Chapter 8: Geologic Time 1. The Art of Time 2. The History of Relative Time 3. Geologic Time 4. Numerical Time 5. Rates of Change Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Geologic Time How long has this landscape looked like this? How can you tell? Will your grandchildren see this if they hike here in 80 years? The Good Earth/Chapter 8: Geologic Time The Art of Time How would you create a piece of art to illustrate the passage of time? How do you think the Earth itself illustrates the passage of time? What time scale is illustrated in the example above? How well does this relate to geological time/geological forces? The Good Earth/Chapter 8: Geologic Time Go back to the Table of Contents Go to the next section: The History of (Relative) Time The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time Paradigm shift: 17th century – science was a baby and geology as a discipline did not exist. Today, hypothesis testing method supports a geologic (scientific) age for the earth as opposed to a biblical age. Structures such as the oldest Egyptian pyramids (2650-2150 B.C.) and the Great Wall of China (688 B.C.) fall within a historical timeline that humans can relate to, while geological events may seem to have happened before time existed! The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time • Relative Time = which A came first, second… − Grand Canyon – B excellent model − Which do you think happened first – the The Grand Canyon – rock layers record thousands of millions of years of geologic history. deposition of the rocks, or the cutting through of those rocks by the river. Why? At which location on the picture above, A or B, are the rocks younger? The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time Red rock units Tan rock units -More complicated histories are represented by multiple events. Important principles: -Above: Explain the history of the Superposition – rocks at the bottom are rocks using the events deposition, the oldest. erosion, and tilting. Refer to the Cross-cutting relationships – older rocks may be cut by younger rocks or features. diagram at left for help. (Hint: there are 4 major events) Inclusion – Younger rocks may incorporate pieces of older rocks. The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time 18th century - James Hutton watched the landscape of his farmland and invented our modern concept of geologic time. Observation: The landscape remained unchanged with the passage of time. Deductions: 1) The same slow-acting geological processes that operate today have operated in the past, meaning it takes a long time to influence the Earth’s surface significantly (Uniformitarianism). 2) All land should be worn flat (erosion) unless some process renews the landscape by forming new mountains (cyclical change). - he called these eroded surfaces, representing gaps in time, unconformities. Controversial resulting message – Earth must be much older than the commonly accepted age of 6,000 years. The Good Earth/Chapter 8: Geologic Time Checkpoint 8.2 Examine the following image of rock layers and answer Questions 1 and 2 about relative time. A E 1. Which statement is most accurate? C a. D is older than B F b. E is older than A D B c. F is older than C The Good Earth/Chapter 8: Geologic Time Checkpoint 8.2 Examine the following image of rock layers and answer Questions 1 and 2 about relative time. 2. When did the tilting of the A E layers occur? a. After A was deposited C F b. Between deposition of layers E and A D B c. Before B was deposited d. Between deposition of layers C and E The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time Grand Canyon Rock Sequence: -Rocks at base are older than rocks at top (superposition). -Examine lowest units – which is older, the schist or the granite? Why? -Schist – metamorphic – thought to have been the root of an ancient mountain belt or volcanic arc. How did the schist/granite get exposed at the surface? The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time Sandstone, shale, limestone progression indicative of passive margin (rising sea level). The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time How can we tell that the volcanism is younger than formation of the sedimentary rocks? The Good Earth/Chapter 8: Geologic Time Checkpoint 8.6 The Power of Fossils Geologists can correlate sedimentary rocks by comparing the fossils found within the rocks Fossils found in many rock layers (long lived species) are difficult to match to layers in other regions. Index fossils: species that existed for a relatively short period of geologic time and found over large geographic areas are the best for precise correlations. Which of the fossils in the diagram at left (1,2, or 3) would make the best index fossil? Why? The Good Earth/Chapter 8: Geologic Time The History of (Relative) Time Fossils of the Grand Canyon support the geologic interpretations Although they do not preserve the body of an organism, tracks are important trace fossils that tell us something about the organisms that left them behind. The Good Earth/Chapter 8: Geologic Time Go back to the Table of Contents Go to the next section: Geologic Time The Good Earth/Chapter 8: Geologic Time Geologic Time (new life) (middle life) (ancient life) Fossils are rare in pre- Proterozoic = “earlier life” Phanerozoic rocks. Phanerozoic = “life revealed” The Good Earth/Chapter 8: Geologic Time Geologic Time Cambrian explosion (542-488 Ma) Explosion of organisms with hard skeletons at beginning of Cambrian Q: Why does this matter? A: Hard parts can be easily preserved as fossils. The Good Earth/Chapter 8: Geologic Time Geologic Time Checkpoint 8.10 Carefully examine the relative positions of the lettered arrows in the following diagram and answer the questions: 1) Which letter corresponds most closely to the first appearance in the rock record of abundant fossils? 2) Which letter corresponds most closely to the extinction of the dinosaurs? The Good Earth/Chapter 8: Geologic Time Geologic Time “The majority of all species that have lived on Earth are now extinct” All major phyla were derived by the Cambrian. The diversity of organisms has increased through time. Q: How would an extinction affect biodiversity? A: Biodiversity decreases after a major extinction event. The Good Earth/Chapter 8: Geologic Time Geologic Time Look at the graph – do you see any patterns? Do they make sense? The Good Earth/Chapter 8: Geologic Time Geologic Time Mass extinctions = events in which large numbers of species die. Fossils found in rocks deposited before a mass extinction event are substantially different from those found in rocks from after the event. The extinct mastodon, a smaller cousin of the mammoth. The Good Earth/Chapter 8: Geologic Time Geologic Time Major mass extinctions throughout geologic history Cretaceous-Tertiary (K-T) extinction (~ 65 Ma): No dinosaurs (except perhaps birds) survived the event. Mammals were able to expand and become the dominant group. WHY? The cause is believed to be impact on earth by a large comet/asteroid. In all, about 75% of all species were destroyed. Permian-Triassic (P-T) extinction (~ 251 Ma): Killed off ~96% of marine species and 70% of land species. Often called “the great dying.” Studying extinction events can shed light on 1) the cause of the extinction, and 2) the response of different types of organisms to such events. Why might some species go extinct while others don’t? The Good Earth/Chapter 8: Geologic Time Geologic Time Concept Survey How might each of the following events have contributed to global changes that could have caused extinctions? A. Assembly of the supercontinent Pangea and creation of a single worldwide ocean B. Thousands of volcanic eruptions over a period of one million years in northeastern Russia C. An impact event (comet/asteroid) The Good Earth/Chapter 8: Geologic Time Go back to the Table of Contents Go to the next section: Numerical Time The Good Earth/Chapter 8: Geologic Time Numerical Time Early methods for determining the age of the Earth were flawed: yielded ages too recent Salinity of oceans – salt delivered to oceans from the continents through streams. Mass of salt in oceans/amount of salt contributed to oceans each year by streams = age of Earth. Age estimate = ~100 million years old. -Flaw – did not take into account the formation of chemical sedimentary rocks which removes salt from the oceans. Conductive cooling of earth – knowing Earth’s volume and properties of rocks, can calculate how long it would take for earth to cool from molten state to present state. Age estimate = ~100 million years old. - Flaws – did not yet know about radioactive decay and the resulting contribution of heat. Nor was the theory of plate tectonics yet proposed, and calculations were made assuming heat was diffused uniformly across the earth’s surface. The Good Earth/Chapter 8: Geologic Time Numerical Time Checkpoint 8.13 Between 1860 and 1920 geologists attempted to estimate the Earth’s age by how long it would take for the thickest sequences of sedimentary rocks to form. Geologists examined sequences of rocks for each geologic period. From the estimated rates for the formation of these units, different scientists estimated ages for Earth ranging from 3 million years to 15 billion years. Explain why these estimates varied over such a wide range. The Good Earth/Chapter 8: Geologic Time Numerical Time Unstable isotopes held the key to the numerical age of the Earth! Isotopes – atoms of the same element with different numbers of neutrons.