Basic Geologic Principles * Principle of Superposition Principle of Original Horizontality Principle of Lateral Continuity Principle of Crosscutting Relationships (*Notes) Principle of Superposition * Younger sedimentary and volcanic rocks are deposited on top of older rocks. The principle of superposition states that in an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become progressively younger towards the top. Exception: overturned rock layers due to folding. (*Notes) Principle of Superposition * Relative Dating Relative dating is used in geology to determine the order of events and the relative age of rocks by examining their position in a sequence. (*Notes) Principle of Superposition OLDER YOUNGER 270 Million Years Old 600+ Million Years Old TIMING: RELATIVE vs. ABSOLUTE Geologic Time 903/es2903page03.cfm?chapter_no=investigation Principle of Original Horizontality * Sedimentary and volcanic rocks are laid down in approximately horizontal layers. This principle is important in the analysis of folded and tilted strata. (*Notes) Originally Horizontal Layers on top of layers Folding & Tilting The Flatirons Boulder, CO Folding & Tilting The Atlas Mountain system of northwest Africa nvestigations/es2903/es2903page04.cfmnvestigations/es2903/es2903page05.cfm?chapter_no=investi gation Folding & Tilting Principle of Lateral Continuity * Sedimentary and volcanic rocks are laid down in layers that are usually much greater in lateral extent than in thickness. -Layers of sediment initially extend laterally in all directions (laterally continuous). -Rocks that are otherwise similar, but are now separated by a valley or other erosion feature, can be assumed to be originally continuous. (*Notes) Principle of Lateral Continuity Principle of Crosscutting Relationships * If a rock unit or geologic feature cuts across another rock unit or geologic feature, it was formed later in geologic time. The principle of cross-cutting relationships states that a rock or fault is younger than any rock (or fault) through which it cuts. (*Notes) Principle of Crosscutting Relationships In a series of horizontal sedimentary beds, there is an igneous dike which cuts vertically through them. The dike is younger than the sediment beds though which it crosses, as the beds would have had to be around before the dike could have intruded. Principle of Crosscutting Relationships Cross Cutting Grand Canyon, Arizona Cross Cutting MOST RECENT Unconformity * A layer of rock is a record of past events. But most rock records are incomplete- there are layers missing. These gaps are called unconformities. Unconformities develop when agents of erosion remove existing rock layers. They also form when a period of time passes without any new deposition occurring to form new layers of rock. (*Notes) Unconformity Unconformities Animation In Figure 2, there are 8 different rock layers seen in this exposure. Layers 1- 3 are horizontal while layers 4-8 are tilted. The contact between the horizontal layers and the lower tilted layers indicates an unconformity where the lower, older layers got tilted, then eroded down to a horizontal surface, followed by deposition of layers 1-3. Processes such a faulting, perhaps associated with mountain building, can create this kind of tilting and unconformity on Earth. Unconformity Unconformities Animation II Unconformity Olympic Coast, Washington: 4th Beach near Kalaloch Basic Geologic Principles