Physical processes create, maintain, and modify Earth’s physical features and environments. Because the physical environment is the essential background for all human activity on Earth, the geographically informed person must understand the processes that produce these features.

Physical processes can be grouped into four categories: those operating in the atmosphere (i.e., climate and meteorology), those operating in the lithosphere (e.g., plate tectonics, erosion, and soil formation), those operating in the hydrosphere (e.g., the circulation of the oceans and the hydrologic cycle), and those operating in the biosphere (e.g., plant and animal communities and ecosystems).

By understanding the interactions within and between these categories of physical processes, the geographically informed person can pose and answer certain fundamental questions: What does the surface of Earth look like? How have its features been formed? What is the nature of these features and how do they interact? How and why are they changing? What are the spatially distinct combinations of environmental features? How are these environmental features related to past, present, and prospective human uses of Earth? The answers to these questions lead to an understanding of how Earth serves as the home of all plants and animals, including humans.

Processes shape and maintain the physical environment. Therefore it is vital that students appreciate the complex relationships between processes and resultant features, and how these relationships give rise to patterns of spatial organizations. For example, in a region such as southern California, the physical landscape is constantly reshaped by a complex set of interacting physical processes: earthquakes, coastal erosion, land subsidence owing to subsurface oil and water extraction, flash floods and landslides caused by heavy rainfall in the spring, and drought and the loss of chaparral vegetation from fire in the dry summer weeks. In turn, these processes show chains of interaction; the chaparral vegetation is the biosphere’s response to the climate and soil. Given the expected variations in rainfall in this Mediterranean climate regime, the chaparral becomes dormant and is prone to fire; however, clearance of the chaparral vegetation, especially in the canyons of steep hills, exposes the surface to flash flooding and soil erosion.

Five basic ideas help to explain the interactions and effects of physical processes. These are known as system, boundary, force, state of equilibrium, and threshold. A system is a collection of elements that are mutually connected and therefore influence one another to form a unified whole (e.g., the hydrologic cycle). Each system has boundaries, either real or arbitrary, within which it operates. Some forces, such as gravity and weather, activate and drive processes; other forces, such as friction, resist change and act to maintain the status quo. Systems exist in different states. When a system is in equilibrium, driving forces such as gravity and resisting forces such as friction are in balance. However, when a threshold—the point at which change may occur—is reached adjustment takes place. For example, an avalanche occurs when gravity, acting on deep layers of snow, overcomes the friction that was holding the snow mass in place (i.e., a state of equilibrium gives way when a threshold is reached). After the avalanche a new state of equilibrium is established.

It is essential that students understand the physical processes that act upon Earth and that such processes affect the choices made by people in different regions of the United States. Knowledge of these processes is required for dealing with such commonplace issues as: evaluating locations of relative safety in an earthquake-prone region; purchasing a home in a flood plain; coping with the threat of sinkholes and subsidence in a landscape underlined by limestone deposits; building a house in an area that has shrink-swell clay soils.

It is also essential that students learn to make intelligent predictions about future events and evaluate the short- and long-term effects of physical events on places and regions. Evaluating reports of world climate change requires knowing the factors that affect climate and weather in general and how the natural environment functions in particular regions. Climate and weather affect more than just personal decision-making on a daily basis. They are major factors in understanding world economic conditions over longer periods. Many important natural resources are formed by physical processes that occur in relatively few places on Earth. Understanding physical processes and the patterns of resources they produce is vital to understanding not only the physical geography of Earth’s surface but also the strategic relationships between nations and world trade patterns.

Understanding physical processes enables the geographically informed person to link the personal with the societal, the short term with the long term, and the local with the global dimensions of Earth.