What Are Geological Processes?

Geological processes, also written geologic processes, are the physical and chemical mechanisms that create, modify, and destroy Earth materials and landforms over time scales ranging from seconds to billions of years. A U.S. Geological Survey inventory of geologic processes covers tectonic movement, volcanic activity, earthquakes, erosion, glaciation, sediment transport and sedimentation, metamorphism, diagenesis, hydrothermal circulation, land subsidence, liquefaction, soil formation, and mass wasting. Geology organizes these mechanisms by their energy source, which separates internal, or endogenic, processes driven by Earth's internal heat and gravitational differentiation from external, or exogenic, processes driven by solar energy, gravity acting at the surface, and the water cycle.

The two classes act in opposition. Internal processes build relief by thickening crust and adding volcanic material, while external processes level it by breaking rock down and moving the debris toward basins. The rock cycle formalizes that exchange, describing how igneous, sedimentary, and metamorphic rocks convert into one another through melting, burial, heating, uplift, and erosion.

Tectonic and Internal Processes

Plate tectonics supplies the framework for internal processes. Lithospheric plates move at rates of a few centimeters per year, driven principally by slab pull at convergent margins and ridge push at spreading centers. Subduction, in which a denser oceanic plate descends beneath an adjacent plate, generates the deepest earthquakes recorded, produces arc volcanism as water released from the sinking slab lowers the melting point of the overlying mantle, and recycles crustal material into the mantle. Divergent boundaries create new oceanic lithosphere, and transform boundaries accommodate lateral slip. Regional metamorphism, batholith emplacement, and isostatic adjustment follow from the same motions.

Weathering and Erosion

At the surface, weathering breaks rock down in place. Mechanical weathering by frost wedging, thermal cycling, salt crystallization, and root growth fractures rock without changing its composition, while chemical weathering by hydrolysis, oxidation, dissolution, and carbonic acid attack converts primary minerals into clays and soluble ions. Chemical weathering of silicate rock is also a long-term sink for atmospheric carbon dioxide, which links the process to climate over geologic time. Erosion then removes the products by flowing water, wind, ice, and gravity-driven mass wasting, the category that includes rockfalls, debris flows, and landslides. Rates depend strongly on climate, relief, lithology, and vegetation cover.

Sedimentation and Deposition

Eroded material travels as bedload, suspended load, and dissolved load until transport energy falls below what is needed to carry it. Sedimentation, the resulting deposition, sorts grains by size and density and builds alluvial fans, deltas, floodplains, dune fields, and deep-sea turbidite sequences. Burial then initiates diagenesis, in which compaction and cementation convert loose sediment into rock, and continued burial passes into metamorphism. The layered products record environmental conditions at the time of deposition, which is the basis of stratigraphy and of the reconstruction of past climate. Educational material from the USGS geology program covers these relationships along with the plate tectonic setting that controls where basins form.

Observation and Measurement

Quantifying process rates is a measurement and instrumentation problem. Seismic networks locate and characterize earthquakes, continuous GNSS stations resolve plate motion to the millimeter, and interferometric synthetic aperture radar maps ground deformation from volcanic inflation, fault slip, subsidence, and landslide creep over wide areas. Airborne and satellite lidar produce repeat topography for erosion and deposition budgets, and multispectral and thermal imaging from platforms documented by NASA's Earth Observatory tracks surface change. Cosmogenic nuclide and radiometric dating supply the absolute time control that turns observed change into a rate.

Applications

Understanding geological processes supports work in several fields, including:

• Seismic and volcanic hazard assessment and early warning
• Landslide and coastal erosion risk mapping for infrastructure siting
• Mineral, groundwater, and hydrocarbon exploration
• Geotechnical engineering for foundations, tunnels, and dams
• Carbon storage and nuclear waste repository site evaluation
• Planetary geology using remote sensing of Mars, the Moon, and icy satellites