Earth as an Evolving Planetary System

Kent C. Condie

• Publisher: Academic Press
• Year: 2005
• ISBN:
  9780080494586
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Earth as an Evolving Planetary System is based on Kent Condie's classic text, Plate Tectonics and Crustal Evolution, which has been revamped and renamed in order to reflect a new emphasis on the evolving interactions of the Earth's systems. This revised volume synthesizes data from the fields of geophysics, oceanography, planetology, and geochemistry.

It features new chapters on the Earth's core, biotic systems, and the supercontinent cycle and mantle plume events. It contains expanded treatment of the evolution of the Earth's crust and mantle, carbon cycle, oxygenation of the atmosphere, and the significance of sulfur isotope fractionation. It also includes new information on mass extinctions and catastrophic events over the last four billion years that have transformed the atmosphere, oceans, and life on Earth. By integrating results from many different disciplines, this important text gives students a broader perspective of the Earth Sciences and shows how specialized data contribute to Earth and planetary history.

This text is designed for advanced undergraduate and graduate students in Earth, Atmospheric, and Planetary Sciences; and scientists in other disciplines who want to look at the Earth with a broader perspective.

* New insight on interaction and evolution of Earth system* Examines the role of castrophic events in Earth's history* New section on the evolution of the mantle

1

Earth Systems

Publisher Summary

This chapter discusses the Earth systems and reviews the basic structure of the Earth as determined primarily from seismology. The state of a system is characterized by a set of variables at any time during the evolution of the system. For the Earth, temperature, pressure, and various compositional variables are most important. In recent years, the Earth has been considered a complex planetary system that evolved over 4.6 billion years of time. It includes reservoirs such as the crust, mantle, and core, and subsystems such as the atmosphere, hydrosphere, and biosphere. Most estimates of the temperature distribution in the Earth are based on one or a combination of two approaches: (1) models of the Earth’s thermal history involving various mechanisms for core formation and (2) models involving redistribution of radioactive heat sources in the Earth by melting and convection processes

Earth as a Planetary System

A system is an entity composed of diverse but interrelated parts that function as a whole (Kump et al., 1999a). The individual parts, often called components, interact with each other as the system evolves with time. Components include reservoirs of matter or energy, described by mass or volume, and subsystems, which behave as systems within a system. In recent years, the Earth has been considered a complex planetary system that evolved over 4.6 billion years of time. It includes reservoirs, such as the crust, mantle, and core, and subsystems, such as the atmosphere, hydrosphere, and biosphere. Because many of the reservoirs in the Earth interact with each other and with subsystems, such as the atmosphere, there is an increasing tendency to consider most or all of the Earth’s reservoirs as subsystems.

The state of a system is characterized by a set of variables at any time during the evolution of the system. For the Earth, temperature, pressure, and various compositional variables are most important. The same thing applies to subsystems within the Earth. A system is at equilibrium when nothing changes as it evolves. If, however, a system is perturbed by changing one or more variables, it responds and adjusts to a new equilibrium state. A feedback loop is a self-perpetuating change and a response in a system to a change. If the response of a system amplifies the change, it is known as a positive feedback loop, whereas if it diminishes or reverses the effect of the disturbance, it is a negative feedback loop. As an example of positive feedback, if volcanism pumps more CO₂ into a CO₂-rich atmosphere of volcanic origin, this should promote greenhouse warming and the temperature of the atmosphere would rise. If the rise in temperature increases weathering rates on the continents, this would drain CO₂ from the atmosphere causing a drop in temperature, an example of negative feedback. Because a single subsystem in the Earth affects other subsystems, many positive and negative feedback loops occur as the Earth attempts to reach a new equilibrium state. These feedback loops may be short lived over hundreds to tens of thousands of years, such as short-term changes in climate, or they may be long lived over millions or tens of millions of years, such as changes in climate related to the dispersal of a supercontinent.

The driving force of planetary evolution is the thermal history of a planet, more fully described in Chapter 10. The methods and rates by which planets cool, either directly or indirectly, control many aspects of planetary evolution. In a silicate-metal planet like Earth, thermal history determines when and if a core will form (Fig. 1.1). It determines whether the core is molten, which in turn determines whether the planet will have a global magnetic field (generated by dynamo-like action in the outer core, as explained in Chapter 5). The magnetic field, in turn, interacts with the solar wind and with cosmic rays, and it traps high-energy particles in magnetic belts around the planet. This affects life because life cannot exist in the presence of intense solar wind or cosmic radiation.

Figure 1.1 Major relationships between thermal and climatic histories of the Earth.

Planetary thermal history also strongly influences tectonic, crustal, and magmatic history (Fig. 1.1). For instance, only planets that recycle lithosphere into the mantle by subduction, as the Earth does, appear capable of generating continental crust and thus having collisional orogens. Widespread felsic and andesitic magmas can only be produced in a plate tectonic regime. In contrast, planets that cool by mantle plumes and lithosphere delamination, as perhaps Venus does today, should have widespread mafic magmas with little felsic to intermediate component. They also appear to have no continents.

So where does climate come into these interacting histories? Climate reflects complex interactions of the ocean-atmosphere system with tectonic and magmatic components, as well as interactions with the biosphere. In addition, solar energy and asteroid or cometary impacts can have severe effects on climatic evolution (Fig. 1.1). The thermal history of a planet affects directly or indirectly all other systems in the planet, including life. The Earth has two kinds of energy sources: those internal to the planet and those external to the planet. In general, internal energy sources have long-term (>10⁶ years) effects on planetary evolution, whereas external energy sources have short-term (<10⁶ years) effects. Gradual increases in solar energy over the last 4.6 Gy have also influenced the Earth’s climate on a long timescale. The most important extraterrestrial effects on planetary evolution, and especially on climate and life, are asteroid and cometary impacts, the effects of which usually last less than 10⁶ years.

Many examples of interacting terrestrial systems are described in later chapters. However, before describing these systems and their interactions, I first need to review the basic structure of the Earth as determined primarily from seismology.

Structure of the Earth

The internal structure of the Earth is revealed primarily by compressional waves (primary waves, or P-waves) and shear waves (secondary waves, or S-waves) that pass through the planet in response to earthquakes. Seismic-wave velocities vary with pressure (depth), temperature, mineralogy, chemical composition, and degree of partial melting. Although the overall features of seismic-wave velocity distributions have been known for some time, refinement of data has been possible in the last 10 years. Seismic-wave velocities and density increase rapidly in the region between 200 and 700 km deep. Three first-order seismic discontinuities divide the Earth into crust, mantle, and core (Fig. 1.2): the Mohorovicic discontinuity, or Moho, defining the base of the crust; the core-mantle interface at 2900 km; and the inner-core-outer-core interface around 5200 km. The core composes about 16% of the Earth’s volume and 32% of its mass. These discontinuities reflect changes in composition, phase, or both. Smaller but important velocity changes at 50 to 200 km, 410 km, and 660 km provide a basis for further subdivision of the mantle, as described in Chapter 4.

Figure 1.2 The distribution of average compressional-wave, or P-wave (V_p), and shear-wave, or S-wave (V_s), velocities and the average calculated density (p) in the Earth. Also shown are temperature distributions for whole-mantle convection (T_w) and layered mantle convection (T_L). LVZ, low-velocity zone.

The major regions of the Earth can be summarized as follows (Fig. 1.2):

1. The crust consists of the region above the Moho and ranges in thickness from about 3 km at some oceanic ridges to about 70 km in collisional orogens.

2. The lithosphere (50–300 km thick) is the strong outer layer of the Earth—including the crust, which reacts to many stresses as a brittle solid. The asthenosphere, extending from the base of the lithosphere to the 660-km discontinuity, is by comparison a weak layer that readily deforms by creep. A region of low seismic-wave velocity and of high attenuation of seismic-wave energy, the low-velocity zone (LVZ), occurs at the top of the asthenosphere and is from 50 to 100 km thick. Significant lateral variations in density and in seismic-wave velocity are common at depths of less than 400 km.

3. The upper mantle extends from the Moho to the 660-km discontinuity and includes the lower part of the lithosphere and the upper part of the asthenosphere. The region from the 410-km to the 660-km discontinuity is known as the transition zone. These two discontinuities, further described in Chapter 4, are caused by two important solid-state transformations: from olivine to wadsleyite at 410 km and from spinel to perovskite with the addition of magnesiowustite at 660 km.

4. The lower mantle extends from the 660-km to the 2900-km discontinuity at the core-mantle boundary. It is characterized mostly by rather constant increases in velocity and density in response to increasing hydrostatic compression. Between 200 and 250 km above the core-mantle interface, a flattening of velocity and density gradients occurs in a region known as the D″ layer, named after the seismic wave used to define the layer. The lower mantle is also referr...

Table of contents

• Cover image
• Title page
• Table of Contents
• Preface
• Chapter 1: Earth Systems
• Chapter 2: The Crust
• Chapter 3: Tectonic Settings
• Chapter 4: The Mantle
• Chapter 5: The core
• Chapter 6: The Atmosphere and Oceans
• Chapter 7: Living Systems
• Chapter 8: Crustal and Mantle Evolution
• Chapter 9: The Supercontinent Cycle and Mantle-Plume Events
• Chapter 10: Comparative Planetary Evolution
• References
• Index