Geologic Time and the Age of the Universe. Illustration of the enormous scale of geologic time

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1 The Rock Record and Geologic Time: Studying Earth History 1 Geologic Time and the Age of the Universe Geologic Time, the vast span of time over which earth processes have occurred, includes more than 4 billion years! 2 Overview of geologic time & the Geologic Time Scale Rates of geologic change (and deep time ) Dating of geologic events Geologic principles used in relative dating Fossils for dating and correlation Radiometric other types of numerical dating Extinction of the dinosaurs Related news, discoveries, and other information (updated often by Dr. Kuehn): Illustration of the enormous scale of geologic time 3 Illustration of the enormous scale of geologic time - continued 4 The Geologic time scale This time scale originally was constructed without knowing the exact ages of the boundaries, only the sequence of rock formations. The divisions are defined based on rocks and fossils found at many locations around the world. Ages in years were added later when it became possible to date many rocks using natural radioactivity. Geologic Time Scale Eras of Time Since the Earth s Beginning March 1913: The first complete geologic timescale is published Not to scale 5 Illustration modified from Fig 9.13 The Rock Record and Geologic Time: Studying Earth History Time scales of geologic processes Days to Months to Years Human lifetime Thousands of years Millions of years Billions of years 6 1

2 7 A single eruption may be short in time, but the lifetime of an entire volcano may be thousands to millions of years. Glaciers 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. 8 9 Geologic History in the Grand Canyon, Arizona The 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. Cross-section of part of the Grand Canyon with rock ages 10 Several unconformities are present, including the 1.2 billion-year Great Unconformity. At the Grand Canyon, the time represented by unconformities exceeds the time recorded by the rocks. The Grand Canyon's 1.2 Billion Year Old Secret Marine fossils Animal tracks Trilobites Plant fossils Fig 8-13 Chernicoff, Geology 2nd edition Development of Geological Thought Approaches 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 catastrophes 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 rocks Development of Geological Thought Approaches 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: presentday processes have occurred throughout geologic time and can be used to explain Earth history Requires a very old Earth 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

3 Development of Geological Thought Modern 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. 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) Dating Rocks and Geologic Events How do we date rocks? What principles and methods are used to determine the ages of earth features? Geologists use two types of ages to describe rocks: Relative ages Answer questions like: - What came first? What came second? Involves putting rocks and geologic events in the order or sequence in which they occurred Numerical 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 years ago The Rock Record and Geologic Time: Studying Earth History Principles used for determining relative ages: Superposition Original Horizontality Cross-Cutting Relationships Inclusions Lateral continuity Superposition: Sedimentary and volcanic rock layers are deposited with the oldest rocks at the bottom and the youngest rocks at the top. Geology - Chernicoff Original Horizontality: Most sedimentary rock units are deposited as horizontal or nearly horizontal layers. When they are found steeply tilted or folded, as in this example, the tilting must have happened after the rocks were formed. 17 Cross-cutting relations: Faults, igneous intrusions, and erosion surfaces which cut across other rocks must have been formed after the rocks which they cut across. 18 Upper: 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 ) Lower: A fault cuts through older sedimentary rocks (from Fig 2.3 Wicander and Monroe, Historical Geology) 3

4 The Rock Record and Geologic Time: Studying Earth History Angular unconformity in the Grand Canyon Note 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 Unconformities: Surfaces of erosion or non-deposition which represent missing records of earth history much like pages missing from a book Unconformities are grouped into three categories (angular unconformity, disconformity, and non-conformity) according to the relationship between the rocks above and below the erosion surface. Deciphering the sequence of events Layer 4 Which fault came first? What is the first event overall? What are the last two events overall? 21 Unconformity 2 What is the sequence of events? Fig Put the following in order: folding deposition of sedimentary layers (A-J) intrusion of pluton and dikes fault movement mystery event: Layer 3 Layer 2 Layer 1 Igneous intrusion (dike and sill) Methods of Numerical Dating Numerical dates can be obtained in a variety of ways, counting tree rings or measuring lichens, for example. The most important source of numerical ages is radiometric dating. Radiometric dating A type of numerical dating that which relies upon the breakdown of unstable (radioactive) isotopes of certain elements and the formation of other isotopes in their place. Examples include: Carbon-14 dating Uranium-lead dating Potassium-argon dating (and related argon-argon dating) Fission track dating Cosmogenic or exposure dating How do scientists determine the age of dinosaur bones? 23 The Acasta Gneiss - Northwest Territories, Canada among the oldest earth rocks discovered so far: 4 billion years old The age comes from radiometric dating using isotopes of uranium and lead and probably represents the timing of the metamorphism. Fig 8-23 Geology 2nd ed Chernicoff 24 4

5 The age of the Earth and solar system is believed to be about the same as the age of the oldest meteorites. Example: the ~4.6 billion year old Hoba meteorite found in Namibia, Africa. 25 The Periodic Table groups elements by similarities in their chemical properties. Most of the known, naturally-occurring elements are present only in trace amounts. Strong tendency to lose outermost electrons Atomic number Chemical symbol Tend to share, gain, or lose electrons Strong tendency to gain electrons 26 Inert (noble) gases Transition elements Lanthanides Fig 8-37 Geology 2nd ed Chernicoff Actinides See: Hydrogen and carbon atoms Modified from Fig Isotopes of Hydrogen Modified from Fig Atomic number = 1 Atomic weight = 2 Atomic number = 1 Atomic number = 1 Atomic weight = 1 Atomic number = 6 Atomic number = 1 Atomic weight = 3 Stable Isotopes of carbon Fig Rate and Progress of Radioactive Decay 30 Stable Unstable (Radioactive) S. Kuehn 5

6 Age = number of half lives elapsed x number of years per half life The amount of time that it takes for half of the radioactive isotope to decay depends on the isotope. For 14 C, the half life is 5,730 years. For 238 U, it is 4.5 billion years. 31 If 12.5% of the parent remains, how many half lives have elapsed? If one half life is 50,000 years, how old is the rock? 32 S. Kuehn Example: 2 half lives x 5,730 years/half life = 11,460 years S. Kuehn Generation of new carbon-14 occurs in the atmosphere. It is then absorbed into plants and animals. The proportion of radioactive carbon in living organisms remains constant during their life. Over time, carbon-14 decays to nitrogen-14. Fig 8-24 Geology 2nd ed Chernicoff Other Numerical Dating Methods Dendrochronology - use of tree rings for dating 35 one varve { 36 Fig Other Numerical Dating Methods Yearly layers: Lakes (varves) Glaciers Fig 8-27 Geology 2nd ed Chernicoff Layers in Greenland GISP2 Ice core - Wikipedia 6

7 Fossils and the History of Life Fossils are remnants of ancient life Fossils include: preserved organisms (body fossils like shells and bones) evidence of biological activity (trace fossils like burrows and tracks) Fossils record the development of life on earth: the earliest forms of life were much simpler than those which came later numerous new species have evolved from earlier ones many species have gone extinct, leaving only remnants The ages of rock units from different places can determined by comparing fossils which they may contain Geologists who study fossils and the history of ancient life are called paleontologists 150 million year old fossil bones at Dinosaur National Monument, Utah Fig 8-1 Geology 2nd ed Chernicoff Fossils - ammonite (left) and petrified wood (right) 39 Trilobite, Fish, and Turtle 40 Via John Moffett on Twitter Well-preserved amphibian Fig 9.4 Many different organisms have lived and died during geologic time. Rocks from different periods of time are characterized by fossils of different organisms Fig 7.3 NAGT lab manual Fossils record the development of new organisms and changes over time. Fossils can also be used to trace the ancestry of modern plants and animals. 7

8 Fossils in rocks record changes in the diversity of life through geologic time. Fossils record not just the development of new types of organisms (evolution), but also the loss of species (extinction). More than once during Earth history, large numbers of species. have been lost over relatively short periods of time. Such events are called mass extinctions. Many scientists consider the Earth to be in the middle of another mass-extinction today, an extinction resulting from human activities. The greatest extinction in Earth history occurred about 250 million years ago. A more famous mass extinction occurred about 65 million years ago and marks the end of the dinosaurs. Fig Extinction of the Dinosaurs (and lots of other stuff too) about 65 million years ago What caused this mass extinction? Two of the leading candidates: - massive volcanic activity (fits better with a slower extinction over hundreds of thousands to millions of years) - meteor/asteroid collision (fits better with rapid extinction) We know that both of these things happened at the time, but which was more important? Why did some organisms survive whereas others went extinct? 44 Death due to lava? (from the related climate effects) Massive lava flows are preserved in the Deccan Traps of India and on the floor of the Indian Ocean. These were erupted during the time of the K-T extinction. The Deccan Traps are one of the largest volcanic provinces in the world, covering an area roughly the size of the states of Washington and Oregon combined. The preserved lava is only a remnant of what was once an even larger volcanic province Death by meteor/asteroid? Photograph by Lazlo Keszthelyi 65 million year old iridium-bearing sediment layer at Bubbio, Italy One possible clue to what may have caused the extinction of the dinosaurs. Chapter 23 Tektites from Thailand Similar glass spheres have been found around the world in 65 million year old sediments. These probably were formed when rock melted by meteor impact was thrown into the air. Fig 1-5b Geology 2nd ed Chernicoff Gravity map of the Chicxulub impact crater Yucatan Peninsula, Mexico This crater may have been produced by a large meteor impact at the end of the Cretaceous Period Mineral grain shattered by meteorite impact - an example of shock metamorphism An impact crater on the Moon for comparison Fig 1-5c Geology 2nd ed Chernicoff 8

9 More to explore: 49 More to explore: 50 What Killed The Dinosaurs? The K-T Extinction Dinosaurs: Facts and Fiction - USGS Origin of Birds Descendents of the Dinosaurs? Winged dinosaurs and the origin of birds U.C. Berkeley Museum of Paleontology Geologic Time - U.S. Geological Survey Fossils, Rocks, and Time - USGS Dinosaur National Monument (home page) (geology) (virtual museum) More to explore: 51 Advances in understanding how evolution works (examples) Evolution and the Fossil Record - AGI & The Paleontological Society (Very Good!) Understanding Evolution, a resource for K-12 teachers Frequently Asked Questions About Evolution PBS The Geological Evolution of the Earth (rocks) 9