Serpentine: The Evolution and Ecology of a Model System - Couverture rigide

Couverture rigide

ISBN 10 : 0520268350 ISBN 13 : 9780520268357

Editeur : University of California Press, 2011

Synopsis

Serpentine soils have long fascinated biologists for the specialized floras they support and the challenges they pose to plant survival and growth. This volume focuses on what scientists have learned about major questions in earth history, evolution, ecology, conservation, and restoration from the study of serpentine areas, especially in California. Results from molecular studies offer insight into evolutionary patterns, while new ecological research examines both species and communities. "Serpentine" highlights research whose breadth provides context and fresh insights into the evolution and ecology of stressful environments.

A propos du livre

À propos de l?auteur

Susan Harrison is Professor of Environmental Science and Policy at the University of California, Davis. Nishanta Rajakaruna is Professor of Botany at the College of the Atlantic.

À propos de la quatrième de couverture

"This outstanding volume brings together leading experts across a broad range of disciplines to bring serpentine into focus, as never before, as a window to understanding major natural processes and patterns in nature. By doing so, the authors illuminate exciting questions and challenges that will serve to inspire and direct much future study of these fascinating systems."—Bruce G. Baldwin, University of California, Berkeley

À propos de la deuxième de couverture

"This outstanding volume brings together leading experts across a broad range of disciplines to bring serpentine into focus, as never before, as a window to understanding major natural processes and patterns in nature. By doing so, the authors illuminate exciting questions and challenges that will serve to inspire and direct much future study of these fascinating systems." Bruce G. Baldwin, University of California, Berkeley

Extrait. © Reproduit sur autorisation. Tous droits réservés.

Serpentine

The Evolution and Ecology of a Model System

By Susan Harrison, Nishanta Rajakaruna

UNIVERSITY OF CALIFORNIA PRESS

Copyright © 2011 the Regents of the University of California
All rights reserved.

ISBN: 978-0-520-26835-7

Contents

Contributors, vii, 
Preface, xi, 
Introduction, xiii, 
PART ONE. SERPENTINE AS A MODEL IN EARTH HISTORY AND EVOLUTION, 
1. Serpentinites and Other Ultramafic Rocks: Why They Are Important for Earth's History and Possibly for Its Future, 3, 
2. Microbes in Extreme Environments: Implications for Life on the Early Earth and Other Planets, 29, 
3. Phylogenetic Patterns of Endemism and Diversity, 49, 
4. Plant Speciation, 71, 
5. Intraspecific Variation, Adaptation, and Evolution, 97, 
6. Genomic Approaches to Understanding Adaptation, 139, 
7. Local Adaptation in Heterogeneous Landscapes: Reciprocal Transplant Experiments and Beyond, 155, 
8. Herbivory and Other Cross-Kingdom Interactions on Harsh Soils, 181, 
9. Invasions and the Evolution of Range Limits, 201, 
PART TWO. SERPENTINE AS A MODEL IN ECOLOGY AND CONSERVATION, 
10. Plant Competition and Facilitation in Systems with Strong Environmental Gradients, 223, 
11. Community Invasibility: Spatial Heterogeneity, Spatial Scale, and Productivity, 237, 
12. Disturbance and Diversity in Low-Productivity Ecosystems, 249, 
13. Plant–Pollinator Interactions in Naturally Fragmented Habitats, 275, 
14. Spatial Ecology: The Effects of Habitat Patch Size, Shape, and Isolation on Ecological Processes, 297, 
15. Systematic Conservation Planning: Protecting Rarity, Representation, and Connectivity in Regional Landscapes, 309, 
16. Biodiversity, Ecosystem Functioning, and Global Change, 329, 
17. Climate Change and Plant Communities on Unusual Soils, 359, 
18. Restoration and Revegetation of Harsh Soils, 383, 
PART THREE. SYNTHESIS, 
19. What Have We Learned from Serpentine in Evolution, Ecology, and Other Sciences?, 417, 
Species Index, 429, 
Subject Index, 435, 

CHAPTER 1

Serpentinites and Other Ultramafic Rocks

Why They Are Important for Earth's History and Possibly for Its Future

Eldridge M. Moores, University of California, Davis

Geology is a historical science, one of the "storytelling sciences," not simply a laboratory science. As such, geologists try to not only understand basic and timeless principles related to the rocks being studied but also give an account of what has happened in the past, and when possible, use this past history to forecast future events (Primack and Abrams, 2006: 17).

Serpentine, strictly speaking, is a mineral. Rocks formed mostly of serpentine are called serpentinites. Serpentine forms chiefly by the alteration (hydration) of the minerals olivine and pyroxene, found mostly in rocks called peridotites, a type of ultramafic rock. The term ultramafic indicates that the rocks are more than 90% olivine and pyroxene; most ultramafic rocks were derived from the Earth's mantle, the layer below the topmost layer or crust (Figure 1.1). Thus, both peridotite and serpentinite are ultramafic.

The olivine- or pyroxene-rich rocks from which serpentinites come are common in the Earth's mantle. Exposures of serpentinite at the Earth's surface indicate special tectonic action to move the rocks from 10–50 km deep to the surface. Serpentinite is relatively widespread in oceanic crust, which comprises about 70% of the Earth's surface. Oceanic crust in the oceans is not more than about 185 million years old. Most exposures of serpentinite at the Earth's continental surface come from ophiolites—exposures of oceanic crust and mantle formed at oceanic spreading centers. This process requires placement of oceanic crust and mantle on the continental crust or in exposure of subduction accretionary complexes above sea level. Mélanges, so-called stratiform mafic-ultramafic complexes, and subcontinental mantle represent subordinate sources of serpentinites.

The general tale of peridotites and their derivative serpentinites involves a long detective story of geologists trying to understand the origin of these rocks. This investigation started in the nineteenth century, continued through the twentieth century and up to the present time. The study of ophiolites led in part to the plate tectonic revolution that transformed our understanding of how the Earth evolved and continues to do so.

In this chapter, I concentrate on the general nature and geologic history of serpentine and its antecedent related rocks and how regional differences in serpentinites relate to the specific history of a particular region. I begin with the basic structure of the Earth and the nature of peridotites and ultramafic rocks. I give the history of the ophiolite concept and how it influenced plate tectonics, an account of our understanding of ophiolites today, ophiolites through time, and other occurrences of serpentinite. The history section partly includes my personal story, as I have worked on ophiolites since the mid-1960s.

GENERAL EARTH STRUCTURE

Earth is composed of a series of layers, determined principally by different chemical and mineral compositions (see Figure 1.1). From the surface to the center, these layers include the crust, the mantle, and the core. The two outer layers, the crust and mantle, are composed mostly of silicate minerals, that is, minerals composed of Si, O, and other elements; carbonate rocks (containing carbon, in addition to oxygen and other elements); and lesser amounts of rocks composed dominantly of oxide minerals, sulfates, phosphates, or related rocks.

The recognition of these layers comes chiefly from the study of the passage through the Earth of seismic waves that are generated in earthquakes. The velocity of seismic waves—that is, how fast seismic energy passes through a rock—varies with respect to the composition of the material, whether it is solid or liquid, its density, and its stiffness. Seismic waves are faster in rocks with olivine and pyroxene than in rocks containing the minerals quartz and feldspar. Molten rock or magma passes seismic energy more slowly than solid rocks; indeed, some types of seismic waves do not pass through liquids at all. Study of these seismic effects, coupled with measurement of the attraction of the Earth's gravity below a point on the surface, as well as slight perturbations in the Earth's magnetic field near the surface, have contributed to the layered model of the Earth.

The boundaries of the layered model are reasonably sharp on a global scale; on a more local scale, they become fuzzy and complex. In places there is considerable mixing of rocks from two layers. This mixing adds to the complexity of analysis of the boundary.

The crust of the Earth consists of two main parts—continental crust and oceanic crust. Continental crust consists of diverse sedimentary, metamorphic, and igneous rocks ranging in age from about 4 billion years old (4 Ga) to recent. Continents average approximately 35 km thick, but range from approximately 15–30 km thick along their margins and in rifted regions (such as the U.S. Basin and Range Province or the East African Rift) to about 70–100 km thickness under high mountain regions, such as the Himalayas or the Andes. The average composition of a continent is approximately that of a granitic rock, with Na approximately equal to K. Minerals in such rocks chiefly include quartz (SiO2), feldspar including potassium feldspar (KAlSi3O8) and plagioclase ((Ca,Na)Si2O6), subordinate mica (biotite or muscovite), and minor iron- and magnesium-bearing minerals, such as amphibole.

As mentioned, oceanic crust underlies some 70% of the area of the Earth's surface. It is thinner than continental crust, about 5–7 km thick on average, and is considerably different in composition from continental crust. A typical oceanic crust comprises a sequence of fine- and coarse-grained rocks of basaltic composition (about 50% SiO2, 10–20% Al2O3, a few percent CaO, and small amounts of K2O and Na2O). The average composition and seismic velocities of oceanic crust are that of a basalt. Chief minerals in basaltic rocks include plagioclase, pyroxene, olivine, and amphibole (another Mg-Fe-bearing mineral).

Both continental and oceanic crust overlie the Earth's mantle. The mantle comprises the main volume of the Earth. At shallow levels it is thought to be mostly composed of olivine, with subordinate pyroxene and spinel, an oxide mineral. The crust together with the uppermost part of the mantle is the lithosphere, a dense, strong layer that forms at mid-oceanic ridges or other spreading centers and thickens away from them to an average thickness of 100 km. Beneath continents, the lithosphere may be as much as 250 km thick.

Beneath the lithosphere is a weak zone in the mantle, the asthenosphere, where the rock is closer to its melting temperature than it is in the overlying lithosphere. It contains the zone on which the plates slide during plate motion. The asthenosphere may attain thicknesses of up to 150 km under the oceans, but it is thinner or possibly absent under continents (e.g., Fjeldskaar, 1994).

Beneath the asthenosphere, the olivine and pyroxenes in mantle rocks change to denser crystalline forms and form a dense layer, the mesosphere, which extends to the core-mantle boundary. The inferred "hot abyssal layer" in the lowermost mantle is a region thought to contain relatively primordial mantle.

Three principal types of plate boundaries exist. Divergent margins, chiefly mid-oceanic ridges, occur where plates move apart and new oceanic lithosphere develops; convergent or subduction margins are where one plate descends beneath another into the Earth's interior (subduction is the term given to the process by which one plate slides beneath another. It is the English translation of the German word Verschluchung, meaning "underthrusting," a term that was used in the early twentieth century to explain the formation of the Alps; Sengor, 1977); conservative or transform margins are where two plates slide past one another without creation or destruction of lithosphere. Figure 1.1 shows these margins schematically, and Figure 1.2 shows the current distribution of continents, oceans, island arcs, and plate boundaries. The plate boundaries are the locations where most of the Earth's earthquakes occur.

An island arc is a curved chain of islands, usually with active volcanoes, that lies above a subduction zone within the oceans. Examples include the Japanese islands, the Philippines, the Aleutians, the Lesser Antilles, and the Marianas. Subduction zones beneath a continent also cause curved chains of volcanoes, such as those on the Kamchatka Peninsula, Mexico and Central America, and the Cascades of the Pacific Northwest, but because they lie on continents, these chains are called continental arcs. Island arcs as defined are distinct from more linear island chains, such as the Hawaiian Islands, that form above a "hot spot" or point-like source of magma that produces a succession of volcanic islands as the plate moves over it. Island arc volcanoes characteristically are cone-shaped, such as Mt. Shasta, and have more Si-rich magmas (andesites) than those of Hawaii-like hot spot islands, which have more rounded or "shield-like" volcanoes and are chiefly of basaltic composition (see later discussion).

The depth of subduction varies from place to place. As illustrated in Figure 1.1, some down-going (subducting) plates descend into the mantle all the way to the core-mantle boundary. Others seem to get stuck at the top of the mesosphere. In other places, down-going plates descend only 100–300 km in depth.

Most of the time, the plates move fairly smoothly, with more or less constant angular velocity with respect to each other. In some places, however, collisions occur that interrupt this smooth motion. For example, a continent on a down-going plate eventually may collide with a subduction zone and subduct a short distance, until the buoyancy of the continental crust arrests the subduction. Modern examples of subduction zone–continent collisions include the northern margin of Australia, which is colliding with the Indonesian subduction zone near Timor, and East China, which is colliding with the east-dipping West Luzon subduction zone near Taiwan.

In other cases, two continents collide with each other. The Alpine-Himalayan mountain belt displays the best examples of such a situation, with continental collisions taking place at present along the Taurus-Zagros Mountains of Turkey, Iraq, and Iran, where Arabia is colliding with Eurasia; and the Himalayas, where India is colliding with central Asia. Previous collisions include the Alps, the Appalachians, and the Urals. Collisions interrupt the smooth action of plates: they change plate motions or the location and nature of boundaries.

PERIDOTITES, SERPENTINITES, AND ASSOCIATED ROCKS

Peridotites are composed of silicon-oxygen–containing minerals called silicates. A peridotite consists principally of olivine, with lesser amounts of one or two pyroxenes and minor oxide and sulfide minerals of chromium, aluminum, and nickel. Olivine is a silicate mineral containing chiefly Mg and Fe, as well as Si and O. Pyroxene comprises chiefly two separate silicate minerals, one containing chiefly Mg and Fe, and the other with significant amounts of Ca as well. Neither olivine nor pyroxene contains large amounts of Al, K, or Na. Olivine, pyroxene, and feldspar are particular silicate minerals with a specific structure and composition.

Serpentinites are called "ultramafic" or "mafic" because they are relatively high in magnesium and iron, as well as silicon, and lack large amounts of aluminum, calcium, sodium, and potassium. As mentioned before, "ultramafic" is reserved for rocks composed of at least 90% olivine and pyroxene or their alteration products, including serpentine; "mafic" refers to rocks that contain olivine and/or pyroxene, with approximately equal amounts of plagioclase feldspar. Ultramafic rocks variously are called peridotite, for a mixture of olivine and pyroxene, and dunite, for a rock composed mostly of olivine. A peridotite, in turn, is a harzburgite if the principal pyroxene is Ca-poor, a lherzolite if there is roughly equal amounts of Ca-poor pyroxene (enstatite) and Ca-rich pyroxene (diopside), and a wehrlite if the pyroxene is mostly diopside. An olivine pyroxenite contains at least 50% pyroxene and a pyroxenite if the pyroxene content is over 90%.

Serpentinites can form in any environment where water and peridotites come in contact with each other at temperatures lower than 500°C. Thus they can form along active plate margins, during emplacement of mantle rocks into the Earth's crust, as will be discussed, or even after emplacement as a result of reaction of peridotite with hot ground water.

Mafic rocks include many diverse types, particularly extrusive basalts, the most common volcanic rock on Earth. Additional rocks include shallow intrusive diabase, a medium fine–grained rock of basaltic composition, and intrusive gabbro, a coarse-grained rock of basaltic composition composed of variable amounts of olivine, pyroxene, and plagioclase.

Ultramafic rocks have high density (3.0–3.3 g/cc), high strength, and high seismic velocities. Hydration of olivine or pyroxene to serpentine in a rock produces a change in the structure of the minerals, weakens the rocks, and lowers its density from 3.3 g/cc for fresh peridotite to 2.4–2.9 g/cc for a serpentinite, depending on the amount of water added. Because serpentine is a "sheet silicate" mineral, similar to mica, zones of planar weakness develop in formerly strong rocks. Under high confining pressure, the rocks maintain considerable strength. At conditions of low confining pressure (near the Earth's surface), serpentinites lose their strength, are easily faulted, and turn into the weak, slippery, sheared bodies that are common in some regions. With low density and planar weakness, serpentinite is easily mobilized in Earth movements and thus becomes detached from its original location. Therefore, its movement may complicate our understanding of the nature and origin of any particular serpentine.

The velocity of seismic waves in the mantle increases sharply from crustal values at the crust-mantle boundary. This sharp increase is called the Moho or M discontinuity after Croatian seismologist Andrija Mohorovicic, who discovered it in 1909.

(Continues...)

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Détails de l'édition

• Éditeur: University of California Press
• Date d'édition: 2011
• Langue: anglais
• ISBN 10: 0520268350
• ISBN 13: 9780520268357
• Reliure: Tapa dura
• Nombre de pages: 460
• Éditeur: Harrison Susan, Rajakaruna Nishanta
• Coordonnées du fabricant: non disponible
• Personne responsable: non disponible