Erosion is the process of moving rock and soil from one location to another, operating across a wide spectrum of time. Whether erosion takes a long time depends entirely on the forces and environment involved. Some landscapes are shaped by processes requiring millions of years to show measurable change, while others can be altered in a matter of hours. Understanding the rate of landscape change requires appreciating the difference between the slow, steady natural background and the speed of catastrophic events and human influence.
Erosion on Geological Time Scales
The common perception of erosion is rooted in the slow, persistent change known as background erosion. This process sets a baseline rate for the planet’s natural denudation. These rates are often so minimal they are difficult to measure without specialized equipment that integrates data over long periods.
For instance, the wearing down of highly resistant rock like granite can proceed at rates of less than one millimeter per thousand years. Chemical weathering, where water and atmospheric components dissolve rock minerals, can take millions of years to fully transform a landscape, such as the gradual dissolution of limestone formations.
Natural Variables That Accelerate Rates
While a geological timescale is the norm, specific natural conditions can accelerate the rate at which landscapes change. The composition of the underlying rock, or substrate, is a major factor, as soft sedimentary materials erode much faster than crystalline rock. Soft shale or jointed sandstone can lose large volumes of material during a single intense storm, while hard granite may only lose the thickness of a credit card over a millennium. Climate also dictates the pace, with warm, humid environments accelerating chemical weathering, and cold climates favoring physical processes like freeze-thaw cycles, where water expands upon freezing. Topography plays an equally significant role, as steep slopes increase the velocity and force of water runoff, which can increase erosion rates compared to low-relief plateaus in the same region.
Rapid Erosion and Human Land Use
The fastest rates of erosion occur when natural forces or human activity overwhelm a landscape’s ability to resist change, shortening the process from centuries to moments. Catastrophic events such as landslides and debris flows move massive volumes of rock and soil instantaneously. A debris flow, a slurry of water, mud, and boulders, can travel down a steep drainage at speeds exceeding 35 miles per hour.
The single greatest accelerator of erosion globally is anthropogenic activity. Intensive agriculture and deforestation are particularly destructive, as they strip vegetation and remove the anchoring root systems that stabilize soil. Studies show that human activities have increased the world’s erosion rate by an estimated 10 to 50 times the natural background rate. Erosion rates on conventionally plowed agricultural land can be 10 to 100 times faster than the natural rate of soil formation. For instance, humans in North America have mobilized as much sediment in the last century as natural processes would have moved in 700 to 3,000 years.
Quantifying Erosion Speed
Scientists use specific metrics and techniques to quantify how quickly erosion is occurring across different timescales. For long-term geological rates, researchers often use cosmogenic radionuclides. These are isotopes produced when cosmic rays strike minerals in the rock surface, allowing scientists to determine the average rate at which the land surface has been lowered over thousands to millions of years.
For studies focused on soil loss and human impact, erosion speed is commonly measured in units of mass per area per time, such as tons per hectare per year (T/ha/yr). Predictive models, like the Universal Soil Loss Equation, use these units and various environmental factors to estimate the average annual soil loss from a particular field. For short-term tracking, methods such as sediment traps and erosion pins provide a direct measurement of material loss or surface lowering over periods ranging from a single season to a few decades.