Geomorphology: the story on HearLore

Geomorphology

In the 5th century BC, a Greek historian named Herodotus stood on the banks of the Nile and made a startling claim that would echo through millennia. He argued that the Nile delta was not a static feature but was actively growing into the Mediterranean Sea, a dynamic process of land creation that challenged the static view of the world held by his contemporaries. This early observation laid the groundwork for a field that would eventually seek to understand the origin and evolution of every topographic and bathymetric feature on Earth. Geomorphology is the scientific study of how physical, chemical, and biological processes shape the surface of our planet, from the deepest ocean trenches to the highest mountain peaks. It is a discipline that bridges the gap between the slow, grinding forces of geology and the rapid, visible changes of weather and life. Geomorphologists do not merely describe what the landscape looks like; they strive to understand why it looks that way, predicting future changes through a combination of field observations, physical experiments, and numerical modeling. This work requires a synthesis of knowledge from physical geography, geology, geodesy, engineering, archaeology, and climatology, creating a broad base of interests that fuels diverse research styles. The Earth's surface is a constant intersection of the lithosphere with the hydrosphere, atmosphere, and biosphere, a dynamic interface where tectonic uplift meets the erosive power of water, wind, and ice. Mountains are uplifted by geologic processes, only to be worn down by denudation, creating sediment that is transported and deposited elsewhere, often to form deep sedimentary basins. This cycle of addition and subtraction defines the history of the terrain, where ice sheets, water, and sediment act as loads that change topography through flexural isostasy. Topography itself modifies the local climate, creating feedback loops where orographic precipitation changes the hydrologic regime, which in turn modifies the topography. These complex interactions mean that the landscape is never truly stable, but is in a state of perpetual flux, shaped by the balance of additive processes like uplift and subtractive processes like erosion.

Ancient Observations

Long before the term geomorphology was coined, scholars across the globe were piecing together the history of the Earth's surface through keen observation and bold hypothesis. In the 4th century BC, the Greek philosopher Aristotle speculated that the seas would eventually fill with sediment while the land lowered, causing land and water to swap places in an endless cycle. This idea was echoed centuries later in the 10th century by the Encyclopedia of the Brethren of Purity, which discussed the cyclical changing positions of land and sea. The medieval Persian Muslim scholar Abū Rayhān al-Bīrūnī, observing rock formations at the mouths of rivers, hypothesized that the Indian Ocean once covered all of India, a radical suggestion for his time. In China, the Song dynasty scientist Shen Kuo made a discovery that would fundamentally alter the understanding of geological time. He found marine fossil shells in a geological stratum of a mountain hundreds of miles from the Pacific Ocean and theorized that the cliff was once the pre-historic location of a seashore that had shifted hundreds of miles over the centuries. He inferred that the land was reshaped by soil erosion and the deposition of silt, observing strange natural erosions of the Taihang Mountains and the Yandang Mountain near Wenzhou. Shen Kuo also promoted the theory of gradual climate change over centuries of time after ancient petrified bamboos were found preserved underground in the dry, northern climate zone of Yanzhou. Even earlier, the scholar-official Du Yu of the Western Jin dynasty predicted that two monumental stelae, one buried at the foot of a mountain and the other erected at the top, would eventually change their relative positions over time. These ancient insights, from the Greek historians to the Chinese statesmen, formed the observational foundation for a science that would not be formally named until the 19th century. The term geomorphology seems to have been first used by Laumann in an 1858 work written in German, but it was John Wesley Powell and W. J. McGee who brought it into general use during the International Geological Conference of 1891. The field grew from the union of geology and geography, evolving from a descriptive study of scenery into a rigorous analytical science.

Common questions

What is the definition of geomorphology and what processes does it study?

Geomorphology is the scientific study of how physical, chemical, and biological processes shape the surface of our planet. It examines features ranging from the deepest ocean trenches to the highest mountain peaks through the interaction of the lithosphere with the hydrosphere, atmosphere, and biosphere.

Who first used the term geomorphology and when was it introduced to the field?

The term geomorphology seems to have been first used by Laumann in an 1858 work written in German. It was brought into general use by John Wesley Powell and W. J. McGee during the International Geological Conference of 1891.

What was the geographical cycle model developed by William Morris Davis and when was it created?

The geographical cycle or cycle of erosion model was developed by William Morris Davis between 1884 and 1899. This theory posited that tectonic uplift creates a river valley that gradually erodes until side valleys flatten the terrain, after which uplift starts the cycle over again.

When did the quantitative revolution in geomorphology begin and who were the key figures involved?

A revolution in geomorphology began in the middle of the 20th century when the field was put on a solid quantitative footing. Key figures included William Walden Rubey, Ralph Alger Bagnold, Hans Albert Einstein, Frank Ahnert, John Hack, Luna Leopold, A. Shields, Thomas Maddock, Arthur Strahler, Stanley Schumm, and Ronald Shreve.

What are the primary agents of erosion and landscape change described in the text?

The primary agents include aeolian processes from wind, rivers and streams that transport sediment, and glaciers that cause abrasion and plucking. Hillslope processes move soil and rock downslope under gravity, while biogeomorphologic processes involve living organisms influencing surface changes.

How does geomorphology apply to the study of other planets and what specific examples are mentioned?

Planetary geomorphology studies the surfaces of other terrestrial planets such as Mars, Venus, Titan, and Iapetus using Earth analogues. Indications of wind, fluvial, glacial, mass wasting, meteor impact, tectonics, and volcanic processes are analyzed to understand the geologic and atmospheric history of those planets.

The Cycle of Erosion

The early 20th century was dominated by the geographical cycle or cycle of erosion model, a broad-scale landscape evolution theory developed by William Morris Davis between 1884 and 1899. This model was an elaboration of the uniformitarianism theory proposed by James Hutton, which posited that a river runs through flat terrain, gradually carving an increasingly deep valley until side valleys erode and flatten the terrain again at a lower elevation. Davis believed that tectonic uplift could then start the cycle over, creating a predictable sequence of landscape development. For decades, geomorphologists sought to fit their findings into this framework, known today as Davisian, and the model became a central pillar of the discipline. However, the Davisian model has been largely superseded today, mainly due to its lack of predictive power and qualitative nature. In the 1920s, Walther Penck developed an alternative model that challenged Davis's rigid sequence. Penck thought that landform evolution was better described as an alternation between ongoing processes of uplift and denudation, rather than a single uplift followed by decay. He emphasized that slope evolution occurs by backwearing of rocks, not by Davisian-style surface lowering, and his science tended to emphasize surface process over understanding in detail the surface history of a given locality. Penck was German, and during his lifetime his ideas were at times rejected vigorously by the English-speaking geomorphology community. His early death, Davis's dislike for his work, and his at-times-confusing writing style likely all contributed to this rejection. Despite the decline of the cycle of erosion model in mainstream geomorphology, it remains part of the science, never proven wrong but neither proven, and is still used to establish denudation chronologies in historical geology. Modern geomorphologists Andrew Goudie and Karna Lidmar-Bergström have praised the model for its elegance and pedagogical value, acknowledging its enduring influence even as the field moved toward more dynamic and quantitative approaches.

The Quantitative Turn

A revolution in geomorphology began in the middle of the 20th century when the field was put on a solid quantitative footing. Following the early work of Grove Karl Gilbert around the turn of the 20th century, a group of American natural scientists, geologists, and hydraulic engineers began to research the form of landscape elements by taking systematic, direct, quantitative measurements. This group included William Walden Rubey, Ralph Alger Bagnold, Hans Albert Einstein, Frank Ahnert, John Hack, Luna Leopold, A. Shields, Thomas Maddock, Arthur Strahler, Stanley Schumm, and Ronald Shreve. They investigated the scaling of measurements to allow the prediction of the past and future behavior of landscapes from present observations. These methods developed into the modern trend of a highly quantitative approach to geomorphic problems, involving fluid dynamics, solid mechanics, geomorphometry, laboratory studies, and full landscape evolution modeling. In Sweden, Filip Hjulström's doctoral thesis, The River Fyris, published in 1935, contained one of the first quantitative studies of geomorphological processes ever published. His students followed in the same vein, making quantitative studies of mass transport, fluvial transport, delta deposition, and coastal processes, which developed into the Uppsala School of Physical Geography. This quantitative revolution allowed geomorphologists to move beyond descriptive accounts and develop laws that govern Earth surface processes. Today, researchers aim to draw out these quantitative laws while recognizing the uniqueness of each landscape. They understand that many geomorphic systems are best understood in terms of the stochasticity of the processes occurring in them, meaning the probability distributions of event magnitudes and return times. This has indicated the importance of chaotic determinism to landscapes, where landscape properties are best considered statistically. The same processes in the same landscapes do not always lead to the same end results, a realization that has shifted the field from seeking a single ideal target form to accepting dynamic changes as an essential part of the landscape's nature.

Winds and Waters

The forces that shape the Earth's surface are diverse and powerful, ranging from the slow grinding of glaciers to the sudden impact of a meteor. Aeolian processes pertain to the activity of the winds, which may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of fine, unconsolidated sediments. Although water and mass flow tend to mobilize more material than wind in most environments, aeolian processes are crucial in arid environments such as deserts, creating features like the Seif and barchan dunes found on the surface of Mars. Rivers and streams are not only conduits of water but also of sediment, mobilizing it as bed load, suspended load, or dissolved load. The rate of sediment transport depends on the availability of sediment itself and on the river's discharge. Rivers are capable of eroding into rock and forming new sediment, setting the base level for large-scale landscape evolution in nonglacial environments. As rivers flow across the landscape, they merge to form drainage systems with four general patterns: dendritic, radial, rectangular, and trellis. Dendritic systems are the most common, occurring when the underlying stratum is stable without faulting. Glaciers, while geographically restricted, are effective agents of landscape change, causing abrasion and plucking of the underlying rock to produce fine sediment termed glacial flour. The debris transported by the glacier, when the glacier recedes, is termed a moraine. Glacial erosion is responsible for U-shaped valleys, as opposed to the V-shaped valleys of fluvial origin. Hillslope processes move soil, regolith, and rock downslope under the force of gravity via creep, slides, flows, topples, and falls, occurring on both terrestrial and submarine slopes. These processes can change the topology of the hillslope surface, which in turn can change the rates of those processes, making hillslope processes an extremely important element of landscapes in tectonically active areas.

Life and Stone

The interaction of living organisms with landforms, or biogeomorphologic processes, is of profound importance for the terrestrial geomorphic system as a whole. Biology can influence very many geomorphic processes, ranging from biogeochemical processes controlling chemical weathering to the influence of mechanical processes like burrowing and tree throw on soil development. Terrestrial landscapes in which the role of biology in mediating surface processes can be definitively excluded are extremely rare, but may hold important information for understanding the geomorphology of other planets, such as Mars. The beaver dams found in Tierra del Fuego constitute a specific form of zoogeomorphology, a type of biogeomorphology that alters the landscape by creating wetlands and changing water flow. Volcanic and plutonic igneous processes can have important impacts on geomorphology, with the action of volcanoes tending to rejuvenize landscapes by covering the old land surface with lava and tephra, releasing pyroclastic material, and forcing rivers through new paths. Plutonic rocks intruding then solidifying at depth can cause both uplift or subsidence of the surface, depending on whether the new material is denser or less dense than the rock it displaces. Tectonic effects on geomorphology can range from scales of millions of years to minutes or less, with earthquakes capable of submerging large areas of land forming new wetlands in minutes. Isostatic rebound can account for significant changes over hundreds to thousands of years, allowing erosion of a mountain belt to promote further erosion as mass is removed from the chain and the belt uplifts. Long-term plate tectonic dynamics give rise to orogenic belts, large mountain chains with typical lifetimes of many tens of millions of years, which form focal points for high rates of fluvial and hillslope processes. Features of deeper mantle dynamics such as plumes and delamination of the lower lithosphere have also been hypothesized to play important roles in the long-term evolution of the Earth's topography, promoting surface uplift through isostasy as hotter, less dense, mantle rocks displace cooler, denser, mantle rocks at depth.