Transcript Slide 1
Stratigraphy The study of rock layers (strata) Learning Objectives: • To understand the terms; principle of original horizontality, cross cutting relationships, principle of uniformitarianism, principle of included fragments and principle of superposition. • To be able to determine the relative age of strata from geologic diagrams or photos using the above principles. Relative Dating: Absolute Dating: Volcanic Eruptions 550 BC and 1300 AD The Principle of Uniformitarianism • The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. • In other words: The past history of the Earth must be explained by what we can see happening today. • Or: “the present is the key to the past” The Concepts of Original Horizontality • The principle of original Horizontality states that the deposition of sediments occurs as horizontal beds. • Observation of modern marine and nonmarine sediments in a wide variety of environments supports this generalisation. The Principle of Superposition • The principle of superposition states that sedimentary rock forms in an undisturbed sequence of layers known as strata. • This means that each stratum (layer) is younger than the strata (layers) beneath it and older than the strata (layers) above it. • Logically a younger layer cannot slip beneath a layer previously deposited. • This principle allows sedimentary layers to be viewed as a form of vertical time line. The Principle of Superposition Stratigraphic Colum Diagram The Principle of Inclusions • The principle of inclusions states that any rock fragment included in another rock must be older than the surrounding rock. • These rock fragments may be included when strata are folded or subject to faulting. The Principle of Cross-cutting Relationships the principle of cross-cutting relationships states that a geologic feature which cuts through another is the younger of the two features. The Principle of Unconformities the principle of Unconformities states that a break in the rock record, may have been due to erosion or lack of deposition So what do they use all this for? Relative Dating: Is the science determining the relative order of past events, without necessarily determining their absolute age. In other words: Geologists can work out rough dates for a rock sequence by considering factors such as; •the average rate of deposition of the different rocks •their composition •the width and extent of the strata •the fossils contained •the periods of uplift and erosion These sequences are then correlated to those of similar age in other regions with the ultimate aim of establishing a consistent geochronology for the entire Earth THE END Answers Q3) (a) (b) Mainly Cross-cutting relationships but maybe also original horizontality and superposition. Answers Q1) What does the term strata mean? • Are beds or layers of sedimentary rock. Answers Q2) What does the term relative dating mean? • Relative dating is determining the relative order of past events, without necessarily determining their absolute age. Criteria – Note to Teacher • 1.7 Rock exposures contain evidence of the sequence of geological events that formed and deformed them. Knowledge and understanding Techniques and skills Geological events are dated and interpreted using: • the principle of Uniformitarianism (the present is the key to the past) • the concepts of original horizontality, lateral continuity and superposition of strata • the dating of rocks on the basis of included fragments, cross cutting relationships (These have been achieved through the PowerPoint and matching activity) • Apply these stratigraphic principles of relative dating to interpret the evidence in rock exposures in the field or in diagrams, photographs, maps or cross sections for the sequence of geological events that formed/deformed them (These have been achieved through the questions worksheet)