Sketch map of Tharsis region of Mars, showing geographic relations between volcanoes and lava flows. Area approximately the same as in figure I-1.

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Contrary to predictions from the 1950s through the mid-1980s, persistent shortages of nonfuel minerals have not occurred, despite prodigious consumption, and world reserves have increased. Global availability of raw materials is relevant to policy decisions regarding mineral development and land use. Justification for environmental protection may exceed that for mining a specific ore body. Demand for environmental accountability is rising worldwide, and new technologies are enabling internalization of costs. Mineral-rich developing nations plagued by inefficient state-owned mining enterprises, high population growth rates, and environmental degradation could realize substantial benefit by reforming government policies to encourage foreign investment in resources and by appropriate allocation of mineral rents.

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The vast volcanic plains and shields of Mars, together with the thermal, spectrographic, and morphological evidence for water ice at the poles, for several percent water in Viking soil samples, for ground ice or permafrost over much of the planet, and for the existence of surface water at some time in the past, suggest that magma and water or ice may have interacted during evolution of the planet's landscape. Relatively small mesas and buttes, with and without summit craters, are remarkably similar to the table mountains of Iceland that formed by subglacial eruption during the late Quaternary period. Table mountains typically comprise foundations of pillow lava and palagonitized tuff breccias (móberg), overlain by subaerial lava flows that commonly culminated in a typical shield volcano; some table mountains, however, failed to reach the subaerial stage and thus lack the cap rock. Subglacial fissure eruptions produced ridges composed of pillow lava and móberg. Conical knobs on steep‐sided Martian plateaus are reminiscent of the small Icelandic shield volcanoes atop móberg pedestals. Candidate table mountains on Mars are especially numerous in the region between latitude 40°N and the margin of the north polar cap, and interaction of lava with a formerly more extensive ice cap may have occurred. More significant is the possibility that Olympus Mons and the broad lobate aureole deposits around its base may have had similar subglacial beginnings. This hypothesis requires an ice cap several kilometers thick in the vicinity of this enormous shield during its initial stages of eruption. The amounts of water ice required do not appear excessive, given the limits to present knowledge of the water budget throughout the planet's history. A mechanism for localizing such ice, however, is required.

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John Young could hardly have known the truth of his prediction when he first set foot on the lunar surface at the Apollo 16 landing site. His mission was the most surprising geologically and has generated the most controversy of all six Apollo landings. The Descartes region of the central lunar highlands, since its first serious consideration as a site for manned exploration 4 years earlier, had been strongly supported as a place to sample volcanic rocks much different from those of the maria and the basin margins. Three days of field exploration ranging 4 to 5 km from the lunar module failed to turn up a single recognizable volcanic rock. Instead, a variety of breccias, complicated beyond belief, were collected from every location. Crystalline rocks were found whose textures were clearly igneous (see frontispiece), but they were not volcanic. And therein lies the heart of the geologic mystery of Descartes."Well it's back to the drawing boards, or wherever geologists go" (T. K. Mattingly, Apollo 16 Command Module Pilot from lunar orbit). PREFACEThis volume contains the final results compiled by the Apollo Field Geology Investigations Team for the Apollo 16 mission. Some of the data presented here were reported in preliminary form shortly after the mission (ALGIT, 1972a(ALGIT, , 1972b AFGIT, 1973; Batson and others, 1972; Muehlberger and others, 1972), but most of the discussion and interpretations that follow are products of individual efforts which have incorporated much of the large body of data available from postmission studies of the rocks, the geophysical and geochemical data, and the extensive collection of photographs taken by the Apollo 16 astronaut crew on the lunar surface and from orbit. The chapter format was chosen to permit individual authors to develop their ideas independently, and we trust this approach will serve to stimulate rather than confuse the reader.Our purpose in this volume is to summarize the field observations at the Apollo 16 site and to bring together the various interpretations placed upon these observations by the astronauts and the Field Geology Team. Much of the extensive geochemical and geophysical data published since 1974 on the Apollo 16 site has not been incorporated or referred to here. The intent is not to provide a grand synthesis but rather to document the local and regional geologic relations and to summarize what inferences can be made from them. Our expectation is that the volume will be used as a reference for researchers desiring more complete information on the geologic context of the Apollo 16 samples and on the interpretations of those intimately involved with the planning, execution, and analysis of the geologic exploration.John Young, Charles Duke, and Kenneth Mattingly deserve special credit for the quality of their performance while exploring this complex area on the surface, from lunar orbit, and later in discourse with the lunar science community. Their continuing interest in the developing story of Descartes began with an unwavering enthusiasm for geo...