Earth Chp 9 Geologic Time

types of fossils within the units NOTES: The units on the Geologic Time Scale were defined using the first occurrence and last appearance of different kinds of organisms.

The subdivisions of the Geologic Time Scale were based on which criteria or characteristics? Hint 1. If Proterozoic means "former life" and Mesozoic means "middle life," what seems to be a common theme among various sections of the time scale?

Granite

Which type of rock is best suited for radiometric dating?

The Principle of Cross-Cutting relationships NOTES: Because the basaltic dike cuts across the sedimentary rocks, the principle of cross-cutting relationships can be used determine that the relative age of the dike must be younger than that of the sedimentary rock it cuts.

While working in the field, you encounter an outcrop in which sandstone, shale, and limestone have been intruded by a basaltic dike. You know that the basaltic dike is younger than the sedimentary rock by applying which law or principle? Hint 1. The same principle that applies to this igneous intrusion could also apply to a fault.

the Paleozoic NOTES: Crinoids, brachiopods, and trilobites have hard parts and they are not similar to organisms that are living today. They typify the marine life of the Paleozoic era.

While working in the field, you find some sedimentary rocks that include hundreds of fossils of crinoids, numerous brachiopods, and a few trilobites. Realizing that these ancient organisms are some of the first with hard parts, and (when taken as a group) are not very similar to animal communities living today, in which unit of geologic time would you place these rocks? Hint 1. The fossil assemblage in this rock is particularly informative. Think about both the types of fossils and their relative abundance. When would all three of these organisms have co-existed in such numbers?

Carbon-14

Archeologists typically use ___ to date artifacts by means of radiometric dating. Hint 1. Most archeological artifacts are geologically very young and consist of bones, charcoal, and cloth.

Law of superposition Principle of Inclusions Principle of Cross Cutting Relationship see image NOTES: As you can see from above, we can determine a sequence of events using the logic of these principles when observing sedimentary rock.

Basic Principles for Relative Geologic Dating Below is a geologic structure that illustrates the various principles of relative dating. You will identify the basic principles used in relative geologic dating by dragging labels to their corresponding targets in the image below. Drag the appropriate labels to their respective targets. Hint 1. Inclusions in sedimentary rock layers According to the principle of inclusions, the layer of rock that has inclusions from another rock layer must be younger. Hint 2. A dike cutting through sedimentary rock layers The rock layers that the dike cuts through must have been there first. This is the principle of cross-cutting relationships.

Original Horizontality NOTES: Unless the beds have been tilted during a tectonic event or were deposited as cross-stratification, the principle of original horizontality confirms that sedimentary rocks are almost always deposited in a horizontal position.

The fact that sedimentary rocks are almost always deposited in a horizontal position reflects which law or principle of geology? Hint 1. This principle helps to unravel complicated sequences of tilted and deformed strata by suggesting what the layers should have looked like when they first formed.

25 Atoms

A container holds 100 atoms of an isotope. This isotope has a half-life of 1.5 months. How many atoms of the radioactive isotope will be in the container after 3 months?

100 atoms NOTES: Atoms of radioactive isotopes turn into atoms of other isotopes. Do radioactive isotopes turn into other atoms, or something else?

A container holds 100 atoms of an isotope. This isotope has a half-life of 1.5 months. How many total atoms will be in the container after 3 months?

The intrusion of I1 or I2 is the youngest event. Without more information, we cannot know which igneous rock is youngest.

A fault (F) breaks three layers of sedimentary rock (S). An igneous intrusion (I1) has broken through the bottommost layer of rock. A second igneous intrusion (I2) has moved up the fault and pooled on top of the uppermost layer of rock. Which event would be considered the youngest?

Fault A is younger than fault B, and both are younger than the five layers of rock

Five layers of rock are cut by two faults. Both faults cut through all five layers of rock. Fault A breaks through to the surface, whereas fault B does not. Which of the following statements about faults A and B is most accurate?

In a floodplain The ocean floor A swamp

Give It Some Thought: Fossilization 9 of 19 Sedimentary rocks may contain evidence of past life in the form of fossils. Physical remains of ancient organisms, such as bones and shells, or indirect evidence, such as plant imprints or animal tracks, are preserved as fossils only in very specific conditions. Sedimentary rocks are exposed at the surface more than igneous and metamorphic rocks. Since they contain fossils and other clues about the geologic past, sedimentary rocks are important in the study of Earth's history. Fossilization Environments In the list below, indicate in which of the environments fossilization is likely to occur. Note that fossils contain organic matter. Select three that apply. Hint 1. Temperature/pressure conditions for the formation of igneous and metamorphic rocks Igneous and metamorphic rocks require high temperature and/or pressure conditions to form. Organic matter will not thrive well under these high temperature/pressure conditions. Hint 2. What fossils are and the conditions for their formation Fossils form from plant and animal remains in many different ways. It is usually the mineral bones or shells of an organism that are preserved, while the soft tissues decay. Fossils are more likely to form in areas where the dead organisms are covered by sediments over time.

Nonconformity

Horizontal cretaceous limestone directly overlying horizontal Cambrian granite is an example of a(n) ____. Hint 1. Breaks in the rock record represent periods of time when deposition ceases, erosion removes rocks, and deposition then resumes. These unconformities have special names based on the types of rocks above and below the break.

The relative ages of the sequence are as follows: sedimentary rocks > dike > normal fault. NOTES: Applying the principle of cross-cutting relationships to this outcrop, the sedimentary rocks must be the first event in the sequence. The dike could then be intruded into the sedimentary rocks and finally the dike and surrounding rocks could be cut by the normal fault.

In the field you encounter a sequence of sedimentary rocks that has a dike running through them. A normal fault crosses the entire sequence, cutting off part of the dike near the surface. Which scenario best describes your observations? (All scenarios list events from oldest to youngest.) Hint 1. Here you have sedimentary rock strata and two cross-cutting phenomena. Which principle would help you determine their age relationships?

In a mountain range. Deep beneath Earth's surface, where sedimentary layers have been buried. NOTES: . You should have a better understanding of the Vishnu Schist in the Grand Canyon, and the processes that led to its formation.

Metamorphic RockPart complete Where else might you expect to find metamorphic rocks like the Vishnu Schist of the Grand Canyon? Hint 1. From sediments to metamorphic rocks. Metamorphic rocks are often derived from sediments and sedimentary rocks that have been subjected to temperature and pressure conditions very different from the ones they formed in at Earth's surface. Where would the higher temperature and pressure conditions occur in a thick sequence of sedimentary rocks?

The organism is buried quickly.

Part complete Which of the below conditions favor fossil preservation? Hint 1. Fossil preservation occurs only when the remains are protected from being destroyed.

Permineralization

Petrified wood is an example of what type of fossil preservation? Hint 1. Petrified wood is created when mineral-rich water permeates wood and precipitates the minerals out of solution to fill the empty pores spaces.

The rate of decay of atoms in container A is greater than the rate of decay of atoms in container B.

Two containers hold the same radioactive isotope. Container A contains 1000 atoms, and container B contains 500 atoms. Which of the following statements about containers A and B is true?

protons; neutrons

Two isotopes of uranium, 238U and 235U, have the same number of ___, but different numbers of ___. Hint 1. Isotopes are atoms that have the same chemical properties and atomic numbers but different atomic masses.

The Vishnu Schist was exposed when sedimentary layers above it were eroded. NOTES: You have correctly identified the reason why the Vishnu Schist is found at Earth's surface.

Vishnu Schist at the Grand CanyonPart complete Which of the following best describes why the Vishnu Schist is visible at Earth's surface? Hint 1. If the Vishnu Schist was formed deep in the crust, how is it found at the surface? What must have happened to the overlying rocks?

Limited to a short span of geologic time but widely distributed

What are the characteristics of an index fossil? Hint 1. An index fossil can be used to correlate the ages of rocks of similar ages in different regions.

Parent isotopes turn into daughter isotopes.

What happens during radioactive decay?

Geologic features that cut through rocks must form after the rocks that they cut through.

What is the principle of cross-cutting relationships?

Sedimentary rocks are close to horizontal when deposited.

What is the principle of original horizontality?

Within a sequence of rock layers formed at Earth's surface, rock layers lower in the sequence are older.

What is the principle of superposition?

the amount of time over which the number of parent isotopes decreases by half

What is the scientific definition of half-life?

Nonconformity NOTES: When older igneous and/or metamorphic rocks are eroded and then covered by flat-lying sedimentary rocks, the resulting feature is known as a nonconformity.

What term describes an erosional surface bounded above by flat-lying sedimentary rocks and below by igneous or metamorphic rocks? Hint 1. Recall the various types of unconformities. Which one would apply to the situation described?

Phanerozoic ====> means "visible life" when translated from Greek. The Phanerozoic represents the last 542 million years of geologic time and contains abundant fossils with hard skeletal parts.

Which eon translates to "visible life" from the Greek? Hint 1. The only way life is directly "visible" in the rock record is through the fossils. When did life on Earth become sufficiently complex to begin leaving abundant fossil traces?

widespread occurrence of hard parts NOTES: The widespread occurrence of hard external and internal skeletons in organisms, and consequently the better probability of their preservation as fossils, defines the beginning of the Phanerozoic 542 million years ago.

Which event is generally used to indicate the beginning of the Phanerozoic eon? Hint 1. What evidence in the rock record separates most units of geologic time? Specifically, what new evidence started showing up in the rock record 542 million years ago?

Epoch NOTES: Of epochs, periods, eras, and eons, epochs are the smallest intervals of geologic time. The spelling of "epoch" (e before p) might help you remember this relationship.

Which is the shortest interval of geologic time? Hint 1. A good way to remember this subdivision might be: "smallest of three, only this e comes before p."

Holocene

Which of the following represents the shortest division of time? Hint 1. The geologic time scale is divided into eons, which are subdivided into eras, which are subdivided into periods, which are subdivided into epochs.

Relative age dating places rocks and events in chronological order but does not provide information about absolute age.

Which of the following statements about relative and absolute age dating is most accurate?

the principle of cross-cutting relationships

Which principle of relative age dating is important for determining the relative age of igneous rock that has intruded into overlying rock?

Superposition NOTES: In a section of flat-lying sedimentary rocks, the law of superposition states that the oldest rocks are found at the bottom of the section and the rocks become increasingly younger as you move toward Earth's surface. This happens because the rocks at the base must have been there prior to the deposition of the rocks above them.

Which principle or law of geology states that in a sequence of undeformed sedimentary rocks, each bed is older than the one above? Hint 1. Recall the basic principles and laws of geology. Which one would apply to the age ordering of layers in sedimentary strata?

Paleozoic, Mesozoic, Cenozoic NOTES: On the Geologic Time Scale, the Paleozoic represents the oldest era and includes organisms that are least similar to those seen today. It is followed by the Mesozoic and then the Cenozoic.

Which represents the correct sequence from oldest to most recent or current eras on the Geologic Time Scale? Hint 1. The prefix meso- means middle. What does paleo- mean? (Think about what paleontologists study.)