IntroductionGeology 200
Study Guide
Unit 13:
The Earth’s Interior
Overview
In Unit 1 we briefly introduced the Earth’s internal structure. We return to this subject in this unit, discussing the evidence about internal structure provided by geophysical data. In later sections of the unit, we deal with the concepts of isostacy and crustal rebound, measurements of gravity, the Earth’s magnetic field, and heat flow within the Earth.
Objectives
After completing this unit, you should be able to
- describe, using diagrams, how seismic waves provide information about the inner structure of the Earth.
- define the term Moho.
- describe the three main types of meteorites.
- explain isostacy, isostatic adjustment, and crustal rebound.
- describe two ways in which a gravity meter can be used to study the crust and upper mantle.
- describe the Earth’s magnetic field, and explain how it is generated.
- explain the variations in heat flow measured at the Earth’s surface.
Section 1: The Interior Structure of the Earth
The reading associated with this section describes how seismic data have provided information about the interior structure of the Earth. As you learned in Unit 1, the Earth is composed of a solid outer crust, a solid mantle with a layer that is partially melted, a liquid metal outer core, and a solid metal inner core. To review, the lithosphere is the crust and the upper mantle, and the asthenosphere is the partially molten layer of the mantle that may be important as a magma generation zone and a zone where it may flow.
Reading Assignment
Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).
- Introduction to Chapter 17: "Earth's Interior and Geophysical Properties" (pp. 412-413).
- “Evidence from Seismic Waves” (pp. 413-415).
- “Earth’s Internal Structure” (pp. 416-421).
- “How Earth's Interior is Studied” (pp. 413).
Study Questions
- Describe how seismic reflections indicate the presence of layers in the Earth’s interior.
- How is the depth to a seismic reflecting surface determined?
- How does seismic reflection differ from seismic refraction?
- How is the depth to a rock boundary determined, using seismic refraction?
- Why do seismic waves follow a curved pathway even through a thin layer of uniform rock?
- How does oceanic crust differ from continental crust? What evidence for this difference do seismic waves supply?
- What is the Moho?
- What two lines of evidence suggest that mantle rock is ultramafic?
- What is the lithosphere? How thick is it?
- How do seismic waves differ between the asthenosphere and the lithosphere? How is this difference explained in terms of the rocks?
- Why is the asthenosphere important, if the rocks in that zone are close to their melting point?
- What may cause the concentric layering of the core of the mantle that is detected by seismic data?
- What evidence for the existence of the core of the Earth is provided by P waves?
- What do S waves indicate about the character of the Earth’s core? What further information do P waves provide?
- What is thought to be the composition of the Earth’s core? On what two lines of evidence is this theory of composition based?
- Describe the three basic types of meteorites.
- What is a carbonaceous chondrite? Why are such objects of particular interest to scientists?
Section 2: Isostasy
Isostasy is the balance between adjacent blocks of brittle crust floating on the plastic mantle. When weight is added or subtracted from a column of rock, isostatic adjustment occurs. The textbook reading associated with this section gives two examples of isostatic adjustment.
Regions that are out of isostatic equilibrium can be detected by gravity measurements. Be certain that you understand the causes of both positive and negative gravity anomalies. While the textbook mentions the negative gravity anomalies found over oceanic trenches, be aware that negative anomalies also occur over caves or salt domes where the density of the rock is less than average.
Reading Assignment
Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).
- “Isostasy” (pp. 421-422).
- “Gravity Measurements” (p. 422-424).
Study Questions
- Explain the concepts of isostasy and isostatic adjustment.
- Given sufficient time, does the reduction in elevation of a mountain above sea level equal the thickness of rock that has eroded? Explain.
- What is crustal rebound?
- What does a gravity meter measure? How can a gravity meter be used to explore local variations in rock density?
- Give two examples of situations in which positive gravity anomalies might occur and two examples in which negative gravity anomalies might occur.
Section 3: The Earth’s Magnetic Field
The reading in this section describes how the Earth’s magnetic field is generated. In the past, the Earth’s magnetic field has periodically reversed its polarity, a phenomenon known as magnetic reversal. We will discuss the importance of magnetic reversals to the development of the theory of plate tectonics in Unit 14.
Reading Assignment
Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).
- “Earth’s Magnetic Field” (pp. 424-429).
Study Questions
- Outline one hypothesis of how the Earth’s magnetic field is generated.
- Draw a sketch of the Earth, showing the magnetic lines of force during normal polarity and during reversed polarity.
- How do rocks record the polarity of the Earth’s magnetic field?
- What do small magnetic anomalies generally indicate? Give three examples of positive magnetic anomalies, and one example of a negative magnetic anomaly.
Section 4: Heat within the Earth
Temperature increases with depth into the Earth. The following reading describes the temperature at the different interior zones of the Earth and discusses the factors that can produce variations in heat flow to the Earth’s surface.
Reading Assignment
Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).
- “Earth's Temperature” (pp. 429-431).
Study Questions
- What is a geothermal gradient? What is the average geothermal gradient near the Earth’s surface (in °C/km)?
- What is the approximate temperature at the boundary between the inner and outer core of the Earth? Why is the inner core still solid at this temperature?
- Give three possible reasons why a region may have a high heat flow.
- Explain why the heat flow through the continents is unexpectedly equal to the heat flow through the sea floor.
Unit 13 Self Test
You have now finished Unit 13, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.