Megascale geologic processes, such as earthquakes, tsunamis, volcanic eruptions, floods, and meteoritic impacts have occurred intermittently throughout geologic time, and perhaps on several planets. Unlike other catastrophes discussed in this volume, a unique process is unfolding on Earth, one in which humans may be the driving agent of megadisasters. Although local effects on population clusters may have been catastrophic in the past, human societies have never been interconnected globally at the scale that currently exists. We review some megascale processes and their effects in the past, and compare present conditions and possible outcomes. We then propose that human behavior itself is having effects on the planet that are comparable to, or greater than, these natural disasters. Yet, unlike geologic processes, human behavior is potentially under our control. Because the effects of our behavior threaten the stability, or perhaps even existence, of a civilized society, we call for the creation of a body to institute coherent global, credible, scientifically based action that is sensitive to political, economic, religious, and cultural values. The goal would be to institute aggressive monitoring, identify and understand trends, predict their consequences, and suggest and evaluate alternative actions to attempt to rescue ourselves and our ecosystems from catastrophe. We provide a template modeled after several existing national and international bodies.

The surface of Mars preserves landforms associated with the largest known water floods. While most of these megafloods occurred more than 1 Ga ago, recent spacecraft images document a phase of outburst flooding and associated volcanism that seems no older than tens of millions of years. The megafloods that formed the Martian outflow channels had maximum discharges comparable to those of Earth’s ocean currents and its thermohaline circulation. On both Earth and Mars, abrupt and episodic operations of these megascale processes have been major factors in global climatic change. On relatively short time scales, by their influence on oceanic circulation, Earth’s Pleistocene megafloods probably (1) induced the Younger Dryas cooling of 12.8 ka ago, and (2) initiated the Bond cycles of ocean-climate oscillation with their associated Heinrich events of “iceberg armadas” into the North Atlantic. The Martian megafloods are hypothesized to have induced the episodic formation of a northern plains “ocean,” which, with contemporaneous volcanism, led to relatively brief periods of enhanced hydrological cycling on the land surface (the “MEGAOUTFLO Hypothesis”). This process of episodic short-duration climate change on Mars, operating at intervals of hundreds of millions of years, has parallels in the Neoproterozoic glaciation of Earth (the “Snowball Earth Hypothesis”). Both phenomena are theorized to involve abrupt and spectacular planet-wide climate oscillations, and associated feedbacks with ocean circulation, land-surface weathering, glaciation, and atmospheric carbon dioxide. The critical factors for megascale environmental change on both Mars and Earth seem to be associated tectonics and volcanism, plus the abundance of water for planetary cycling. Some of the most important events in planetary history, including those of the biosphere, seem to be tied to cataclysmic episodes of massive hydrological change.

Abstract Diagenetic reactions are characterized by mineral dissolution and precipitation. Both can occur separately or simultaneously and can involve one or more minerals. The process, either dissolution or precipitation, can vary over many orders of magnitude, from sub-millimeter to kilometer scale. The dominant scale for diagenetic purposes, however, would seem to be from millimeters (mm) to meters (m), with the mm scale more common in fine grained rocks such as shales and siltstones, and the meter scale more common in sandstones. Megascale phenomena (kilometers) appear to be related to fluid movement along faults and fractures and are associated with tectonic events and thermal anomalies. There is also a slow, persistent, large-scale fluid movement associated with the dewatering of mudstones that begins on initial deposition and persists until effective porosity is lost (Bonham, 1980; Bjørlykke, 1983). The mechanics of this dewatering are reasonably well understood, at least relative to the chemical consequences, but it is still an area of active investigation, particularly as it may involve overpressure development in young, subsiding basins such as the Gulf Coast or the San Joaquin Basin of California. The importance of this dewatering flow is that it is pervasive, it involves huge volumes of rock, and if it becomes focused even slightly, it can concentrate large fluxes of fluid through limited volumes of rock.