We must be cognizant of the fact that conditions were different at different times in Earth’s history, and take that into account when interpreting the rock record.
Despite the different past conditions on Earth as a whole, there still exist environments today where some of these conditions are present. These environments are like little samples of what Earth used to be like. This means we can still use present conditions to inform us about the past, but we have to think carefully about ways that such environments today differ from the ancient environments that no longer exist.
It is only within the last 50 years or so that we have been able to answer questions like, “How did that mountain range get there?” and “Why do earthquakes happen where they do?” The theory of plate tectonics– the idea that Earth’s surface is broken into large moving fragments, called plates– profoundly changed our perspective on how the Earth works. Figure 1.8 shows Earth’s 15 largest tectonic plates, along with arrows indicating the plates’ direction of motion, and how fast they go. (Longer arrows mean faster motion.) There are many more plates on Earth that are too small to show conveniently in Figure 1.8. A more detailed map of Earth’s tectonic plates can be found at here.
Prior to plate tectonics, we made observations but could only guess at mechanisms. It was like watching the hands on a clock and trying to guess what moves them. After plate tectonics it was like being able to open the clock and not only watch the gears turn, but realize for the first time that there are such things as gears. Plate tectonics not only explains why things have happened, but also allows us to predict what might happen in the future.
Plate tectonics is covered in more detail later, however the key point is that Earth’s outer layer consists of rigid plates that are constantly interacting with each other as they move around the Earth. The boundaries of plates move away from each other in some places, collide in others, and sometimes just slide past each other (illustrated by the red arrows in Figure 1.8). The plates can move because they are floating on a layer of weak rock that deforms as the plates travel, much the same way the filling in a peanut butter and jelly sandwich allows you to slide the top layer of bread across the bottom layer.
Whether the plates move away from each other, collide, or just slide past each other determines things like the locations of mountain belts and volcanoes, where earthquakes happen, and the shapes and sizes of oceans and continents.
Cottrell, M. (2006) History of Saskatchewan. Retrieved 26 August 2017. Visit the website