{"id":60,"date":"2021-09-16T19:28:23","date_gmt":"2021-09-16T19:28:23","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accphysicalgeography\/chapter\/1-6-geological-time-physical-geology-2nd-edition\/"},"modified":"2022-02-02T16:37:20","modified_gmt":"2022-02-02T16:37:20","slug":"1-6-geological-time-physical-geology-2nd-edition","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/chapter\/1-6-geological-time-physical-geology-2nd-edition\/","title":{"raw":"1.6 Geological Time \u2014 Physical Geology \u2013 2nd Edition","rendered":"1.6 Geological Time \u2014 Physical Geology \u2013 2nd Edition"},"content":{"raw":"
\r\n\r\nIn 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 \u2014 no prospect of an end. [1]<\/sup>\u00a0<\/em>Of course he wasn\u2019t 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\u2019t 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 Ma<\/span><\/strong> (for mega annum<\/em> or \u201cmillions of years\u201d) or 4.57 Ga<\/span><\/strong> (for giga<\/em>annum<\/em> or billions of years). More recent dates can be expressed in ka<\/span><\/strong> (kilo<\/em>annum<\/em>); 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.\r\n
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\r\n\r\nTo help you understand the scientific notation for geological time\u2014which is used extensively in this book\u2014write the following out in numbers (for example, 3.23 Ma = 3,230,000 years).\r\n
    \r\n \t
  1. 2.75 ka<\/li>\r\n \t
  2. 0.93 Ga<\/li>\r\n \t
  3. 14.2 Ma<\/li>\r\n<\/ol>\r\nWe use this notation to describe geological events in the same way that we might say \u201cthey arrived at 2 pm.\u201d\u00a0 For example, we can say \u201cthis rock formed at 45 Ma.\u201d But this notation is not used to express elapsed time. We don\u2019t say: \u201cI studied for 4 pm for that test.\u201d And we don\u2019t say: \u201cThe dinosaurs lived for 160 Ma.\u201d\u00a0 Instead, we could say: \u201cThe dinosaurs lived from 225 Ma to 65 Ma, which is 160 million years.\u201d\r\n\r\nSee Appendix 3 for Exercise 1.3 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\nUnfortunately, knowing how to express geological time doesn\u2019t 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\u2019ll see in other places, or even later in this book, this time scale is linear throughout its length, meaning that 50 Ma during the Cenozoic<\/span><\/strong> is the same thickness as 50 Ma during the Hadean<\/span><\/strong>\u2014in each case about the height of the \u201cM\u201d 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 \u201cCenozoic.\u201d Most other time scales have earlier parts of Earth\u2019s 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.\r\n
    \r\n\"Geological\r\n<\/a>\r\n
    Figure 1.6.1 The geological time scale. [Image Description]<\/a><\/div>\r\n<\/div>\r\nTo create some context, the Phanerozoic<\/span><\/strong> 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. In fact, large organisms\u2014those that leave fossils visible to the naked eye\u2014have 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<\/em> has existed since approximately 2.8 Ma, or 0.06% (1\/1,600th) of geological time.\r\n\r\nGeologists (and geology students) need to understand geological time. That doesn\u2019t 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.\r\n\r\nFor example, the Atlantic Ocean between New York City and northwestern Africa has been getting wider at a rate of about 2.5 centimeters (cm) per year. Imagine yourself taking a journey at that rate\u2014it 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 kilometers (km)!\r\n\r\nA 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\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 on Table 1.1.\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 1.1 A summary of some important geological dates expressed as if all of geological time was condensed into one year.<\/caption>\r\n
    [Skip Table]<\/a><\/td>\r\n<\/tr>\r\n
    Event<\/th>\r\nApproximate Date<\/th>\r\nCalendar Equivalent<\/th>\r\n<\/tr>\r\n
    Formation of oceans and continents<\/td>\r\n4.5 to 4.4 Ga<\/td>\r\nJanuary<\/td>\r\n<\/tr>\r\n
    Evolution of the first primitive life forms<\/td>\r\n3.8 Ga<\/td>\r\nearly March<\/td>\r\n<\/tr>\r\n
    Formation of Colorado's 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\n0.6 Ga or 600 Ma<\/td>\r\nNovember 15<\/td>\r\n<\/tr>\r\n
    Animals first crawled onto land<\/td>\r\n360 Ma<\/td>\r\nDecember 1<\/td>\r\n<\/tr>\r\n
    The Present-Day Rocky Mountains started to form<\/td>\r\n90 Ma<\/td>\r\nDecember 25<\/td>\r\n<\/tr>\r\n
    Extinction of the non-avian dinosaurs<\/td>\r\n65 Ma<\/td>\r\nDecember 26<\/td>\r\n<\/tr>\r\n
    Beginning of the Pleistocene ice age<\/td>\r\n2 Ma or 2000 ka<\/td>\r\n8 p.m., December 31<\/td>\r\n<\/tr>\r\n
    Retreat of the most recent glacial ice<\/td>\r\n14 ka<\/td>\r\n11:58 p.m., December 31<\/td>\r\n<\/tr>\r\n
    Arrival of the first people in North America<\/td>\r\n10 ka<\/td>\r\n11:59 p.m., December 31<\/td>\r\n<\/tr>\r\n
    Arrival of the first Europeans on the west coast of what is now Canada<\/td>\r\n250 years ago<\/td>\r\n2 seconds before midnight, December 31<\/td>\r\n<\/tr>\r\n<\/thead>\r\n<\/table>\r\n
    \r\n
    \r\n\r\nAlong with being one of the seven natural wonders of the world, the Grand Canyon serves as one of the ultimate natural laboratories in which vast amounts of geological time can be studied. Now, imagine yourself starting at the top of the canyon and you are able to embark on a 12.6 km trek to the bottom of the canyon where the Colorado River is. When you start the hike, you'll start at sedimentary rocks that are dated to be 270 Ma. When you reach the bottom, the oldest rocks are an age of 2 Ga (they are metamorphic rocks). With this distance and age data, see if you can answer the following:\r\n
      \r\n \t
    1. As you hike, how far back in time would you go with each meter along the trail? Give your answer in yr\/m (years per meter).<\/li>\r\n \t
    2. The elevation at the top of the canyon is 2085 m, with the elevation being 734 m at the river. In knowing that the Colorado River has carved the Grand Canyon through time, at an average rate of 0.5 mm\/yr (millimeters per year), how many years has it taken for the canyon to form?<\/li>\r\n<\/ol>\r\nHelpful hints: 1 km = 1,000 m and 1 m = 1,000 mm\r\n\r\nSee Appendix 3 for Exercise 1.4 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n

      Image descriptions<\/h3>\r\nFigure 1.6.1 image description:<\/strong> 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]<\/a>\r\n

      Media Attributions<\/h3>\r\n