ESS 101 B Laboratory 7 QUIZ: Geologic Time Questions and Correct Answers | Latest Update 2026/2027 | Graded A+ (100% Verified)

ESS 101 B Laboratory 7 QUIZ: Geologic Time Questions and Correct Answers | Latest Update 2026/2027 | Graded A+ (100% Verified) Research into geologic history depends on geological observations (like rock samples where we can measure physical or chemical properties). If you were a geological researcher trying to study the oldest crust on earth, where would you expect to find rock samples to support your work? On the ocean floor because that is where new crust forms. On the ocean floor because it has the lowest elevation. Correct! On a continental plate because continents are not easily subducted. On a continental plate because they have the steepest slopes.  Figure 7-2: Geologic Time Scale. (Ka = Kilo-annum = thousands of years, Ma = Mega-annum millions of years, Ga = Giga-annum = billions of years) Geologists (and many other scientists who study the past) use the geologic time scale to describe different periods of earth’s history. The geologic time scale divides earth’s history into a series of chronologic subdivisions that are defined by important geologic events, such as the oxygenation of earth’s atmosphere or the evolution of different species. The names and descriptions of each type of subdivision are listed below and are included as headings in figure 7-2. Eon: eons are defined by important planetary developments in earth’s history, like the development of multicellular life. Era: each era describes a length of time with different dominant lifeforms Period: periods are defined by evolutionary changes in the fossil record Epoch: epochs are differentiated by other detectable changes in the geologic record  Question 6 0.5 / 0.5 pts The distinction between eons is based on very broad planetary developments, like the formation of an oxygenated atmosphere or the emergence of multicellular life. For example, the Phanerozoic eon reflects all time in earth’s history since the appearance of shells in the fossil record. Other subdivisions of the geologic time scale (including eras, periods, and eons) require more specific observations based on the existence of specific species or geochemical conditions. Why are there no subdivisions for the Hadean, Archean, or Proterozoic eons? (Select all that apply) Correct! Rocks on earth are continually created and destroyed, which means most rocks formed during these eons no longer exist Correct! Life on earth was not abundant prior to the Phanerzoic Eon Correct! Because they are so old, we are more uncertain about the age of rocks from these eons  1. Determining Relative Ages Relative dating techniques use physical and/or chemical observations about a rock to provide an age that is relative to another rock or process age. These observations allow us to say one rock is older than another, but they do not tell us exactly when the rock formed. For example, when we looked at clastic sedimentary rocks, we knew that the clasts were older than the cement that held them together. These same principles apply to very large-scale geologic formations.  Question 7 0.5 / 0.5 pts During the Proterozoic era, earth’s atmosphere evolved alongside cyanobacteria on earth’s surface. Imbalances in the production and uptake of free oxygen led to fluctuations in the atmospheric and oceanic oxygen content. For about 800,000,000 years, this imbalance caused alternating deposits of iron oxides (black bands) and iron-rich silica (red bands) on the ocean floor, creating a banded iron formation. Which of the following could be said of this formation pictured above? This rock formation is older than the Phanerozoic because it couldn’t have formed in a fully oxygenated environment. This rock is younger than the Hadean because it formed in the oceans, which are thought to have formed at the onset of the Archean. With the information provided, it is only possible to assign this formation a relative age. Correct! All of the answers are correct.  Stratigraphic Principles The way in which rocks are layered or positioned can tell us a lot about the relative ages of different rock units. There are several principles, first proposed by Nicholas Steno in 1669, that help us date geologic rock units in a relative sense. These principles are necessary to unravel the history of the earth that is contained in the geologic record and are used every day by geoscientists. (1) The Principle of Original Horizontality says that the layers of sedimentary and volcanic rock are generally deposited as horizontal layers (Figure 7-3). There are exceptions to the rule, such as rock deposited on the sides of mountains, but such rocks are unlikely to be preserved in the geologic record compared to flat-lying layers deposited in broad basins. Foreset beds of a delta are also deposited as non-horizontal layers. If rocks are not found in horizontal layers, it is usually because subsequent geologic processes have tilted or folded those strata (layers). This law applies to all rocks that form at earth’s surface, such as sedimentary rocks and many volcanic rocks. For example, sediment is originally deposited in horizontal layers within a depositional basin and subsequently lithified into sedimentary rock. Figure 7-3 : The Principle of Original Horizontality. (2) The Principle of Superposition is based on the observation that each individual layer at the surface is deposited on top of the previous layer. Therefore, unless the layers have been overturned by later geologic processes, the oldest layers are at the bottom and the youngest layers are at the top (Figure 7-4).