5 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
Abbreviation Meaning Example
Ga giga annum
 or billions of years
Earth is 4.57 Ga old.
Ma mega annum
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.

Geologic Society of America Geologic Time Scale, 2012
Figure 1.6 Geologic Society of America Geologic Time Scale, 2012. Source: Walker, J.D., Geissman, J.W., Bowring, S.A., and Babcock, L.E., compilers (2012) Geologic Time Scale v. 4.0: Geological Society of America, doi: 10.1130/2012.CTS004R3C. Download PDF

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 Nova Scotia’s oldest rocks 1.5 Ga end of April
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 groundbreaking 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[1].  Charles Lyell, also a Scottish geologist, paraphrased this idea as “the present is the key to the past” in his book Principles of Geology.[2]  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.

The mud cracks in this photo from Flagstaff, Arizona, formed when mud puddles tried in the hot summer sun. Similar mud cracks preserved in the rocks in the read of the photo show (coincidentally) that Arizona had a similar climate in the past. Source: Jason Loxton (2019) CC by 4.0.

Plate Tectonics

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.

Figure 1.8 Earth’s fifteen largest tectonic plates. Black arrows show the direction of plate motions. The length of the arrow indicates velocity. Red arrows show how plates move relative to each other. Source: Steven Earle (2015) CC BY 4.0. view source Modified after U. S. Geological Survey (1996) Public Domain view original

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

  1. Read James Hutton's abstract at http://bit.ly/1j6tIAN. Note that the typeface prints an "s" like an "f."
  2. The 7th edition of Charles Lyell's Principles of Geology (1847) can be found at http://bit.ly/1l3T6Zh