CHAPTER 8: GEOLOGIC EVENTS AND TIME
               This chapter focuses on determining the relative ages of rock units from field relationships and 
            how we put together a geologic history or sequence of events. You will also learn how we determine 
            the numerical ages of rock units.
            (Note: Terms in red and italics appear as entries in the companion glossary.)
               Since the late 1700’s, geologists have been determining the relative ages of rock formations 
            based on their relationships with other rock formations and geologic features. Relative ages answer 
            the simple question of whether one geologic feature (rock formation, fault, erosion surface, etc.) is 
            older or younger than some other geologic feature. Another type of age is a numerical age, where 
            the ages of geologic events are expressed as numbers of years before present. Numerical ages for 
            relatively recent events can sometimes be associated with actual human observations, associations 
            with known historic events with calendar ages such as a volcanic eruption, or the counting of annual 
            layers in trees or sediment from the last ~12,000 years. In this case, numerical ages are often known 
            with certainty and are referred to as absolute ages. However, this does not work for most pre-
            historic events. For less than a century, geologists have had numerical dating techniques based on 
            chemical principles (decay of radioactive isotopes) at their disposal. These techniques provide 
            numerical age estimates back many millions of years for rock formations such as those in the Fells.
            8.1 RELATIVE DATING OF EVENTS
               Past geologic processes can be complex, but they can usually be broken down into a sequence of 
            events that brings some order to them. Using simple outcrop observations, we can usually 
            determine the relative ages of different rock formations. We apply fundamental principles that were 
            first established in the 1600’s by Nicholas Steno and late 1700’s by James Hutton, a Scottish 
            geologist. This essentially marks the beginning of geology as a science, and it greatly changed our 
            perception of the age of the Earth and the duration of geologic events. If you would like to know 
            more about James Hutton and the beginnings of geology as a science, there is an excellent three-
            part series on the history of geology as a science and the development of fundamental principles 
            that was made for the BBC. It is called “Men of Rock” (BBC, 2010) and it has three parts, each about 
            an hour long, that give you the early history and more about the field of geology in Scotland. AND,
            they are entertaining!
               Although Hutton used all the fundamental principles, they were not widely published or studied 
            until John Playfair (1802), and later Charles Lyell, summarized Hutton’s work in more available and 
            easily read books. Lyell’s publications, Principles of Geology (Lyell, 1830, 1832, 1833), are generally 
            regarded as the first textbooks in geology. These widely circulated publications stated all the 
            fundamental principles, which we still use today to determine the relative ages of geologic features 
            and events. The fundamental principles may seem obvious today but in the late 18th century they 
            were a breakthrough in geologic and scientific thought. They lifted constraints placed on science 
            and geology by religious doctrines.
            8.1.1 Geologic Events
               The simplest geologic events are the formation of rock or other geologic units. The formation of 
            an igneous rock unit refers to the time of magma solidification or crystallization to produce solid 
            rock. In the case of sedimentary rocks, it is the time when sediment was deposited. This will also be
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      the age of the fossils found in a sedimentary rock since they become a part of the rock when 
      sediment accumulates and buries organisms or their parts to eventually preserve them as fossils. 
      The age of metamorphic rocks can be trickier because geologists would not only like to know when 
      metamorphism occurred that changed the rock but also the age of the protolith. These two times 
      can be greatly different, and in the case of some metamorphic rocks that are metamorphosed 
      multiple times we may be trying to determine the ages of several events (protolith formation, 
      metamorphic event 1, metamorphic event 2, etc.).
        Several types of events, such as metamorphism, operate on rock formations that have already 
      been formed. Rocks can also be deformed by tilting of layers, folding, faulting, or fracturing, which 
      all represent geologic events. Rock units can also be weathered and eroded, and these events 
      represent times in which rock units were exposed at Earth’s surface. Weathering and erosion can 
      produce surfaces cut into rocks by removal of weathered material (erosion surfaces); thereby 
      destroying a part of the geologic record. This boundary represents a time gap in the geologic record 
      or a break in what is recorded by the rocks, known as an unconformity. Unconformities can also be 
      the result of a period of non-deposition in a sedimentary rock sequence. Technically, an 
      unconformityis: a surface that represents a break in time and is overlain by a sedimentary unit, 
      lava flow, or pyroclastic deposit. Unconformities will be reviewed in more detail below.
        To show the relationships between different rock formations and their relative ages, geologists 
      often look at rock formations in cross section, or side view. This allows you to better see the relative 
      positions of rock units and the character of their contact relationships. It also provides a better 
      perspective for determining relative ages. Cross section views can sometimes be seen on 
      photographs or on drawings that depict field relationships as might be drawn for notes in a field 
      book. Camera images may be very useful, but many times they do not clearly show the observed 
      rock types very well and a drawing can better record observations. Symbols for rock types and colors 
      are used to tell various rock units apart. Figure 8.1 shows some standard symbols that are used in 
      the Earth and Ocean Sciences Department at Tufts University to indicate various rock types in cross 
      sections. Using this symbology, it is possible to draw cross section sketches (interpretive diagrams) 
      that show the basic rock types and relationships seen in the field.
      Figure 8.1 – Standard 
      rock symbols used to 
      draw geologic cross 
      sections in the Dept. of        Five different 
      Earth and Ocean                 symbols used for 
      Sciences at Tufts               intrusive igneous 
      University. These 
      symbols are used by             rocks
      many others, and we 
      have adopted the 
      standard forms.
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      8.1.2 The Fundamental Principles
        In the late 1700’s and early 1800’s, natural scientists, who were the first geologists, formalized the 
      set of rules, or fundamental principles, that could be applied to the formation of rock units and their 
      relative ages. The development of the fundamental principles occurred at about the same time as the 
      formalization of the laws of mathematics, chemistry, and physics that supplanted explanations based 
      on divine intervention and catastrophism (large supernatural events) that relied on traditional stories 
      in the Old Testament of the Bible related to Earth’s creation and Noah’s Flood. The fundamental 
      principles were an outgrowth of advances in scientific reasoning that were applied to rocks, and they 
      were consistent with observations made on modern processes that form rocks, especially 
      sedimentary rocks and the rocks formed by volcanic eruptions. It took early geologists longer to 
      understand intrusive igneous rocks because their formation was not something that they could 
      observe at Earth’s surface. None of the fundamental principles are rocket science, but the formulation 
      of these principles back in the late 18th century triggered a revolution in how we perceived the 
      formation of Earth and its history and age, and it established the science of geology. In addition to 
      providing a rational explanation of geologic events and their sequence, uniformitarianism gave early 
      geologists a sense of the vast amount of time in Earth’s history. 
        The use of observations of modern processes to understand the character of ancient events is 
      what is called the principle of uniformitarianism. Technically, uniformitarianism says that geologic 
      events of the past were governed by the same laws of mathematics, chemistry, and physics that 
      govern processes today. While conditions in the past may have been different, for example, there may 
      have been less oxygen in the atmosphere, or temperatures may have been different, the laws of 
      mathematics, chemistry, and physics that operate today are also applicable to ancient Earth systems. 
      Uniformitarianism is often simplified to a cliché phrase: “the present is the key to the past”. While 
      there is some truth to this expression in terms of some similarities between modern and ancient 
      rock-forming processes, the present is never an exact analog for the past because of differing 
      conditions through geologic time.
        In the early formulation of fundamental principles by Nicholas Steno in the 1600’s, sedimentary 
      rocks got much of the attention. Bedded sedimentary rock units were recognized to have been 
      deposited as nearly horizontal layers, or beds, like modern sediments, with only a few exceptions. 
      This is known as the principle original horizontality (Fig. 8.2). Sedimentary layers that we see tilted or 
      folded today were not deposited in that configuration and therefore must have been tilted or folded 
      at some later time after their deposition as horizontal layers. Exceptions to this rule are sedimentary 
      structures called crossbeds (see Fig. 8.3), which are formed on the dipping faces of channel bars in 
      rivers or sand dunes, and deposits left by mass movement such as landslides and mudflows. Mass 
      movement deposits, which are not laid down layer upon layer, are frequently poorly sorted without 
      well-defined bedding and can be chaotic with irregular surfaces.
      Figure 8.2 – The principle of original 
      horizontality says that sedimentary rock 
      units were laid down as nearly 
      horizontal layers with a few exceptions 
      such as crossbeds and some mass 
      movement deposits. Shown here are 
      dipping beds of the Chinle Formation 
      east of the Grand Canyon in Arizona 
      that were tilted after being deposited 
      horizontally.
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        Sediments and sedimentary rock layers or beds are always deposited on some older surface. 
      Another way of saying this is that younger sedimentary rock units are superposed on older units. 
      This is the principle of superposition (Fig.8.3), which is Nicholas Steno’s great contribution. Early 
      naturalists also came to understand that sedimentary rock units, like modern sedimentary layers, did 
      not form with abrupt ends where we see them exposed in cliff faces or other outcrops. Instead, they 
      were once more continuous and today have been truncated by erosion at Earth’s surface. This is the 
      principle of continuity (Fig. 8.4). Sedimentary rock units are also made of particles that had to form 
      at some time prior to the deposition of the sediment. They are pieces of other rock formations that 
      must have been older. For example, if a conglomerate has pebbles made of granite, the pebbles of 
      granite come from a granite rock formation that is older than the time at which the conglomerate 
      was deposited. It indicates that there is granite older than the conglomerate. If the granite pebbles 
      are somewhat unique, it can tell us specifically which granite is older than the conglomerate. This 
      applies to the sand grains in sandstone, the silt particles in siltstone, and the clay particles in shale. 
      They are all from eroded rock formations older than the sedimentary rock in which they are found. 
      This is the principle of derivation – a sedimentary rock unit is younger than the rock units that are 
      the sources of the particles that make up the sedimentary rock unit (Fig. 8.5)
                           Figure 8.3 (left) – The principle of superposition says that 
                           sedimentary rock units are always laid down on older rock 
                           units or sediment. A sequence of sedimentary beds or 
                           layers, therefore, is always oldest at the bottom and 
                           youngest at the top. Shown here are horizontal beds of 
                           sandstone, each containing crossbeds, that rest one upon 
                           another with the oldest unit at the bottom and youngest 
                           unit at the top. The rock unit is the Navajo Sandstone in 
                           Zion National Park in Utah. Note the car for scale.
      Figure 8.4 (above right) – The principle of continuity states that sedimentary layers found truncated at Earth’s 
      surface once extended in all directions until they thinned to nothing or reached the edges of their sedimentary 
      basin. Shown here is the upper 2/3 of the Grand Canyon, which was once composed of horizontal layers that 
      were more continuous prior to erosion. The same rock units are found on both sides of the canyon.
                      Chapter 8 - Events and Time   4