Lab 8 - Geologic Time - Handout

GEOL101 Dynamics of the Earth – Fall 2023 Name: Emily Thomson Laboratory 8 Geologic Time . . . keeps on ticking, ticking Section: Learning Outcomes: ● Differentiate absolute and relative dating ● Describe and apply the six principles of relative dating to reconstruct geologic events ● Differentiate between the various types of unconformities ● Determine the relative age of a sample by comparing its fossil composition using the principles of faunal succession Introduction One of the most fundamental techniques in geology is dating rocks and putting geologic events in their proper historical sequence. There are two ways by which this can be accomplished. Absolute dating involves determining the age of the rock in years (e.g., 125,000 years; 2.6 billion years). The most common form of absolute dating is radiometric dating, where specific minerals are analyzed to determine (1) the amount of remaining original “parent” isotope (e.g., Uranium-235) incorporated during the rock’s formation to (2) the amount of initially-absent “daughter” isotope (e.g., Lead-207) produced by the progressive radioactive decay of the parent over time. This method involves complex instrumentation and careful selection and screening of samples. We won’t cover absolute dating more in this laboratory, but it is important to realize that geoscientists apply this technique to various mineral-isotope systems to estimate the absolute age of geologic events on timescale ranging from thousands to billions of year. Even if you cannot tell the precise age of a given rock, you can determine the order in which a series of geologic events occurred and place a relative age on that rock. This is called relative dating which is the most fundamental concept in geology. Absolute dating techniques have only been around since the late 1960’s, but geologists have been putting relative ages on rocks since the 1700’s. Early geologists used the principle of faunal succession and other principles of relative dating to determine the relative age of a rock. In fact, fossils and the principles of relative dating were used to create the geologic time scale long before we knew the absolute age of the earth. In this lab we are going to focus on principles of relative dating. Geologic Time Scale The geologic time scale, shown on the next page, reflects how scientists from all over the world have worked as a community for over 150 years to divide the ancient record of life and our planet into highly resolved intervals of geologic time. Most of the major boundaries between these time intervals are based upon major changes in the fossil record; for example, the Cretaceous-Paleogene (K-P) boundary is defined by the extinction of the dinosaurs from a meteorite impact over 65 million years ago. Thus, the geologic time scale acts as both a calendar and a file cabinet for our growing understanding of the history and dynamics of our complex Earth system. Notice that the time scale on the next page is divided into finer and finer time intervals from right to left; for example, the Cenozoic Era includes the Paleogene , Neogene , and Quaternary Periods , while the Paleogene Period includes the Paleocene , Eocene , and Oligocene Epochs . An interesting fact about the geologic time scale is that most of it was developed long before the discovery of radioactivity – a discovery that subsequently allowed geoscientists to add absolute dates to all the boundaries largely defined initially by fossils. This lab is being used and was modified with permission from Gary Jacobson of Grossmont Community College.

Principles of Relative Dating A few foundational principles make distinguishing older from younger events relatively simple. Most of these make use of the common sense fact that if event “A” does something to event “B”, then event “B” must have already been in existence and therefore must be older. Below are six principles to help you order geologic events in time. Superposition : Rocks deposited on the earth’s surface form layers that are older on the bottom and younger on top. Thus, in Figure 1, layer E is the oldest and A is the youngest. This is true for undisturbed sedimentary and extrusive igneous rocks. Superposition does not apply to intrusive igneous rocks and rocks that have been overturned by folding or displaced by faults. Note that Sill F in Figure 2 would actually be younger than layers A, B and C, which lie above it. This is assuming Figure 1 is the starting condition for Figure 2. Original Horizontality : Rocks on the earth’s surface are originally deposited in essentially horizontal layers (Figure 1). Therefore non-horizontal rocks indicate that some younger event has disturbed their original horizontality. In Figure 2, folding would be younger than layers A-E, but not necessarily younger than Sill F, because intrusive igneous rocks do not need to be originally horizontal. Original Continuity: Rocks deposited on the earth’s surface form layers that continue laterally in all directions until they thin out as a result of non-deposition, or until they reach the edge of the basin in which they are deposited. Intrusive igneous bodies such as dikes, sills and laccoliths also have a degree of original continuity, but they may terminate by tapering-out between the rocks that enclose them (note Sill F in Figure 2). Also, rocks that appear tilted or folded (Figure 2) indicate a tectonic or folding event has occurred and the event is younger than the rocks themselves.

Figure 1: Superposition Figure 2: Original Horizontality and Continuity Cross-cutting Relations: Geological features are younger than the features they cut. The rule applies to intrusive igneous bodies, faults and erosion surfaces. Thus the erosion surface in Figure 3 is younger than the units it cuts. When a molten rock (magma) pushes through (intrudes) a body of rocks, the resulting igneous rocks must be younger than those rocks which were intruded. Sill F (Figure 2) must be younger than the units above and below it. Figure 3: Surface Erosion Figure 4: Angular Unconformity Unconformities: If a surface of erosion becomes buried, as G has done in Figure 4, then the feature is called an unconformity . An unconformity is a break in time. They can occur for a variety of reasons, but they always result from an interruption in sedimentation. There are three types of unconformities: 1. If the layers below the unconformity are non-parallel to the erosion surface, the structure is called an angular unconformity (Figure 4). 2. If the layers below the unconformity are parallel to the unconformity but there is a break in time or an erosional surface, the structure is called a disconformity (Figure 5). In figure 5 we know that there is an unconformity above unit C because Dike x stops at the top of unit C. Dikes and sills don’t normally stop intruding exactly at a contact between two units. Because of this we can interpret the contact between units C and G to be an erosional surface or unconformity, more specifically a disconformity. 3. A nonconformity overlies metamorphic or plutonic igneous rocks (Figure 6). In other words any place where sedimentary rocks come in contact with crystalline rocks (metamorphic or igneous).