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...