SIDEBAR 1–1 The Critical Zone: Earth’s Surface and Near-Surface Environment
The surface and near-surface environment sustains nearly all terrestrial life. The rapidly expanding needs of society give special urgency to understanding the processes that operate within this Critical Zone (see figure below). Population growth and industrialization are putting pressure on the development and sustainability of natural resources such as soil, water, and energy. Human activities are increasing the inventory of toxins in the air, water, and land, and are driving changes in climate and the associated water cycle. An increasing portion of the population is at risk from landslides, flooding, coastal erosion, and other natural hazards.
The Critical Zone is a dynamic interface between the solid Earth and its fluid envelopes, governed by complex linkages and feedbacks among a vast range of physical, chemical, and biological processes. These processes can be organized into four main categories: (1) tectonics driven by energy in the mantle, which modifies the surface by magmatism, faulting, uplift, and subsidence; (2) weathering driven by the dynamics of the atmosphere and hydrosphere, which controls soil development, erosion, and the chemical mobilization of near-surface rocks; (3) fluid transport driven by pressure gradients, which shapes landscapes and redistributes materials; and (4) biological activity