Lab5_geoTime_wksht_FA2023

34 Name GEOSCIENCE 001 FALL 202 3 LAB 5: GEOLOGIC TIME Section Date 7KLV ODE LV PRGLILHG IURP ³*HRORJ\ IURP ([SHULHQFH´ E\ (± .LUVWHQ 3HWHUV DQG /DUU\ (± 'DYLV² IURP ³,QYHVWLJDWLQJ (DUWK´ E\ &± Gil Wiswall and Charles H. Fletcher, and by the geologic history lab developed by Michael Harris of the Department of Geology at James Madison University. Introduction Large sequences of layered sedimentary rocks can represent millions of years of elapsed time. Each distinct layer or bed in a sequence is called STRATUM, and multiple layers are collectively called STRATA. The study of sequences of strata is called STRATIGRAPHY. A sequence of strata may also be referred to as a STRATIGRAPHIC SECTION. Extrusive igneous rocks such as lava flows and ash falls may also form as beds and can occur as discrete layers in a sequence of sedimentary rocks or they may cut across pre-existing layers of rock. All sequences of rock world-wide are sometimes collectively referred to as the ROCK RECORD. Each rock type and geologic structure in a sequence represents a different geologic event. As geologists we want to understand the history of the Earth and the rate of geologic processes. To do this, we must be able to date stratigraphic sections and the rock record. This lab provides exercises to investigate the two types of dating that geologists use, RELATIVE and NUMERICAL or ABSOLUTE DATING. Relative Dating RELATIVE DATING refers to the placing of events in the order in which they occurred without any understanding of the actual time or absolute time during which any one event occurred. In other words, we can discuss that a certain event happened first, or previous to the next event, or another event FRXOGQ¶W KDYH RFFXUUHG XQWLO RWKHU HYHQWV KDG± $ VHW RI YHU\ VLPSOH SULQFLSOHV LQ FRQMXQFWLRQ ZLWK FDUHIXO observation allows the determination of the relative order of geologic events represented by the rocks in an exposed stratigraphic section. Principles of Geology An understanding of stratigraphy begins by recognizing certain principles. Natural processes such as erosion, deposition, and plate tectonics, and the natural laws of gravity and isostasy that have produced the current features of the Earth, are thought to have operated in the same way in the distant past as they do now. This idea is known as the PRINCIPLE OF UNIFORMITARIANISM and was first stated by James Hutton in the 18 th century. Given an understanding of uniformitarianism, the relative timing of geologic events can be determined by applying other simple ideas, collectively known as the PRINCIPLES OF GEOLOGY which include: 1) THE PRINCIPLE OF ORIGINAL HORIZONTALITY ± sediments that are deposited or SUHFLSLWDWHG RQ WKH (DUWK¶V VXUIDFH DUH GRQH VR LQ PRVWO\ KRUL]RQWDO OD\HUV± 7KXV² LI WKH URFNV are noted to be tilted, folded, or metamorphosed, then these events must have followed deposition and lithification;

35 2) THE PRINCIPLE OF SUPERPOSITION ± in a series of layered rocks that have accumulated RQ WKH (DUWK¶V VXUIDFH² WKH ROGHVW URFNV DUH DW WKH ERWWRP RI WKH VHTXHQFH DQG WKH \RXQJHVW DUH at the top; 3) THE PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS ± any geologic feature that is crosscut or modified by another feature must be the older unit, i.e. , the crosscutting feature is the younger feature, it needed something to already be there for it to cut across. THINGS TO REMEMBER ± each of the rock types, sedimentary, igneous, or metamorphic, represent a unique geologic event, which is referred to using the appropriate terminology that reflects the process(es) that formed it, i.e.; o Sedimentary rocks are deposited, so we refer to the DEPOSITION of a sedimentary rock ( e.g., deposition of shale or sandstone) o Plutonic rocks are intruded, so we refer to the INTRUSION of a plutonic rock ( e.g., intrusion of granite or mafic dike) o Volcanic URFNV DUH HUXSWHG RQWR WKH (DUWK¶V VXUIDFH² VR ZH UHIHU WR WKH (5837,21 RI D volcanic rock ( e.g., eruption of basalt or rhyolite) o Metamorphic rocks form through the METAMORPHISM of a protolith. The protolith, not the metamorphic rock, was deposited, intruded, or extruded. We report the event that formed the protolith, AND THEN the metamorphic event. The metamorphism itself is considered a separate event. Deformation and erosion of rocks are also geologic events. Some but not all of the rock units may be folded or faulted, and the discussion of the relative ages of these deformation events follows the same as if talking about the rock units. Erosion events are also discussed as events relative to other events. Unconformities Unconformities are (usually) irregular contacts between strata in a stratigraphic sequence produced during periods of erosion or non-deposition. These contacts, thin boundaries between layers of rock, represent episodes of missing information, where there are no recorded rock units. Unconformities are labeled according to the nature of the strata above and below the unconformity. There are three types: 1) DISCONFORMITY ± a boundary between parallel, undeformed layers of rock, usually formed due to erosion or non-deposition; 2) NONCONFORMITY ± a boundary between layers of sedimentary rock overlying igneous or metamorphic rocks. This relationship usually indicates that the underlying igneous or metamorphic rocks were exposed before being buried and deposited upon; 3) ANGULAR UNCONFORMITY ± a boundary between layers of sedimentary rock overlying tilted or folded beds. This relationship suggests that the older rocks were deformed, exposed, and then deposited upon.

37 FIGURE 2

38 I. RELATIVE DATING Use the figures to determine the relative order of geologic events. List the events and corresponding HYLGHQFH IRU HDFK HYHQW± 'RQ¶W VNLS DQ\ VWHSV DQG UHPHPEHU WKDW WKH ROGHVW HYHQW VKRXOG EH OLVWHG DW WKH bottom of the table (i.e., start with #1). USE FIGURE 1 FOR THIS PROBLEM Event Description Evidence 9 8 7 6 5 4 3 2 1 USE FIGURE 2 FOR THIS PROBLEM Event Description Evidence 9 8 7 6 5

40 1 2 3 2 4 2 5 2 6 2 7 2 8 2

41 Absolute Dating: Half-life of 40 K 'RQ¶W XVH WKH GHFD\ HTXDWLRQ ± this exercise is to use the graph! Potassium 40 is a naturally radioactive element that decays to 40 Ar with a half-life of just over 100 million years (1.192 x 10 8 years). Potassium is an abundant element in the crust and is found in a wide range of igneous and metamorphic rocks. Consequently, this decay series is important for dating a wide variety of geologic events. 1. Column I in Table 2 indicates the number of half-lives that have passed since the geologic clock started. Calculate the time represented for each half-life interval. Enter your result in column II. 2. Assume that a biotite crystal originally contained 3000 atoms of 40 K (N o = 3000). Determine the number of 40 K atoms remaining after each half-life interval. Enter your results in column III. 3. Calculate the number of daughter atoms after each half-life interval. Enter the results in column IV. 4. For each half-life interval, add the number of 40 K atoms in column III to the number of 40 Ar atoms in column IV. Enter your answer (N o ) in column V. 5. Plot your results in the graph provided below ( plot atoms of 40 K on the y-axis ). 6. The curve you generated is an exponential decay curve, which can be used to estimate the age of a rock or mineral that originally contained 3000 atoms of 40 K. a. If a mineral now contains 500 40 K atoms, how old is it? b. What is its age if it now contains 2750 40 Ar atoms? 7. It is possible to date radiometrically rocks and minerals that contain naturally radioactive elements. They are the basis for quantifying the timing and duration of geologic events and have produced an entire new subdiscipline of geology. a. What quantities can we measure? b. What quantity must we assume was initially zero?

43 III. COMBINING RELATIVE AND ABSOLUTE DATING TECHNIQUES First, determine the ages of the following units in Figure 3: Granodiorite, Granite, and Basalt. ROCK ISOTOPE HALF-LIFE FRACTION OF ELAPSED HALF- LIVES AGE Granodiorite 238U/206Pb 4,500,000,000 0.09 Granite 235U/207Pb 710,000,000 0.46 Basalt 235U/207Pb 710,000,000 0.36 Use these dates and relative dating to answer the following questions: 1 . a) How old is the unconformity underneath the sandy shale (this should be a range)? b) Approximately how long did this period of erosion and non-deposition last? 2 . Using the radiometric ages of the intrusive rocks in the diagram, how long did it take for conglomerate, shale, and sandstone below the unconformity to be deposited? 3 . What sort of depositional environment existed immediately after the basalt flow?

44 Figure 3