along the front of the fluid plume; this is what happens to oil and gas. The pressure of the aqueous fluid drives hydrocarbons into reservoir rocks where they collect and remain, if a seal successfully traps them. Otherwise, the pressure of groundwater and their own buoyancy will force them to the surface.

Understanding the role of fluids in the crust necessitates analysis of the interacting thermal, chemical, mechanical, and hydrological processes. Researchers believe that role may extend to influencing, perhaps even initiating, tectonic events. Extremely high pore-fluid pressures, which are characteristic of actively tectonic regions, may facilitate major crustal movements. Frictional resistance to slippage and faulting becomes negligible in certain cases of high pore-fluid pressure.

While seismologists analyze the influence of fluids on fault susceptibility, research continues to reveal how aqueous fluids move in subduction zones and return to the surface through volcanic eruptions. Aqueous fluids also cycle through crustal material at ocean spreading centers, spewing from vents loaded with particulate matter as "black smokers." On continents such fluids bubble to the surface as geothermal springs and geysers.

Ores, those materials that contain valuable metals or other materials, can form by many concentration processes involving chemical reaction with water. Water can seep through soil horizons, leaching solutes away and leaving residual materials such as the bauxite deposits that form aluminum ore. When it reaches solid bedrock, water sustains weathering of minerals and carries away the residue. Deep within the crust, water percolates through the metamorphic zones where igneous intrusions shoulder into the native rocks and contributes to the process of change. And where hot igneous rock and cool saline water make contact along the 40,000-km length of the oceans' spreading centers, researchers can watch minerals precipitate. These observations support analogies that help describe processes characterizing other areas of igneous activity, such as volcanic arcs and continental rifts. Eventually, even the oldest water returns to the surface through uplift, exposure, and erosion; then it quickens again and churns through a shorter episode along the surface.

But once the fluids reach the surface, they do not cease their interaction with the rocks. Oceans continue to pound against the shores, as waves and rocks break each other. Rivers erode the mountains and carry them away. Rain pelts against outcrops, dislodging a grain at a time. Rainwater seeps into fractures and pores, expands on freezing, and thus weathers the rocks mechanically. Water flowing along the surface also dissolves rocks through chemical weathering, forming sinkholes, caverns, caves.

Geologists have always had an appreciation of the links between the solid-earth and its fluid envelopes, but they are now realizing that those envelopes permeate the patches of soil that clothe the continents and the ooze that shrouds the ocean floor. Hydrogen, oxygen, and carbon compose a major proportion of the elements that circulate through the air, ocean, and crust in a variety of fluid forms—coupling into compounds, migrating through pores, dissolving, and precipitating. That circulation of hydrous fluids, in different phases and forms, through the few kilometers between the mantle and space makes this planet what it is.

WATER RESOURCES

The Earth is the water planet. This claim is not based on a mere fluid veneer. The distinctive features that set the Earth apart from other solid planets—the deep, wide oceans; the abundance of living beings; even the buoyant, mutable, silica-rich continents—can be attributed to circulation, and concentration, of water. In the narrow view, water is the most critical resource required for human survival. In the wider view, it is a necessary component of many earth subsystems (Figure 4.2).

Most water exists, rich with salt, in the oceans, and many theorists agree that this has been the rule for nearly all of earth history. Theories proposing a hydrous mantle that slowly generated the ocean waters by gradual degassing do not resolve problems associated with retention of large amounts of water-bearing minerals within the host mantle. Water is continuously cycling through the shallow mantle (Figure 4.3). Lithospheric slabs plunge down into the mantle at subduction zones, notably along the deep oceanic trenches. These slabs, detected by instruments that sense density anomalies, contain water in the form of hydrous minerals—inorganic chemical compounds that incorporate the components of water. The volatile water returns to the surface in complex processes associated with subduction zones, resulting in volcanic arcs that arise where one ocean plate subducts beneath another ocean plate or in volcanic spines that run along the edge of a continent where an oceanic plate subducts beneath it. Certain ancient volcanoes, such as those found in the South African interior, generated material containing hydrous minerals that came from



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