{"id":62,"date":"2017-08-27T16:05:29","date_gmt":"2017-08-27T20:05:29","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/1-5-three-big-ideas-geological-time-uniformitarianism-and-plate-tectonics\/"},"modified":"2021-05-17T16:40:08","modified_gmt":"2021-05-17T20:40:08","slug":"1-5-three-big-ideas-geological-time-uniformitarianism-and-plate-tectonics","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/physicalgeologyh5p\/chapter\/1-5-three-big-ideas-geological-time-uniformitarianism-and-plate-tectonics\/","title":{"raw":"1.5 Three Big Ideas: Geological Time, Uniformitarianism, and Plate Tectonics","rendered":"1.5 Three Big Ideas: Geological Time, Uniformitarianism, and Plate Tectonics"},"content":{"raw":"In geology there are three big ideas that are fundamental to the way we think about how Earth works.\u00a0 The ideas are like the sound track to a movie\u2014sometimes we might not even notice them, but they nevertheless affect our perception of what's happening.\u00a0 In the rest of this book these ideas may be mentioned explicitly in some cases, but in other cases they won't be discussed by name, and it will be helpful for you to realize that they're relevant.\r\n

Geological Time (Deep Time)<\/h1>\r\nEarth is approximately 4.6 billion years old (4,600,000,000 years), which is a long time for geological events to unfold and changes to happen. The changes themselves might be tiny\u2014over a year, a chemical reaction might eat away a few layers of atoms at the surface of a rock. Over hundreds of millions of years, however, the chemical reaction could cause a mountain range to crumble into grains of sand, and be swept away by rivers.\r\n\r\nFor geologists who study very, very slow processes, 10 million years might be a short time, and 1 million years might be trivial.\u00a0 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.\r\n\r\nAs 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.\r\n
\r\n\r\nNeed Some Help Visualizing Geological Time?<\/strong>\r\n\r\nWatch the animation Four Ways to Understand the Earth\u2019s Age <\/em>to see four analogies for geological time.\r\n\r\nhttps:\/\/youtu.be\/tkxWmh-tFGs\r\n\r\n<\/div>\r\n \r\n

Expressing Geological Time in Numbers<\/h2>\r\nSpecial notation is used for geological time because, as you might imagine, writing all those zeroes can become tiresome.\u00a0 Table 1.1 shows common abbreviations you will see throughout this book.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 1.1 Abbreviations Used to Describe Geological Time<\/strong><\/th>\r\n<\/tr>\r\n
Abbreviation<\/strong><\/th>\r\nMeaning<\/strong><\/th>\r\nExample<\/strong><\/th>\r\n<\/tr>\r\n
Ga<\/td>\r\ngiga annum\r\n<\/em>\u00a0or billions of years<\/td>\r\nEarth is 4.6 Ga old.<\/td>\r\n<\/tr>\r\n
Ma<\/td>\r\nmega annum\r\n<\/em> or millions of years<\/td>\r\nEarth is 4,600 Ma old.<\/td>\r\n<\/tr>\r\n
ka<\/td>\r\nkilo annum<\/em> or thousands of years<\/td>\r\nThe last glacial cycle ended 11,700 years ago, or 11.7 ka.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n
\r\n\r\nHow Many Years Is That?<\/strong>\r\n\r\n[h5p id=\"116\"]\r\n\r\n<\/div>\r\n

Expressing Geological Time Using the Geological Time Scale<\/h2>\r\nThe geological time scale<\/strong> (Figure 1.7) is a way of breaking down geological time according to important events in Earth's history.\u00a0 Time is divided into eons<\/strong>, eras<\/strong>, periods<\/strong>, and epochs<\/strong>, and these intervals are referred to by names rather than by years.\u00a0 Giving intervals of geologic time names rather than using numbers makes sense because we won't always know the age in years (the absolute<\/strong> age<\/strong>) of a rock or fossil, but we can place it in context based on our knowledge of the geological record.\u00a0 We can describe its relative age <\/strong>by saying that it's older than or younger than another rock or fossil.\r\n<\/strong>\r\n\r\n[caption id=\"attachment_870\" align=\"alignnone\" width=\"2200\"]\"Geologic<\/a> Figure 1.7<\/strong> 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><\/em>[\/caption]\r\n\r\n
[h5p id=\"117\"]<\/div>\r\n \r\n\r\nThe tricky thing about the geologic time scale is that the boundaries are always changing.\u00a0 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.\u00a0 Sometimes the original reason for defining a boundary no longer holds, but we agree to use it anyway.\u00a0 For example, the Phanerozoic Eon (the last 542 million years) is named for the time during which visible (phaneros<\/em>) life (zoi<\/em>) is present in the geological record, and its start was meant to mark the first appearance of these organisms. We now have evidence that large organisms\u2014those that leave fossils visible to the naked eye\u2014have existed longer than that, first appearing by 600 Ma at the latest.\r\n\r\n \r\n
\r\n\r\nAn Early Definition of the Proterozoic<\/strong>\r\n\r\nNotice that in Figure 1.7 the Proterozoic Eon precedes the Phanerozoic Eon. This was not always the case. Figure 1.8 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.\r\n\r\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"700\"]\"Classification<\/a> Figure 1.8<\/strong> An excerpt from the periodical The Annals and Magazine of Natural History<\/em> (1879) in which the name \"Proterozoic\" is assigned to the Cambrian, Ordovician, and Silurian periods instead of to the time preceding the Cambrian. Source: Karla Panchuk (2017) CC BY 4.0 Read the book<\/a><\/em>[\/caption]\r\n\r\n<\/div>\r\n \r\n

A Way To Think About Geological Time<\/h2>\r\nA useful mechanism for understanding geological time is to scale it down into one year. The origin of the solar system and Earth at 4.6 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\u2019s 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\u2019s history, as expressed on this time scale, are summarized in Table 1.2.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 1.2\u00a0 Some Important Dates Expressed As If All of Geological Time Were Condensed Into One Year<\/strong><\/th>\r\n<\/tr>\r\n
Event<\/strong><\/th>\r\nApproximate Date<\/strong><\/th>\r\nCalendar Equivalent<\/strong><\/th>\r\n<\/tr>\r\n
Formation of oceans and continents<\/td>\r\n4.5 - 4.4 Ga<\/td>\r\nfirst week of January<\/td>\r\n<\/tr>\r\n
Evolution of the first primitive life forms<\/td>\r\n3.8 Ga<\/td>\r\nend of February<\/td>\r\n<\/tr>\r\n
Formation of British Columbia\u2019s oldest rocks<\/td>\r\n2.0 Ga<\/td>\r\nJuly<\/td>\r\n<\/tr>\r\n
Evolution of the first multi-celled animals<\/td>\r\n600 Ma<\/td>\r\nbeginning of November<\/td>\r\n<\/tr>\r\n
Animals first crawled onto land<\/td>\r\n360 Ma<\/td>\r\nend of November<\/td>\r\n<\/tr>\r\n
Vancouver Island reached North America and the Rocky Mountains were formed<\/td>\r\n90 Ma<\/td>\r\nDecember 16<\/td>\r\n<\/tr>\r\n
Extinction of the non-avian dinosaurs<\/td>\r\n66 Ma<\/td>\r\nDecember 18<\/td>\r\n<\/tr>\r\n
The last time atmospheric CO2<\/sub> levels were above 400 ppm (Tripati et al., 2009)<\/td>\r\n16 Ma<\/td>\r\n6:20 p.m. December 30<\/td>\r\n<\/tr>\r\n
Beginning of the Pleistocene ice age<\/td>\r\n2 Ma<\/td>\r\n10:10 p.m., December 31<\/td>\r\n<\/tr>\r\n
Oldest radiocarbon date from people living in Canada (British Columbia)<\/td>\r\n13.8 ka<\/td>\r\n11:58 p.m., December 31<\/td>\r\n<\/tr>\r\n
First indication of fossil fuel impacts on atmospheric CO2<\/sub> levels (~280 ppm)<\/td>\r\n221 years ago<\/td>\r\n1.5 seconds before midnight, December 31<\/td>\r\n<\/tr>\r\n
Atmospheric CO2<\/sub> levels exceed 400 ppm<\/td>\r\n8 years ago<\/td>\r\n0.06 seconds before midnight, December 31<\/td>\r\n<\/tr>\r\n
Source: Karla Panchuk (2021) CC BY 4.0, modified after Steven Earle (2015) CC BY 4.0 view original<\/a>\r\n<\/em><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Uniformitarianism<\/h1>\r\nUniformitarianism<\/strong> is the notion that the geological processes occurring on Earth today are the same ones that occurred in the past.\u00a0 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.\u00a0 While this idea might not seem remarkable today, it was ground breaking and even controversial for its time.\u00a0 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.\u00a0 For some, this implied questioning their most deeply held religious beliefs.\r\n\r\nThe idea of uniformitarianism can be traced back to James Hutton, first in a paper he read to the Royal Society in 1784, and later in his book Theory of the Earth<\/em>, first published in 1788. (Read this book<\/a> at Project Gutenberg.) In 1788 he explained, \"In examining things present, we have data from which to reason with regard to what has been; and, from what has actually been, we have data for concluding with regard to that which is to happen hereafter.\"\r\n\r\nCharles Lyell, also a Scottish geologist, expanded on Hutton's ideas and incorporated those of may other early geologists in his own book Principles of Geology<\/em><\/a>. According to Lyell, \"religious prejudice\" was a major stumbling block to the acceptance of these ideas, but also Hutton and other scientists should have written less wordy books, and included more pictures.\r\n\r\nCharles Lyell's thinking about uniformitarianism is paraphrased as \"the present is the key to the past.\" As he put it in his own less-wordy more-pictures book, a belief in the \"permanency of the laws of nature\" means that a geologist\r\n

\"will deem it incumbent on him to examine with minute attention all the changes now in progress on the Earth, and will regard every fact collected respecting the causes in diurnal action, as affording him a key to the interpretation of some mystery in the archives of remote ages.\"<\/em><\/p>\r\nIn other words, learning how things work today will tell you about how they worked in the past.\r\n\r\nParaphrasing uniformitarianism as \"the present is the key to the past\" has led some to view it as an oversimplification, because not all geological processes occurring today occurred at all times in the geological past.\u00a0 Some important chemical reactions that happen on Earth's surface today require abundant oxygen in the atmosphere, and could not have occurred prior to Earth developing an oxygen-rich atmosphere.\u00a0 Furthermore, 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.\r\n\r\nDespite 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 tiny samples of what Earth used to be like.\u00a0 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.\r\n\r\nArguably, saying that \"the present is the key to the past\" is an oversimplification on Lyell's part isn't exactly fair to Lyell, given his phrasing about the \"permanency of the laws of nature.\" (Author's note: I have been unable to find that exact \"key to the past\" quote so far, so at this point we have to allow for the possibility that Lyell didn't oversimplify at all, and he's getting in trouble for what someone else said. I will update this if I find out differently). If we take his meaning to be that the basic way that physics and chemistry work has always been the same, then we are still using that principle to interpret the Mars rocks in Figure 1.2<\/a>. Imagine what Lyell would have thought about this idea being used to infer the presence of an ancient river on Mars!\r\n\r\n \r\n

\r\n\r\nCheck Your Understanding\r\n<\/strong>\r\n\r\n[h5p id=\"118\"]\r\n\r\n<\/div>\r\n

Plate Tectonics<\/h1>\r\nThe theory of plate tectonics<\/strong>\u2014the idea that Earth's surface is broken into large moving fragments, called plates<\/strong>\u2014profoundly changed our perspective on how the Earth works. Before plate tectonic theory, we couldn't answer questions like \"How did that mountain range get there?\" and \"Why do earthquakes happen where they do?\" This critical framework is also very recent\u2014the papers that led to the widespread acceptance of plate tectonics were published at about the same time that we got cordless power tools, contact lenses, and satellite television.\r\n\r\nEarth has 15 large tectonic plates (Figure 1.9), and numerous smaller ones. (A more detailed map of Earth's tectonic plates can be found at here<\/a>.) Tectonic plates move in a variety of directions and at a variety of speeds, although on average they move a few millimetres per year. In Figure 1.9, the black arrows show the direction of motion. The size of arrows indicates how fast the plate is moving. Plates with arrows going in more than one direction are rotating. Plate tectonic traffic is complicated!\r\n\r\nThe explanatory power of plate tectonic theory comes from what happens when plates interact along their margins. Plate boundary interactions (red arrows) include collisions (\u2192 \u2190<\/strong><\/span>), separation (\u2190<\/strong> \u2192<\/strong><\/span>), or sliding along each other (\u2197 \u2199<\/strong><\/span>).\r\n\r\n \r\n\r\n[caption id=\"attachment_60\" align=\"aligncenter\" width=\"650\"]\"A<\/a> Figure 1.9<\/strong> 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<\/a> Modified after U. S. Geological Survey (1996) Public Domain view original<\/a><\/em>[\/caption]\r\n\r\nBefore plate tectonic theory, we made observations but could only guess at mechanisms.\u00a0 It was like watching the hands on a clock and trying to guess what moves them.\u00a0 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<\/em> things have happened, but also allows us to predict what might happen in the future.\r\n\r\nPlate 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.\u00a0 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.\r\n\r\nWhether 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.\r\n\r\n \r\n
\r\n\r\nCheck Your Understanding<\/strong>\r\n\r\n[h5p id=\"119\"]\r\n\r\n<\/div>\r\n \r\n\r\n ","rendered":"

In geology there are three big ideas that are fundamental to the way we think about how Earth works.\u00a0 The ideas are like the sound track to a movie\u2014sometimes we might not even notice them, but they nevertheless affect our perception of what’s happening.\u00a0 In the rest of this book these ideas may be mentioned explicitly in some cases, but in other cases they won’t be discussed by name, and it will be helpful for you to realize that they’re relevant.<\/p>\n

Geological Time (Deep Time)<\/h1>\n

Earth is approximately 4.6 billion years old (4,600,000,000 years), which is a long time for geological events to unfold and changes to happen. The changes themselves might be tiny\u2014over a year, a chemical reaction might eat away a few layers of atoms at the surface of a rock. Over hundreds of millions of years, however, the chemical reaction could cause a mountain range to crumble into grains of sand, and be swept away by rivers.<\/p>\n

For geologists who study very, very slow processes, 10 million years might be a short time, and 1 million years might be trivial.\u00a0 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.<\/p>\n

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.<\/p>\n

\n

Need Some Help Visualizing Geological Time?<\/strong><\/p>\n

Watch the animation Four Ways to Understand the Earth\u2019s Age <\/em>to see four analogies for geological time.<\/p>\n