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Chapter 8: Problem 6

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

Short Answer

Expert verified
Fossil 1 is the best choice for an index fossil. Fossil 3 is least characteristic of specific geologic conditions.

Step by step solution

01

Understand Index Fossils

Index fossils are fossils used to define and identify geologic periods. They must meet certain conditions: they should be geographically widespread, easy to identify, and have existed for a short period of time.
02

Examine Fossils Observed

We have three fossils to consider: Fossil 1, Fossil 2, and Fossil 3. From the context, we need to determine which of these meets the conditions for being an index fossil.
03

Determine Index Fossil Criteria

A good index fossil should be found in all four rock locations (Sandstone A, Shale A, Shale B, and Sandstone B) and ideally only in a particular time frame. This means it should not be found in every type of rock endlessly.
04

Analyze Diagram Details

The diagram shows that Fossil 1 is found predominantly within a specific type of rock and appears to correspond frequently in the locations across different rock types, which is a key indicator meeting index fossil standards.
05

Conclusion on Least Characteristic Fossil

The fossil least characteristic of specific geologic conditions would appear in many different geological settings or across all layers. The diagram suggests Fossil 3 does not specifically characterize a single set of geologic conditions as it spans more diverse rock types.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Geologic Conditions
Geologic conditions are the environmental settings in which rocks form. Understanding these conditions is crucial when analyzing rock formations and fossils. Different geological environments lead to the formation of distinct rock types and subsequently, the preservation of various fossils.

Some common geologic conditions include:
  • Marine environments, where limestone is often formed from the accumulation of shells and skeletons of sea organisms.
  • Desert environments, which are characterized by sandstone that has formed from the cementation of sand after being deposited by wind or water.
  • Swamp environments, leading to the formation of coal from the accumulation and compression of plant material.
Understanding the specific geological conditions helps in identifying the type of fossils that are likely to be found in those rocks. For instance, marine fossils are typically found in limestone due to its oceanic origin.
Rock Types
Rocks are categorized into three main types: igneous, sedimentary, and metamorphic. Each type is formed by different geological processes.

When surveying outcrops for fossil examination, it's crucial to know the rock type as this influences fossil preservation:
  • Igneous Rocks: Formed by the cooling and solidifying of magma or lava. They rarely contain fossils because they often form at temperatures too high for life to survive.
  • Sedimentary Rocks: Created by the accumulation of sediments, these rocks are most likely to contain fossils. Examples include limestone, shale, and sandstone.
  • Metamorphic Rocks: These rocks originate from rocks that have been transformed by extreme heat and pressure. Although they can contain fossils, the metamorphic process can often obliterate fossil evidence.
Differentiating between these rock types is essential when identifying fossils in field studies.
Fossil Identification
Identifying fossils is a key component of paleontology, allowing scientists to reconstruct past environments and biological histories. Fossils are the preserved remains or traces of organisms that lived in the past, typically found within sedimentary rocks.

Key characteristics for identifying fossils include:
  • Shape and Structure: Detailed examination of the fossil's shape and structure helps in identifying the organism it represents. Each organism has unique anatomical features that allow for species-level identification.
  • Geological Context: The specific geological layer in which a fossil is found provides vital clues about its age and the environment it lived in. This is particularly important for index fossils, which are used to date rock layers.
  • Comparative Analysis: Fossils are often compared to known specimens in fossil databases or collections to assist in accurate identification.
Fossil identification not only aids in understanding Earth's history but is also a crucial step in selecting suitable index fossils for stratigraphic studies.

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Most popular questions from this chapter

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

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.

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.

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
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