Chapter 8

Construct a diagram that illustrates a cross section of six rock units that would account for the features listed below (not in order). Clearly label your units. Remember, these events are not in order-you must determine the order of events based on the descriptions. a) Rhyolite cross-cuts and covers all units except sandstone. b) Dark, fine-grained igneous rock cross-cuts and covers conglomerate and older units. c) Oldest rocks are made of black, biochemical layers that were later tilted. d) Coarse-grained clastic rock is deposited immediately over coal. e) Opaque chemical sedimentary rock is deposited directly over basalt. f) River cuts partially into limestone. g) Medium-grained clastic rock is deposited over small-grained, high-silica extrusive rock.

Geologists look for similar rock types or fossils to tell them that geologic environments were similar between two widely spaced locations. What are some examples of modern environments that have characteristic types of plants and animals?

Outcrops of rock are examined in four different locations in a state. The rock types and the fossils they contain are illustrated in the following diagram. Which fossil would be the best choice to use as an index fossil for these rocks? Which fossil is least characteristic of a specific set of geologic conditions? a) Fossil 1 b) Fossil 2 c) Fossil 3 \(\therefore\) Sandstone A Shale A Shale B Sandstone B \(\square\) Limestone B Coal Gneiss \(\quad\) Limestone C Fossil 1 (C) Fossil 2 Fossil 3

The debate whether dinosaurs went extinct due to a large space rock that struck the Earth \(65.5\) million years ago (MYA) may have been answered with the discovery of a distinctive brow horn from a Ceratopsian dinosaur just 13 centimeters (5.1 inches) below the K-T boundary-the distinct layer of geological sediments separating the Cretaceous and Tertiary periods. Rocks laid down \(65.5\) MYA show a thin layer abundant in rare elements like Iridium, spherules and shocked Quartz that could only have come from a meteorite impact. Since no fossils have ever been found in sediments above the \(\mathrm{K}-\mathrm{T}\) boundary, conventional wisdom has it that the end of dinosaurs came with an asteroid impact that caused firestorms, acid rain and a nuclear winter that blotted out the Sun. But that theory had a hole in it. The fossil record showed an apparent lack of dinosaur fossils in the last few million years leading up to the impact, suggesting that the "three meter gap" proves that dinosaurs went extinct long before the catastrophic impact. Scientists working in the Hell's Creek formation in the Montana badlands say they have resolved that dispute. Writing in the Royal Society journal Biology Letters, the paleontologists report on the new discovery of the closest dinosaur fossil ever found to the \(\mathrm{K}\)-T boundary. The ancient remains uncovered in Montana belong to the last known dinosaur to ever walk the planet and gives weight to the theory that dinosaurs were in fact wiped out by an asteroid impact. All other dinosaur fossils found are either much older, or were unearthed after being washed from their original graves into much younger sediments, long after they died. The fossil is most likely of an adult triceratops, a dinosaur growing up to 30 feet long and weighing up to 13 tons. The nearly 18 -inch fossilized brow horn was found just 5 inches below the \(\mathrm{K}-\mathrm{T}\) boundary. "This is the youngest dinosaur that has been discovered in situ. Others can be found in younger deposits, but those have been put there by geological processes and are actually much older" said Tyler Lyson, a paleontologist at Yale University. The discovery undermines the theory that gained ground in the \(1980 \mathrm{~s}\), that dinosaurs died out due to climate change or rising sea levels long before the planet was struck by a space rock. The theory carried some weight due to a lack of fossils found within the "three meter gap" of the \(\mathrm{K}-\mathrm{T}\) boundary. The finding "demonstrates that dinosaurs did not go extinct prior to the impact and that at least some dinosaurs were doing very well right up until we had the impact," Lyson told the Guardian. Gaps in the fossil record-which is patchy at best-are nothing new. Another, covering tens of millions of years, is clearly evident in the Hell's Creek formations some 60 meters ( 200 feet) below the K-T boundary. But because similar dinosaur fossils are found both above and below this gap it is assumed the absence of fossils has more to do with geological processes, or simply blind prospecting luck, than any extinction event and subsequent miraculous reintroduction. The "three meter gap" prior to the \(\mathrm{K}-\mathrm{T}\) boundary is unique because dinosaur fossils never reappear in the geological record. Dr. Paul Barrett of the Natural History Museum said the discovery was strong evidence that dinosaurs were killed off in North America by a catastrophic event, but the evidence is not conclusive globally. "It shows that in this part of the world dinosaurs were still viable and still roaming around at the time the meteorite hit. But what it doesn't tell us is what was going on in the rest of the world, and it could be that in other parts of the world dinosaurs were dying out at different rates and for different reasons because of other things going on at the time," he told BBC News. He argues that just one brow horn discovery doesn't resolve the dispute over dinosaur extinction. Source: Scientists Find Fossil Below K-T Boundary" by Lawrence LeBlond. RedOrbit.com, July 13, 2011. Reprinted by permission. http://www.redorbit.com/news/science/2078377/ scientists_find_fossil_below_kt_boundary/ 1\. What are the key observations mentioned in this article (select all that apply)? a) Dinosaurs were wiped out by a meteor impact b) A dinosaur fossil was found within a few centimeters of the KT boundary c) No in-situ, non-bird dinosaur fossils are found above the KT boundary d) Climate change played a major role in the extinction of dinosaurs 2\. What hypothesis is best supported from the observations? a) Dinosaurs across the planet were killed off by a meteor impact. b) Dinosaurs in North America were killed off by a meteor impact. c) Dinosaurs across the planet were not killed off by a meteor impact. d) Dinosaurs in North America were not killed off by a meteor impact.

Suppose that all of geologic time were proportional to the length of a football field ( 100 yards). Earth would have formed at the opposing team's goal line ( 100 yards), and the present day would represent the home team's goal line ( 0 yards). Metaphor equation Metaphor value \(=(\) years before present \(/\) age of Earth \() \times\) metaphor maximum Example Oldest fossil bacteria \(=3,500\) million years old Age of Earth \(=4,600\) million years Metaphor maximum \(=100\) yards Metaphor value \(=(3,500,000,000 / 4,600,000,000) \times 100=76\) yards Key metaphor dimensions 100 yards \(=4,600\) million years 10 yards \(=460\) million years 1 yard \(=46\) million years 1 foot \(=15.3\) million years 1 inch \(=1.3\) million years Calculate the yardage of the extinction at the end of the Paleozoic era. Then fill in the blank cell in the table and label the following figure. Develop your own metaphor for geologic time and describe it. Choose some of the most significant geologic events from the geologic timescale and convert them into your own metaphor equation. Don't try to be too detailed in your analysis. The intention here is to recognize the length of the geologic timescale and the relative positions of key events. Approximate lengths, distances, heights, widths, depths, sizes, time periods, and so on are okay as long as you recognize the relative proportions of the time intervals. \begin{tabular}{|l|l|l|} \hline Distance from home goal line & Time, million years & Event \\ \hline 76 yards & 3,500 & Oldest fossil bacteria \\ \hline 26 yards & 1,200 & Oldest known animal fossil (jellyfish) \\ \hline 12 yards & 542 & Hard skeletons become common (fossils) \\ \hline 10 yards & 458 & First land plants (mosses) \\ \hline & 251 & Widespread extinction ends Paleozoic era \\ \hline \(1.4\) yards & 66 & Dinosaurs become extinct \\ \hline \(0.00036\) inch & \(0.00051\) & Columbus landed, 1492 \\ \hline \end{tabular}

Radioactive isotopes in clastic sedimentary rocks always predict an age that is a) older than the sedimentary rock. b) younger than the sedimentary rock. c) correct for the sedimentary rock. The isotope of element X has 15 protons, 17 neutrons, and 15 electrons. The element has an atomic number of and a mass number of a) \(15 ; 32\) c) \(17 ; 47\) b) \(17 ; 15\) d) \(15 ; 30\) If radioactive decay began with 400,000 parent isotopes, how many would be left after three half-lives? a) 200,000 c) 50,000 b) 100,000 d) 25,000

The half-life of a radioactive isotope is 500 million years. Scientists testing a rock sample discover that the sample contains three times as many daughter atoms as parent isotopes. What is the age of the rock? a) 500 million years c) 1,000 million years b) 1,500 million years d) 2,500 million years

We daily encounter evidence of things that have changed over time. For example, an instructor finds a stick of chalk that has become too small to use, or a student finds that jeans have become so worn that a hole has formed in the fabric. Identify three examples of everyday objects that change over time but at different rates. For example, something that is used up or worn out in a matter of days (for example, chalk), months (for example, jeans), or years.

We have presented an Earth history stretching back \(4.6\) billion years. Has the history of life on Earth been more affected by rare, high-magnitude events or frequent, low-magnitude processes? Justify your choice.