Name: GEOLOGIC HISTORY This lab is being used and was modified with permission from Gary Jacobson of Grossmont Community College. INTRODUCTION: One of the most fundamental techniques in geology is dating rocks and putting geologic events in their proper sequence. There are two ways by which this can be accomplished. Absolute dating involves being able to date how many years old a rock is. Although this may sound simple, determining the numerical age of a rock is difficult to accomplish because several criteria must be met. Radiometric dating is the most commonly used way to absolute date a rock. The method is limited because you can only date a rock which has formed directly from cooling magma or a recrystallized metamorphic rock. However, even if you cannot tell the precise age of a 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. 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 be older. 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 reverse 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 AE, but not necessarily younger than Sill F, because intrusive igneous rocks do not need to be originally horizontal. 1 Figure 2: Original Horizontality and Original Continuity Figure 1: Superposition 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 taperingout 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. 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. When an earthquake breaks a group of rocks, a fault forms. More on faults later. 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. 2 1. If the layers below the unconformity are non-parallel to the erosion surface, the structure is called an angular unconformity (Figure 4). Figure 5: Disconformity Figure 6: Nonconformity 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 is 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). Law of Inclusions: Inclusions
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