Week 6 Lab Assignment
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University of California, Los Angeles *
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Course
70B
Subject
Geology
Date
Apr 3, 2024
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4
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CLUSTER 70B: Evolution of the Cosmos and Life
Name:______________________
WEEK 5 LABORATORY/DISCUSSION WORKSHEET
INTERPRETING GEOLOGIC HISTORY LAB ASSIGNMENT Part 1: Relative and Absolute Dating Determining relative ages of rocks based on their physical relationships 1.
List the relative ages of the geological features labeled on the diagram above:
Oldest
_______ _______ _______ _______ _______ _______ _______ _______ _______ _______ _______ _______ _______ Youngest Determining Absolute Ages by Radiometric Dating
You can only use radioactive dating on igneous rock (exception: 14
C dating which can be used on formerly living material, but this has a short time range). This is because radioactive dating determines how long ago that mineral was formed. It tells you when the volcanic
rock formed. In particular, it tells you when the crystals within the volcanic rock formed, but this is about the same as the age of the whole volcanic rock itself. You cannot use this method for dating sediments or sedimentary rock directly because sediments are derived from a wide range of source rocks, each of which has a different age. So one part of the sedimentary rock might yield a radioactive age of 3 billion years, another part gives 2 million years, and yet another gives 100,000 years. To determine the ages of sedimentary rocks (and hence of the fossils within them), you need to find volcanic rocks just above and/or below the sedimentary rocks of interest. 1.
List the relative ages of the geological features labeled on the diagram above: Oldest
_______ _______ _______ _______ _______ _______ _______ _______ _______ _______ _______ _______ _______ Youngest
3. Consult the diagram above. If the Zoroaster Granite (a kind of volcanic rock) were dated and found to be 2 billion years old and the igneous sill F were dated at 1.5 billion years old, what could you say about the absolute ages of the following:
a)
Vishnu Schist, K ________________________________ b)
Gravel M ______________________________________ c)
Bright Angel Shale H ____________________________ Part 2: Biostratigraphy Biostratigraphic units
(biozones) are bodies of strata that are defined or characterized on the basis of their contained fossils. A biostratigraphic unit may be based on a single taxon, on combinations of taxa, on relative abundances, on specified morphological features, or on variations in any of the many other features related to the content and distribution of fossils in strata. One type of biostratigraphic unit can be defined when a taxon disappears from the rock record, like the extinction of the ornithischian dinosaurs or the trilobites. This type of unit is called a Last Appearance Datum. The last appearance of a taxon sets a limit on the youngest age of rocks that the fossil can be found within, and the First Appearance Datum sets a limit on the oldest rocks. 1. What are the age and geographic ranges of trilobites (
Trilobita
)?
First appearance: Last appearance: Geographic range: Total time: 2. What are the age and geographic ranges of ornithischian dinosaurs (
Ornithischia
)? First appearance: Last appearance: Geographic range: Total time: 3.
What are the age and geographic ranges of Mariella
, a Cretaceous ammonite? First appearance: Last appearance: Geographic range: Total time: 4.
If you found a Trilobita fossil, about how old would the rocks that contained it be in millions of years? How confident are you in that age range? 5.
If you found an Ornithischia fossil, about how old would the rocks that contained it be in millions of years? How confident are you in that age range?
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6.
If you found a Mariella
fossil, about how old would the rocks that contained it be in millions of years? How confident are you in that age range? 7.
An index fossil is a fossil that can be used to determine the age of the sediments in which it is found. Good index fossils are wide ranging across the globe, can be found in a variety of habitats, have short geologic time ranges, and are easily identifiable/visually distinct. Which species, Trilobita
, Ornithischia
, or Mariella
, is a better index fossil and why?