1 Lab 3: Rock Record Case Study At any one particular location on the Earth, rocks beneath the surface are composed of many different layers of rocks which include igneous, metamorphic and sedimentary rock types.

1 Lab 3: Rock Record Case Study At any one particular location on the Earth, rocks beneath the surface are composed of many different layers of rocks which include igneous, metamorphic and sedimentary rock types. The rock record is the pattern of rocks observed at a particular location. By observing the relationships between and among the layers of rocks in the pattern, geologists can piece together the order of geologic events that lead to the creation of that particular rock pattern. In other words, we can use the rocks to understand how the geology in that place changed over time. This process of identifying rock layers and determining their ages relative to one another is called relative dating. Examining rocks and relative dating is very important for identifying geologic hazards such as those from earthquakes, volcanoes, tsunamis and landslides. First, we can use the rock record to identify where faults reach the surface so we know where we might expect earthquakes to occur as shown in Figure 1A. Second, we can put together an eruptive history of a volcano (see Figure 1b) to understand what volcanic hazards that particular volcano produced in the past and then use that knowledge to understand what it may be capable of in the future. Third, tsunamis tend to leave marks on the surface of the earth that are preserved in the rock record so we can use this information to identify when past tsunami’s occurred and how far inland those past tsunami’s came, similar to the last exercise you did for the Japan Case Study. Finally, evidence for landslides can also be found in the rock record. We can use that information to tell us about which areas are prone to landslide events, how often they might happen, how big the events are, etc. Figure 1: Outcrop (a) of rock layers broken by a rupture of a fault and a section (b) of rock layers which indicate a sequence of events in a volcanic eruption. A few notes about terminology. When sediments are transported to a place and remain there, we say they are ‘deposited’ there – like when you deposit a check at the bank. The phrase ‘Layer A was deposited first’, means sediments of the composition of Layer A were deposited at this location first. This usually means the bottom of that layer was at the surface of the earth at one time. An exception to that statement exists for igneous rocks which intrude (or enter forcefully) into sedimentary, igneous or metamorphic rocks typically while they are beneath the surface. Sedimentary rocks are usually indicated by flat layers relative to the surface of the Earth. Metamorphic rocks are usually indicated by layers that are folded, faulted, or tilted. Igneous rocks usually form irregular shapes in the middle of other units if they cool beneath the surface and flat or wavy layers if they were deposited on the surface after an eruption. Now that we know how to identify rocks from the Rock Cycle Lecture, and we know how to look at the rock record, we now need to understand how to read the order of events using the relative dating technique. (a) (b) 2 Here are a few simple relationships geologists use as guides when using the relative dating technique. 1) The Law of Superposition. According to this law, the youngest rocks are found on the top. In this example, rock layer 1 was deposited first, then layer 2 after layer 1, then layer 3 after layer 2, and finally layer 4 on top which is the youngest in this sequence of rocks. 2) Original Horizontality. According to this principle, all layers are deposited flat. Then they were folded, tilted or otherwise deformed after they were deposited flat. In this example, rock layer 1 was deposited first, then layer 2 after layer 1, then layer 3 after layer 2, and finally layer 4 on top which is the youngest in this sequence of rocks. After all of that, the rocks were then folded which means they were squished at edges. 3) Cross-cutting relationships. According to this principle, all layers are deposited flat. Then they were intruded by a different feature. Therefore the intruded feature is younger than the rocks it is intruding into. In this example, rock layer 1 was deposited first, then layer 2 after layer 1, then layer 3 after layer 2. Then, layer 4 intruded into layers 1, 2, and 3. So it is younger than all 3 of those layers. Finally, layer 5 was deposited on top which is the youngest in this sequence. 3 4) Unconformities. According to this principle, erosion of old layers occurs before the deposition of new layers. This results in the old layers having a wavy top. In this example, rock layer 1 was deposited first, then layer 2 was deposited after layer 1, then layer 3 after layer 2, and then layer 4 after layer 3. Layers 1-4 were then titled, and the surface was eroded leaving the wavy line marked with number 5. Then layer 6 was deposited. Afterwards, layer 7 was deposited and is the youngest layers in this sequence. Here is an example: Put the 5 geologic events in order, from youngest to oldest, that made this rock pattern. Use the letters to refer to each layer and call the youngest event number 1. By the Law of Superposition, the oldest event created layer C at the very bottom. The next event created layer B because it sits on top of layer C, again by the Law of Superposition. The third event deposited layer A because it sits on top of layer B, again by the Law of Superposition. Then the event that deposited layer D occurred because it crosscuts layers C, B, and A. Finally, movement along the fault line shown in black created the displacement indicated by the letter E because E cuts through layers A, B and D which are younger than C. So to order from 4 youngest to oldest we reverse that series of events we just figured out: E was the youngest so it is event #1, D = #2, A =#3, B = #4, C is the oldest and event #5. 1. E (youngest) 2. D 3. A 4. B 5. C (oldest) 5 Relative Dating Exercise Here is a pattern that is a bit more complicated. Place these 13 events in their relative order from oldest to youngest. In other words, the oldest event happened first and will be event number 1 in your sequence. Here are the events: a. deposition of layer A b. deposition of layer B c. deposition of layer C d. deposition of layer D e. deposition of layer E f. deposition of layer F g. movement along the fault G h. deposition of layer H i. deposition of layer I j. erosion to create unconformity J k. intrusion to create layer K l. erosion to create canyon L m. tilting of layers E, F, H and I You will put this event order in the Lab 3 exercise on BlackBoard.

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