**Relative Dating** **Lab Objective:** To interpret cross-sectional diagrams, and to use logic and basic principles to order geologic events. **Introduction:** Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists had no way of determining the exact age of Earth materials. Relative dating techniques, however, have been used for hundreds of years and are instrumental in determining the relative order of past events (i.e., the age of one object in comparison to another), without necessarily determining their exact (or even estimated) age. Even though relative dating methods can only determine the sequential order in which a series of events occurred, they remain incredibly useful in understanding Earth processes. The activity below will help you practice putting geologic events in chronological order. Are you likely to use this skill in some future career? Probably not (unless you become a geologist), but that is not the point. It is about coming to logical conclusions, and defending those conclusions with observations and evidence. Good luck. **Methods:** Use geologic principles to determine the sequence of events for each cross-sectional diagram provided. **Step 1:** Use the numbers to the right of each diagram to list each of the events as they occurred (with 1 being the oldest/first thing to occur). **Step 2:** Justify/defend your interpretation by listing the geologic principle that helped you arrive at that conclusion in the space provided. **Tips on How to Determine the Sequence of Events:** 1. Visually locate all unconformities (periods of erosion) and note the rocks that they separate. 2. Visually identify all sedimentary layers. 3. Visually identify all intrusive igneous rocks and faults. Identify other cross-cutting relationships such as tilting, folding, or other deformations. 4. Try starting with the oldest and deepest features you can identify and work your way up through the younger features closer to the surface. When you get stuck, approach the problem from the other direction, identifying the youngest features, and work your way back through time. Hopefully, these two techniques will help you decipher the diagrams. **Example:** - The diagram includes a cross-section with various geological features labeled in order: - **#1: Layers** - Principle: Superposition - **#2: Magma Intrusion** - Principle: Cross-Cutting - **#3: Erosion **Cross-section #1:** This diagram illustrates different layers of rock strata that are interrupted by a diagonal fault line. The layers are horizontal with alternating colors: - Top layer: Light gray - Second layer: Dark blue - Third layer: Light gray with line patterns - Fourth layer: Dark red - Bottom layer: Brick red **Event and Principle:** - **#1:** - **#2:** - **#3:** **Cross-section #2:** This section displays a similar sequence of horizontal strata, featuring an intrusion that cuts through the layers. The intrusion is irregular and filled with a textured pattern to signify a different material (likely magma that later solidified). - Top layer: Light gray - Second layer: Dark blue - Third layer: Light gray with line patterns - Fourth layer: Dark red - Bottom layer: Brick red - Intrusion: Light gray with dotted texture **Event and Principle:** - **#1:** - **#2:** **Cross-section #3:** Here, the diagram shows inclined layers with some faulting. The layers are depicted in angled orientations indicative of geological tilting and deformation. - Top layer: Light gray - Second layer: Dark red - Third layer: Brick red with a stippled pattern - Fourth layer: Blue stripes - Fifth layer: Light gray - Sixth layer: Dark blue **Event and Principle:** - **#1:** - **#2:** - **#3:** - **#4:** These cross-sections are useful for studying principles of relative dating, where the sequence of geological events is interpreted and analyzed.