:06 nul LTE4 web.ung.edu 9/358 1.2.1 Relative Time and Geologic Laws The methods that geologists use to establish relative time scales are based on Geologic Laws. A scientific law is something that we understand and is proven. It turns out that, unlike math, it is hard to prove ideas in science and, therefore, Geologic Laws are often easy to understand and fairly simple. Before we discuss the different geologic laws, it would be worthwhile to briefly introduce the differ- ent rock types. Sedimentary rocks, like sandstone, are made from broken pieces of other rock that are eroded in the high areas of the earth, transported by wind, ice, and water to lower areas, and deposited. The cooling and crystallizing of mol- ten rock forms igneous rocks. Lastly, the application of heat and pressure to rocks creates metamorphic rocks. This distinction is important because these three different rock types are formed differently and therefore, need to be inter- preted differently. The Law of Superposition states that Youngest in an undeformed sequence of sedimentary rocks the oldest rocks will be at the bottom of the sequence while the youngest will be on top. Imagine a river carrying sand into an ocean, the sand will spill out onto the ocean floor and come to rest on top of the seafloor. This sand was deposited after the sand of the Oldest seafloor was already deposited. We can then Figure 1.2 | Block diagram showing the create a relative time scale of rock layers relative age of sedimentary layers based from the oldest rocks at the bottom (labeled on the Law of Superposition. #1 in Figure 1.2) to the youngest at the top of an outcrop (labeled #7 in Figure 1.2). 6 5 4 3 Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 Page 4 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY The Law of Original Horizontality states that undeformed sedimentary rock are deposited horizontally. The deposition of sediment is controlled by gravi- ty and will pull it downward. If you have muddy water on a slope, the water will flow down the slope and pool flat at the base rather than depositing on the slope itself. This means that if we see sedimentary rock that is tilted or folded it was first deposited flat, then folded or tilted afterward (Figure 1.3). The Law of Cross-Cutting Uplift and tilting states that when two geologic fea- tures intersect, the one that cuts across the other is younger. In es- sence, a feature has to be present before something can affect it. For example, if a fault fractures through a series of sedimentary rocks those sedimentary rocks must be older than the fault (Figure 1.4). Figure 1.3 Sedimentary rocks are deposited horizontally such that if the layers are tilted or folded it must have occur One other feature that can be following deposition. useful in building relative time Author: Bradley Deline scales is what is missing in a se- Source: Original Work quence of rocks. Unconformi- ties are surfaces that represent significant weathering and ero- sion (the breakdown of rock and Fault movement of sediment) which re- sult in missing or erased time. Erosion often occurs in elevated areas like continents or moun- tains so pushing rocks up (called uplifting) results in erosion and destroying a part of a geologic se- Igneous quence; much older rocks are then Intrusion exposed at the earth’s surface. If the area sinks (called subsidence), Figure 1.4 Block diagrams showing the Law of Cross-Cutti then much on the right the geological features (fault License: CC BY-SA 3.0 7 / / ensured.Overton of these newly one I intrusion across the sedimentary layers and mi 5:07 .ILTEC web.ung.edu NTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY 10/358 Nonconformity Intrusion of igneous rock into sedimentary rocks Uplift and erosion Subsidence and deposition Intrusive Igneous Rock Angular Unconformity Deposition of Uplift, tilting, and sedimentary rocks erosion Subsidence and deposition Disconformity Deposition of sedimentary rocks Uplift and erosion Subsidence and deposition Figure 1.5 Block diagrams showing the formation of the three types of Unconformities. The three unconformities differ based on the type of rock underneath the erosion surface. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 then the orientation of the layers is important. If the rocks below the erosion sur- face are not parallel with those above, the surface is called an Angular Uncon- formity. This is often the result of the rocks below being tilted or folded prior to the erosion and deposition of the younger rocks. If the rocks above and below the erosion surface are parallel, the surface is called a Disconformity. This type of surface is often difficult to detect, but can often be recognized using other informa- tion such as the fossils discussed in the next section. Page 6 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY Using these principles we can look at a series of rocks and determine their rel- ative ages and even establish a series of events that must have occurred. Common events that are often recognized can include 1) Deposition of sedimentary layers, 2) Tilting or folding rocks, 3) Uplift and erosion of rocks, 4) Intrusion of liquid mag- ma, and 5) Fracturing of rock (faulting). Figures 1.6 and 1.7 show how to piece to- gether a series of geologic events using relative dating. Building a Relative Time Sequence Step 1. Identify and number all of the sedimentary layers 5:07 | LTE CU web.ung.edu 11/358 Using these principles we can look at a series of rocks and determine their rel- ative ages and even establish a series of events that must have occurred. Common vents that are often recognized can include 1) Deposition of sedimentary layers, 2) ſilting or folding rocks, 3) Uplift and erosion of rocks, 4) Intrusion of liquid mag- ma, and 5) Fracturing of rock (faulting). Figures 1.6 and 1.7 show how to piece to- gether a series of geologic events using relative dating. Building a Relative Time Sequence Step 1. Identify and number all of the sedimentary layers. m Step 2. Identify any other geologic events Step 3. Place the sedimentary Layers and geologic events in order based on the Geologic Laws. 1. Deposit sedimentary layers 1-4. 2. Igneous Intrustion. 3. Uplift and Erode – Disconformity. 4. Subsidence and Deposit layers 5-7. 5. Uplift and Erode Disconformity Igneous Intrusion Figure 1.6 An example showing how to determine a relative dating sequence of events from a block diagram. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 Page 7 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY Building a Relative Time Sequence Step 1. Identify and number all of the sedimentary layers. Step 2.Identify any other geologic events Step 3. Place the sedimentary Layers and geologic events in order based on the Geologic Laws. Tilting Angular Unconformity 1. Deposit sedimentary layers 1-6. 2. Fold and Fault layers 1-6. 3. Uplift and Erode – Angular Unconformity 4. Subsidence and Deposit layers 7-10 5. Tilt Layers 1-10. lift and Erode. 5:07 LTE CU web.ung.edu INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY А 13/358 L.3 LAB EXERCISE Part A – Relative Time Relative time is an important tool for ge- ologist to quickly construct series of events, especially in the field. In the following sec- tion, apply what you have learned regarding relative time to the questions below. 1. In Figure 1.8, which of the following rock layers is the oldest? Figure 1.8 Block diagram to use to answer questions 1 and 2. Author: Bradley Deline a. A b. B c. C Source: Original Work License: CC BY-SA 3.0 2. Which Geologic Law did you use to come to the conclusion you made in the previous question? a. The Law of Superposition b. The Law of Cross-Cutting c. The Law of Original Horizontality d. Unconformities B. Unconformity C. Sedimentary Rock Layer A. Igneous Intrustion Unconformities 1 and 2 Metamorphic Rocks Figure 1.9 Block diagram to use to answer questions 3, 4, and 5. Unconformities are shown in brown. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 3. In Figure 1.9, which of the following geologic structures is the youngest? a. A b. B c. Page 19 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY

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:06 nul LTE4 web.ung.edu 9/358 1.2.1 Relative Time and Geologic Laws The methods that geologists use to establish relative time scales are based on Geologic Laws. A scientific law is something that we understand and is proven. It turns out that, unlike math, it is hard to prove ideas in science and, therefore, Geologic Laws are often easy to understand and fairly simple. Before we discuss the different geologic laws, it would be worthwhile to briefly introduce the differ- ent rock types. Sedimentary rocks, like sandstone, are made from broken pieces of other rock that are eroded in the high areas of the earth, transported by wind, ice, and water to lower areas, and deposited. The cooling and crystallizing of mol- ten rock forms igneous rocks. Lastly, the application of heat and pressure to rocks creates metamorphic rocks. This distinction is important because these three different rock types are formed differently and therefore, need to be inter- preted differently. The Law of Superposition states that Youngest in an undeformed sequence of sedimentary rocks the oldest rocks will be at the bottom of the sequence while the youngest will be on top. Imagine a river carrying sand into an ocean, the sand will spill out onto the ocean floor and come to rest on top of the seafloor. This sand was deposited after the sand of the Oldest seafloor was already deposited. We can then Figure 1.2 | Block diagram showing the create a relative time scale of rock layers relative age of sedimentary layers based from the oldest rocks at the bottom (labeled on the Law of Superposition. #1 in Figure 1.2) to the youngest at the top of an outcrop (labeled #7 in Figure 1.2). 6 5 4 3 Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 Page 4 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY The Law of Original Horizontality states that undeformed sedimentary rock are deposited horizontally. The deposition of sediment is controlled by gravi- ty and will pull it downward. If you have muddy water on a slope, the water will flow down the slope and pool flat at the base rather than depositing on the slope itself. This means that if we see sedimentary rock that is tilted or folded it was first deposited flat, then folded or tilted afterward (Figure 1.3). The Law of Cross-Cutting Uplift and tilting states that when two geologic fea- tures intersect, the one that cuts across the other is younger. In es- sence, a feature has to be present before something can affect it. For example, if a fault fractures through a series of sedimentary rocks those sedimentary rocks must be older than the fault (Figure 1.4). Figure 1.3 Sedimentary rocks are deposited horizontally such that if the layers are tilted or folded it must have occur One other feature that can be following deposition. useful in building relative time Author: Bradley Deline scales is what is missing in a se- Source: Original Work quence of rocks. Unconformi- ties are surfaces that represent significant weathering and ero- sion (the breakdown of rock and Fault movement of sediment) which re- sult in missing or erased time. Erosion often occurs in elevated areas like continents or moun- tains so pushing rocks up (called uplifting) results in erosion and destroying a part of a geologic se- Igneous quence; much older rocks are then Intrusion exposed at the earth’s surface. If the area sinks (called subsidence), Figure 1.4 Block diagrams showing the Law of Cross-Cutti then much on the right the geological features (fault License: CC BY-SA 3.0 7 / / ensured.Overton of these newly one I intrusion across the sedimentary layers and mi 5:07 .ILTEC web.ung.edu NTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY 10/358 Nonconformity Intrusion of igneous rock into sedimentary rocks Uplift and erosion Subsidence and deposition Intrusive Igneous Rock Angular Unconformity Deposition of Uplift, tilting, and sedimentary rocks erosion Subsidence and deposition Disconformity Deposition of sedimentary rocks Uplift and erosion Subsidence and deposition Figure 1.5 Block diagrams showing the formation of the three types of Unconformities. The three unconformities differ based on the type of rock underneath the erosion surface. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 then the orientation of the layers is important. If the rocks below the erosion sur- face are not parallel with those above, the surface is called an Angular Uncon- formity. This is often the result of the rocks below being tilted or folded prior to the erosion and deposition of the younger rocks. If the rocks above and below the erosion surface are parallel, the surface is called a Disconformity. This type of surface is often difficult to detect, but can often be recognized using other informa- tion such as the fossils discussed in the next section. Page 6 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY Using these principles we can look at a series of rocks and determine their rel- ative ages and even establish a series of events that must have occurred. Common events that are often recognized can include 1) Deposition of sedimentary layers, 2) Tilting or folding rocks, 3) Uplift and erosion of rocks, 4) Intrusion of liquid mag- ma, and 5) Fracturing of rock (faulting). Figures 1.6 and 1.7 show how to piece to- gether a series of geologic events using relative dating. Building a Relative Time Sequence Step 1. Identify and number all of the sedimentary layers 5:07 | LTE CU web.ung.edu 11/358 Using these principles we can look at a series of rocks and determine their rel- ative ages and even establish a series of events that must have occurred. Common vents that are often recognized can include 1) Deposition of sedimentary layers, 2) ſilting or folding rocks, 3) Uplift and erosion of rocks, 4) Intrusion of liquid mag- ma, and 5) Fracturing of rock (faulting). Figures 1.6 and 1.7 show how to piece to- gether a series of geologic events using relative dating. Building a Relative Time Sequence Step 1. Identify and number all of the sedimentary layers. m Step 2. Identify any other geologic events Step 3. Place the sedimentary Layers and geologic events in order based on the Geologic Laws. 1. Deposit sedimentary layers 1-4. 2. Igneous Intrustion. 3. Uplift and Erode – Disconformity. 4. Subsidence and Deposit layers 5-7. 5. Uplift and Erode Disconformity Igneous Intrusion Figure 1.6 An example showing how to determine a relative dating sequence of events from a block diagram. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 Page 7 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY Building a Relative Time Sequence Step 1. Identify and number all of the sedimentary layers. Step 2.Identify any other geologic events Step 3. Place the sedimentary Layers and geologic events in order based on the Geologic Laws. Tilting Angular Unconformity 1. Deposit sedimentary layers 1-6. 2. Fold and Fault layers 1-6. 3. Uplift and Erode – Angular Unconformity 4. Subsidence and Deposit layers 7-10 5. Tilt Layers 1-10. lift and Erode. 5:07 LTE CU web.ung.edu INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY А 13/358 L.3 LAB EXERCISE Part A – Relative Time Relative time is an important tool for ge- ologist to quickly construct series of events, especially in the field. In the following sec- tion, apply what you have learned regarding relative time to the questions below. 1. In Figure 1.8, which of the following rock layers is the oldest? Figure 1.8 Block diagram to use to answer questions 1 and 2. Author: Bradley Deline a. A b. B c. C Source: Original Work License: CC BY-SA 3.0 2. Which Geologic Law did you use to come to the conclusion you made in the previous question? a. The Law of Superposition b. The Law of Cross-Cutting c. The Law of Original Horizontality d. Unconformities B. Unconformity C. Sedimentary Rock Layer A. Igneous Intrustion Unconformities 1 and 2 Metamorphic Rocks Figure 1.9 Block diagram to use to answer questions 3, 4, and 5. Unconformities are shown in brown. Author: Bradley Deline Source: Original Work License: CC BY-SA 3.0 3. In Figure 1.9, which of the following geologic structures is the youngest? a. A b. B c. Page 19 INTRODUCTORY GEOLOGY INTRODUCTION TO PHYSICAL GEOLOGY

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