1The Rock Record and Geologic Time: Studying Earth History• Overview of geologic time• Rates of geologic change (and “deep time”)• Dating of geologic events1• Geologic principles used in relative dating• Fossils for dating and correlation• Radiometric other types of numerical dating• The Geologic Time Scale • Some events in earth historyGeologic Time and the Age of the UniverseGeologic Time, the vast span of time over which earth processes have occurred, includes more than 4 billion years!2Illustration of the enormous scale of geologic time3Illustration of the enormous scale of geologic time - continued42The Rock Record and Geologic Time: Studying Earth HistoryTime scales of geologic processesDays to Months to YearsHuman lifetime5Thousands of yearsMillions of yearsBillions of yearsA single eruption may be short in time, but the lifetime of an entire volcano may be thousands to millions of years.Understanding Earth6Glaciers move slowly in human terms. Their movements are almost imperceptible from one day to the next, but given enough time they can carve great valleys and strip away entire mountain ranges. 7Understanding EarthThe rock layers of the Grand Canyon record a great deal of geologic time. The granite, schist, and gneiss found at the bottom of the canyon and are more than 1 billion years old. These are overlain by successively younger sedimentary rocks. The Kaibab limestone at the top of the canyon is about 250 million years old.Geologic History in the Grand Canyon, ArizonaSeveral unconformities are present and8are present, and the time they represent exceeds the time recorded by the rocks.Understanding Earth3Cross-section of part of the Grand Canyon with rock agesPlant fossilsAnimal tracks9Fig 8-13 Chernicoff, Geology 2nd editionMarine fossilsTrilobitesDevelopment of Geological ThoughtApproaches to Interpreting the rock record – continued• Catastrophism• Proposed by Baron Georges Cuvier, a French zoologist• Explained the both the geologic history and the biologic history of the Earth through a series of sudden, widespread catastrophesI l d d i j t t hi t•Included six major catastrophic events• Each catastrophe produced major changes in a short period of time• Compatible with a young age for the Earth• Geologic evidence failed to support the idea – For example, it was recognized that many more than six catastrophes were needed to explain observed rocksDevelopment of Geological ThoughtApproaches to Interpreting the rock record – continued• Principle of Uniformitarianism• A central concept of modern geology• In the 1700’s, James Hutton proposed the basic premise: present-day processes have occurred throughout geologic time and can be used to explain Earth history•Requires a very old Earthequ es a ve y o d a t• Hutton saw geologic processes as operating in cycles: a mountain range could be worn away, its material could be deposited elsewhere, and those sediments could be pushed upward to form another mountain range• On the basis of his field observations and the experiments of others, Hutton recognized that igneous rocks were formed by cooling of molten rock, an idea known as plutonism• Hutton’s ideas were popularized largely by Charles Lyell who published a book, Principles of Geology, in 1830Development of Geological ThoughtModern concept of Uniformitarianism• Allows for the rates, intensities, and extents of processes to vary over time• e.g. volcanism• e.g. ice ages• Allows for large, catastrophic events as well as slow change•volcanic eruptions, large storms, meteorite impacts, etc. arevolcanic eruptions, large storms, meteorite impacts, etc. are seen as normal processes• rivers, for example, shape the Earth through day-to-day sediment transport and erosion as well as the occasional flood• Used to interpret the rock record by applying our understanding of processes that we can observe today• Used to predict future geologic events (e.g. earthquakes, opening/closing of oceans, building/wearing away of mountains)4Dating Rocks and Geologic EventsHow do we date rocks? What principles and methods are used todetermine the ages of earth features? Geologists use two types of ages to describe rocks:Relative ages13• Answer questions like: - What came first? What came second?• Involves putting rocks and geologic events in the order or sequence in which they occurredNumerical ages (also called “absolute” or “specific” ages) • Answer questions like: - When did it happen? How old is it?• Involves assigning an age which indicates a number of yearsagoThe Rock Record and Geologic Time: Studying Earth HistoryPrinciples used for determining relative ages:Superposition14Original HorizontalityCross-Cutting RelationshipsInclusionsLateral continuity15Superposition:Sedimentary and volcanic rock layers are deposited with the oldest rocks at the bottom and the youngest rocks at the top. Geology - ChernicoffOriginal Horizontality:Most sedimentary rock units are deposited as horizontal or nearly horizontal layers. When they are found steeply tilted or folded, as in this lhili16example, the tilting must have happened after the rocks were formed. Understanding Earth5Cross-cutting relations:Faults, igneous intrusions, and erosion surfaces which cut across other rocks must have been formed after the rocks which they cut across.17Upper: In this example, the sedimentary rocks were formed first. The dike was intruded later. The erosion which formed the valley happened even later.(from Fig 4.16 Understanding Earth)Lower: A fault cuts through older sedimentary rocks(from Fig 2.3 Wicander and Monroe, Historical Geology)The Rock Record and Geologic Time: Studying Earth HistoryUnconformities: Surfaces of erosion or non-deposition which represent i i d f th hi t h lik i i18missing records of earth history much like pages missing from a bookUnconformities are grouped into three categories (angular unconformity, disconformity, and non-conformity) according to the relationship between the rocks above and below the erosion surface.Angular unconformity in the Grand CanyonNote the horizontal layers above and the tilted layers below. The rocks above the unconformity are more than 200 million years younger than the rocks below.Fig 9.7 Understanding Earth19Layer 4Layer 3Unconformity 2• Which fault came first?• What is the first event overall?• What are the last two events overall?Deciphering