The petrologic and geochemical characteristics of the East Pacific Rise and other actively spreading ridges provide important direct evidence concerning the composition of the Earth’s upper mantle because mid-ocean ridge basalts are thought to represent partial melts of upper mantle material rising beneath mid-ocean ridges. Since the characteristics of the upper mantle can be used to constrain models for the accretion and differentiation history of the Earth as well as the nature of physical and chemical processes taking place in the mantle, MORB (midocean ridge basalt) and other oceanic basalts play an important role in constraining models of mantle convection, continent formation, and related questions. This chapter describes the petrologic and geochemical characteristics of lavas from the axes of East Pacific spreading ridges, primarily the East Pacific Rise (EPR) between the Tamayo Fracture Zone and the equator, and the Galapagos spreading center (GSC). The Juan de Fuca Ridge and the Gulf of California are considered in other chapters of this book (Johnson and Holmes, this volume; Lonsdale, this volume). Figure 1 shows locations of the principal sampling areas along the EPR and the GSC and Figure 2 shows the distribution of dredge hauls in the eastern Pacific. Until very recently, the spreading centers of the eastern Pacific were very poorly sampled compared to the Mid-Atlantic Ridge. The Juan de Fuca and Gorda Ridges are reasonably well sampled (Melson and others, 1976; Delaney and others, 1982; Davis and Clague, 1987; Dixon and Clague, 1986). The Gulf of California is mostly covered with sediment, but basalt has been recovered by drilling (Saunders and others, 1982; Lonsdale, this volume). The data base for the GSC includes analyses from Melson and others (1976), Schilling and others (1976), Fornari and others (1983), Christie and Sinton (1981), White and others (1987), and references therein. For the EPR, detailed studies have been conducted at 23°N near the Tamayo fracture zone (Bender and others, 1984), 21°N (Moore and others, 1977; Juteau and others, 1980; Hawkins and Melchior, 1980), 12° to 13°N (Hekinian and others, 1983; Batiza and Vanko, 1984; Macdougall and Lugmair, 1986), and 8° to 9°N (Batiza and others, 1977; Batiza and Johnson, 1980; Natland and Melson, 1980; Morel and Hekinian, 1980). Many of these papers include analyses of volcanic rocks from the EPR axis, as do Engel and Engel (1964), Engel and others (1965), Bonatti (1967), Kay and others (1970), and Sun and others (1979). Recent expeditions have greatly increased the number of dredge hauls of the EPR axis at 10° to 12°N (Thompson and others, 1985), 5°30′ to 14°30′N (Langmuir and others, 1986) and overlapping spreading centers (OSCs) at 12°54′N, 9°03′N, 5°30′N, and 3°57′N (Natland and others, 1986), but these studies are not yet fully complete.

Abstract Failed rifts are inactive mid-ocean ridge segments abandoned during processes that change the spreading geometry of lithospheric plates (Table 1). Morphologically, they resemble active ridge crests, and it seems in some cases that their failure or abandonment was closely coupled to the propagation of a neighboring active rift. In such cases, the propagating rift advances into old lithosphere formerly created by the dying rift, while the dying or abandoned rift ceases to function as a spreading center. A transient and complex transform-type plate boundary forms between the advancing rift and the dying one, as discussed by Hey and others (1986). For propagating/retreating rifts (PRRs) (discussed fully in Hey and others, this volume) and migrating overlapping spreading centers (OSCs), also called nontransform offsets, (Rea, 1978; Lonsdale, 1983; MacDonald and Fox, 1983; Macdonald, this volume; Fox and Gallo, this volume), the distance separating the coupled propagating and failing rifts is usually less than or about 50 km. It is possible that this relatively small distance between the advancing and retreating rifts leads to close coupling of ridge behavior and ease in establishing a transform-type boundary between them. If even smaller rift offsets such as small non-overlapping offsets (SNOOs) (Batiza and Margolis, 1986) and deviations from axial linearity (DevALs) (Langmuir and others, 1986) migrate, it would be expected that close coupling of advancing and dying rifts and the ease of creating a transient plate boundary between them would be correspondingly enhanced due to the close proximity (<3 km) of the two rifts and the hot, thin, weak lithosphere between them

Abstract The part of the Pacific covered in this volume contains a wide variety of constructional volcanic features: volcanic rises (Hess Rise), volcanic ridges (Cocos Ridge), large seamount chains (Line Islands, Pratt-Welker chain, the Fieberling Chain, and the Musician seamounts), small chains like those west of the Juan de Fuca Ridge, guyots, and many seamounts that are not members of linear chains but instead are distributed in patchy clusters or small groups or are isolated. In the eastern Pacific, unlike the western and south Pacific, volcanic topography is not dominated by large linear island and seamount chains (Menard, 1964). Instead, the most abundant seamounts of the eastern Pacific are clustered and isolated volcanoes. The purpose of this chapter is to discuss the characteristics, distribution, and origin of the volcanoes and groups of volcanoes in the eastern Pacific exclusive of the Hawaii-Emperor chain, which is discussed else-where (Clague, this volume). Some of the volcanoes and linear groups of the eastern Pacific are probably of hotspot origin; most probably are not. The origin of many of these not-hotspot volcanoes is probably linked to mid-ocean ridge volcanism, but others may originate on old lithosphere far from active ridge crests. Oceanic volcanoes of non-hotspot character are almost certainly of several types, inasmuch as there are many tectonic environments in which a favorable combination of magma availability and plumbing systems can lead to the formation of volcanoes. These conditions may prevail for a variety of reasons in both near–ridge crest and off-ridge locations. The study of seamount volcanism thus contributes directly to solution of a variety of problems concerning the characteristics, origin, and evolution of oceanic lithosphere.