Surface of the Earth

Geomorphology (from Greek: γῆ, ge, "earth"; μορφή, morfé, "form"; and λόγος, logos, "study") is the scientific study of landforms and the processes that shape them. Geomorphologists seek to understand why landscapes look the way they do, to understand landform history and dynamics, and to predict future changes through a combination of field observations, physical experiments, and numerical modeling. Geomorphology is practiced within physical geography, geology, geodesy, engineering geology, archaeology, and geotechnical engineering, and this broad base of interest contributes to a wide variety of research styles and interests within the field.

Overview[link]

The surface of Earth is modified by a combination of surface processes that sculpt landscapes, and geologic processes that cause tectonic uplift and subsidence. Surface processes comprise the action of water, wind, ice, fire, and living things on the surface of the Earth, along with chemical reactions that form soils and alter material properties, the stability and rate of change of topography under the force of gravity, and other factors, such as (in the very recent past) human alteration of the landscape. Many of these factors are strongly mediated by climate. Geologic processes include the uplift of mountain ranges, the growth of volcanoes, isostatic changes in land surface elevation (sometimes in response to surface processes), and the formation of deep sedimentary basins where the surface of Earth drops and is filled with material eroded from other parts of the landscape. The Earth surface and its topography therefore are an intersection of climatic, hydrologic, and biologic action with geologic processes.

The broad-scale topographies of Earth illustrate this intersection of surface and subsurface action. Mountain belts are uplifted due to geologic processes. Denudation of these high uplifted regions produces sediment that is transported and deposited elsewhere within the landscape or off the coast.^[1] On progressively smaller scales, similar ideas apply, where individual landforms evolve in response to the balance of additive processes (uplift and deposition) and subtractive processes (subsidence and erosion). Often, these processes directly affect each other: ice sheets, water, and sediment are all loads that change topography through flexural isostasy. Topography can modify the local climate, for example through orographic precipitation, which in turn modifies the topography by changing the hydrologic regime in which it evolves. Many geomorphologists are particularly interested in the potential for feedbacks between climate and tectonics mediated by geomorphic processes.^[2]

In addition to these broad-scale questions, geomorphologists address issues that are more specific and/or more local. Glacial geomorphologists investigate glacial deposits such as moraines, eskers, and proglacial lakes, as well as glacial erosional features, to build chronologies of both small glaciers and large ice sheets and understand their motions and effects upon the landscape. Fluvial geomorphologists focus on rivers, how they transport sediment, migrate across the landscape, cut into bedrock, respond to environmental and tectonic changes, and interact with humans. Soils geomorphologists investigate soil profiles and chemistry to learn about the history of a particular landscape and understand how climate, biota, and rock interact. Other geomorphologists study how hillslopes form and change. Still others investigate the relationships between ecology and geomorphology. Because geomorphology is defined to comprise everything related to the surface of Earth and its modification, it is a broad field with many facets.

Practical applications of geomorphology include hazard assessment (such as landslide prediction and mitigation), river control and stream restoration, and coastal protection.

History[link]

With some notable exceptions (see below), geomorphology is a relatively young science, growing along with interest in other aspects of the earth sciences in the mid-19th century. This section provides a very brief outline of some of the major figures and events in its development.

Ancient geomorphology[link]

The first theory of geomorphology was arguably devised by the polymath Chinese scientist and statesman Shen Kuo (1031-1095 AD). This was based on his observation of marine fossil shells in a geological stratum of a mountain hundreds of miles from the Pacific Ocean. Noticing bivalve shells running in a horizontal span along the cut section of a cliffside, he 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 and formed by soil erosion of the mountains and by deposition of silt, after observing strange natural erosions of the Taihang Mountains and the Yandang Mountain near Wenzhou. Furthermore, he promoted the theory of gradual climate change over centuries of time once ancient petrified bamboos were found to be preserved underground in the dry, northern climate zone of Yanzhou, which is now modern day Yan'an, Shaanxi province.

Early modern geomorphology[link]

The first use of the word geomorphology was likely to be in the German language when it appeared in Laumann's 1858 work. Keith Tinkler has suggested that the word came into general use in English, German and French after John Wesley Powell and W. J. McGee used it in the International Geological Conference of 1891.^[3]

An early popular geomorphic model was the geographical cycle or the cycle of erosion, developed by William Morris Davis between 1884 and 1899. The cycle was inspired by theories of uniformitarianism first formulated by James Hutton (1726–1797). Concerning valley forms, uniformitarianism depicted the cycle as a sequence in which a river cuts a valley more and more deeply, but then erosion of side valleys eventually flatten the terrain again, to a lower elevation. Tectonic uplift could start the cycle over. Many studies in geomorphology in the decades following Davis' development of his theories sought to fit their ideas into this framework for broad scale landscape evolution, and are often today termed "Davisian". Davis' ideas have largely been superseded today, mainly due to their lack of predictive power and qualitative nature, but he remains an extremely important figure in the history of the subject.

In the 1920s, Walther Penck developed an alternative model to Davis', believing that landform evolution was better described as a balance between ongoing processes of uplift and denudation, rather than Davis' single uplift followed by decay. However, due to his relatively young death, disputes with Davis and a lack of English translation of his work his ideas were not widely recognised for many years.

These authors were both attempting to place the study of the evolution of the Earth's surface on a more generalized, globally relevant footing than had existed before. In the earlier parts of the 19th century, authors - especially in Europe - had tended to attribute the form of landscape to local climate, and in particular to the specific effects of glaciation and periglacial processes. In contrast, both Davis and Penck were seeking to emphasize the importance of evolution of landscapes through time and the generality of Earth surface processes across different landscapes under different conditions.

Quantitative geomorphology[link]

While Penck and Davis and their followers were writing and studying primarily in Western Europe, another, largely separate, school of geomorphology was developed in the United States in the middle years of the 20th century. Following the early trailblazing work of Grove Karl Gilbert around the turn of the 20th century, a group of natural scientists, geologists and hydraulic engineers including Ralph Alger Bagnold, John Hack, Luna Leopold, Thomas Maddock and Arthur Strahler began to research the form of landscape elements such as rivers and hillslopes by taking systematic, direct, quantitative measurements of aspects of them and investigating the scaling of these measurements. These methods began to allow prediction of the past and future behavior of landscapes from present observations, and were later to develop into what the modern trend of a highly quantitative approach to geomorphic problems. Quantitative geomorphology can involve fluid dynamics and solid mechanics, geomorphometry, laboratory studies, field measurements, theoretical work, and full landscape evolution modeling. These approaches are used to understand weathering and the formation of soils, sediment transport, landscape change, and the interactions between climate, tectonics, erosion, and deposition.

Contemporary geomorphology[link]

Today, the field of geomorphology encompasses a very wide range of different approaches and interests. Modern researchers aim to draw out quantitative "laws" that govern Earth surface processes, but equally, recognize the uniqueness of each landscape and environment in which these processes operate. Particularly important realizations in contemporary geomorphology include:

1) that not all landscapes can be considered as either "stable" or "perturbed", where this perturbed state is a temporary displacement away from some ideal target form. Instead, dynamic changes of the landscape are now seen as an essential part of their nature.^[4][5]

2) that many geomorphic systems are best understood in terms of the stochasticity of the processes occurring in them, that is, the probability distributions of event magnitudes and return times.^[6] This in turn has indicated the importance of chaotic determinism to landscapes, and that landscape properties are best considered statistically.^[7] The same processes in the same landscapes does not always lead to the same end results.

Processes[link]

Grand Canyon, Arizona

Modern geomorphology focuses on the quantitative analysis of interconnected processes. Modern advances in geochronology, in particular cosmogenic radionuclide dating, optically stimulated luminescence dating and low-temperature thermochronology have enabled us for the first time to measure the rates at which geomorphic processes occur on geological timescales.^[8][9] At the same time, the use of more precise physical measurement techniques, including differential GPS, remotely sensed digital terrain models and laser scanning techniques, have allowed quantification and study of these processes as they happen.^[10] Computer simulation and modeling may then be used to test our understanding of how these processes work together and through time.

Geomorphically relevant processes generally fall into (1) the production of regolith by weathering and erosion, (2) the transport of that material, and (3) its eventual deposition. Although there is a general movement of material from uplands to lowlands, erosion, transport, and deposition often occur in closely spaced tandem all across the landscape.

The nature of the processes investigated by geomorphologists is strongly dependent on the landscape or landform under investigation and the time and length scales of interest. However, the following non-exhaustive list provides a flavor of the landscape elements associated with some of these.

Primary surface processes responsible for most topographic features include wind, waves, chemical dissolution, mass wasting, groundwater movement, surface water flow, glacial action, tectonism, and volcanism. Other more exotic geomorphic processes might include periglacial (freeze-thaw) processes, salt-mediated action, or extraterrestrial impact.

Fluvial processes[link]

The geomorphology of the large Pantanal wetland in South America is dominated by fluvial processes.

Rivers and streams are not only conduits of water, but also of sediment. The water, as it flows over the channel bed, is able to mobilize sediment and transport it downstream, either 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.^[11]

Rivers are also capable of eroding into rock and creating new sediment, both from their own beds and also by coupling to the surrounding hillslopes. In this way, rivers are thought of as setting the base level for large scale landscape evolution in nonglacial environments.^[12][13] Rivers are key links in the connectivity of different landscape elements.

As rivers flow across the landscape, they generally increase in size, merging with other rivers. The network of rivers thus formed is a drainage system and is often dendritic (tree-like), but may adopt other patterns depending on the regional topography and underlying geology.

Eolian processes[link]

Wind-eroded alcove near Moab, Utah

Eolian processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth. Winds 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, eolian processes are important in arid environments such as deserts.^[14]

Mesquite Flat Dunes in Death Valley looking toward the Cottonwood Mountains from the north west arm of Star Dune (2003)

Hillslope processes[link]

Example of mass wasting at Palo Duro Canyon, Texas

Soil, regolith, and rock move downslope under the force of gravity via creep, slides, flows, topples, and falls. Such mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, Mars, Venus, Titan and Iapetus.

Ongoing hillslope processes can change the topology of the hillslope surface, which in turn can change the rates of those processes. Hillslopes that steepen up to certain critical thresholds are capable of shedding extremely large volumes of material very quickly, making hillslope processes an extremely important element of landscapes in tectonically active areas.^[15]

On Earth, biological processes such as burrowing or tree throw may play important roles in setting the rates of some hillslope processes.^[16]

Glacial processes[link]

Features of a glacial landscape

Glaciers, while geographically restricted, are effective agents of landscape change. The gradual movement of ice down a valley causes abrasion and plucking of the underlying rock. Abrasion produces 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.^[17]

The way glacial processes interact with other landscape elements, particularly hillslope and fluvial processes, is an important aspect of Plio-Pleistocene landscape evolution and its sedimentary record in many high mountain environments. Environments that have been relatively recently glaciated but are no longer may still show elevated landscape change rates compared to those that have never been glaciated. Nonglacial geomorphic processes which nevertheless have been conditioned by past glaciation are termed paraglacial processes. This concept contrasts with periglacial processes, which are directly driven by formation or melting of ice or frost.^[18]

Tectonic processes[link]

Tectonic effects on geomorphology can range from scales of millions of years to minutes or less. The effects of tectonics on landscape are heavily dependent on the nature of the underlying bedrock fabric that more less controls what kind of local morphology tectonics can shape. Earthquakes can, in terms of minutes, submerge large areas of land creating new wetlands. Isostatic rebound can account for significant changes over thousand or hundreds of years, and allows 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 and thus long-term sediment production.

Features of deeper mantle dynamics such as plumes and delamination of the lower lithosphere have also been hypothesised to play important roles in the long term (> million year), large scale (thousands of km) evolution of the Earth's topography (see dynamic topography). Both can promote surface uplift through isostasy as hotter, less dense, mantle rocks displace cooler, denser, mantle rocks at depth in the Earth.^[19][20]

Igneous processes[link]

Some landscapes are dominated by igneous processes like Villarrica National Park in the picture.

Both volcanic (eruptive) and plutonic (intrusive) igneous processes can have important impacts on geomorphology. The action of volcanoes tends to rejuvenize landscapes, covering the old land surface with lava and tephra, releasing pyroclastic material and forcing rivers through new paths. The cones built by eruptions also build substantial new topography, which can be acted upon by other surface processes. 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.

Biological processes[link]

Beaver dams, as this one in Tierra del Fuego, constitute a specific form of zoogeomorphology, a type of biogeomorphology

The interaction of living organisms with landforms, or biogeomorphologic processes, can be of many different forms, and is probably 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, to even controlling global erosion rates through modulation of climate through carbon dioxide balance. 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.^[21]

Scales in geomorphology[link]

Different geomorphological processes dominate at different spatial and temporal scales. Moreover, scales on which processes occur may determine the reactivity or otherwise of landscapes to changes in driving forces such as climate or tectonics.^[22] These ideas are key to the study of geomorphology today.

To help categorize landscape scales some geomorphologists might use the following taxonomy:

• 1st - Continent, ocean basin, climatic zone (~10,000,000 km²)
• 2nd - Shield, e.g. Baltic Shield, or mountain range (~1,000,000 km²)
• 3rd - Isolated sea, Sahel (~100,000 km²)
• 4th - Massif, e.g. Massif Central or Group of related landforms, e.g., Weald (~10,000 km²)
• 5th - River valley, Cotswolds (~1,000 km²)
• 6th - Individual mountain or volcano, small valleys (~100 km²)
• 7th - Hillslopes, stream channels, estuary (~10 km²)
• 8th - gully, barchannel (~1 km²)
• 9th - Meter-sized features

Overlap with other fields[link]

There is a considerable overlap between geomorphology and other fields. Deposition of material is extremely important in sedimentology. Weathering is the chemical and physical disruption of earth materials in place on exposure to atmospheric or near surface agents, and is typically studied by soil scientists and environmental chemists, but is an essential component of geomorphology because it is what provides the material that can be moved in the first place. Civil and environmental engineers are concerned with erosion and sediment transport, especially related to canals, slope stability (and natural hazards), water quality, coastal environmental management, transport of contaminants, and stream restoration. Glaciers can cause extensive erosion and deposition in a short period of time, making them extremely important entities in the high latitudes and meaning that they set the conditions in the headwaters of mountain-born streams; glaciology therefore is important in geomorphology.

See also[link]

References[link]

• Edmaier, Bernhard (2004). Earthsong. London: Phaidon Press. ISBN 0-7148-4451-9. 
• Chorley, Richard J.; Stanley Alfred Schumm and David E. Sugden (1985). Geomorphology. London: Methuen. ISBN 0-416-32590-4. 
• Committee on Challenges and Opportunities in Earth Surface Processes, National Research Council (2010). Landscapes on the Edge: New Horizons for Research on Earth's Surface. Washington, DC: National Academies Press. ISBN 0-309-14024-2. 
• Selby, Michael John (1985). Earth's changing surface: an introduction to geomorphology. Oxford: Clarendon Press. ISBN 0-19-823252-7. 
• Kondolf, G. Mathias; Hervé Piégay (2003). Tools in fluvial geomorphology. New York: Wiley. ISBN 0-471-49142-X. 
• Needham, Joseph (1954). Science and civilisation in China. Cambridge, UK: Cambridge University Press. ISBN 0-521-05801-5. 
• Scheidegger, Adrian E. (2004). Morphotectonics. Berlin: Springer. ISBN 3-540-20017-7. 
• Selby, Michael John (1985). Earth's changing surface: an introduction to geomorphology. Oxford: Clarendon Press. ISBN 0-19-823252-7. 

External links[link]

• International Association of Geomorphologists
• Geomorphology in the Association of American Geographers
• British Society for Geomorphology
• Association of Polish Geomorphologists
• German Geomorphologists Group (Deutscher Arbeitskreis fuer Geomorphologie
• Model of landscape evolution by William Morris Davis (by GEOMORPHLIST)
• The Geographical Cycle, or the Cycle of Erosion (1899)
• Geomorphology from Space (by NASA)
• USDA-NRCS Web Soil Survey Survey of surficial geologic deposits and geomorphology across the U.S.
• The American Geophysical Union Earth and Planetary Surface Processes focus group

v t e Subfields of physical geography Biogeography Climatology / Paleoclimatology Coastal geography Edaphology Geomorphology Glaciology Hydrology / Hydrography Landscape ecology Limnology Oceanography Palaeogeography Pedology/Soil science Quaternary science

v t e Geologic Principles & Processses Stratigraphic Principles Principle of original horizontality Law of superposition Principle of lateral continuity Principle of cross-cutting relationships Principle of faunal succession Principle of inclusions and components Walther's law Petrologic Principles Intrusive Extrusive Exfoliation Weathering Pedogenesis Diagenesis Compaction Metamorphism Geomorphologic Processes Plate tectonics Salt tectonics Tectonic uplift Subsidence Marine transgression Marine regression Sediment transport Fluvial processes Aeolian processes Glacial processes Mass wasting processes

Arthur Holmes
Arthur Holmes around age 22
Born	14 January 1890
Died	20 September 1965
Nationality	British
Institutions	Durham University (1924-1942), Edinburgh University (1943-1956)
Alma mater	Imperial College

Arthur Holmes (14 January 1890 – 20 September 1965) was a British geologist. As a child he lived in Low Fell, Gateshead and attended the Gateshead Higher Grade School (later Gateshead Grammar School).^[1]

Age of the earth[link]

Holmes was a pioneer of geochronology, and performed the first uranium-lead radiometric dating (specifically designed to measure the age of a rock) while an undergraduate at the Royal College of Science (now Imperial College) in London, assigning an age of 370 Ma to a Devonian rock from Norway. This result was published in 1911,^[2] after his graduation in 1910. By 1911 he had already spent six months in Mozambique prospecting for minerals. While abroad he had contracted blackwater fever and malaria so severe that a note of his death was sent home by telegraph. However, he returned home and recovered – though suffering life-long recurrences of the illness.

1912 saw Holmes on the staff of Imperial College, publishing his famous booklet The Age of the Earth in 1913 (he estimated the Earth's age to be 1,600 Ma). He obtained his doctorate (of Science) in 1917 and in 1920 joined an oil company in Burma as chief geologist. The company failed, and he returned to England penniless in 1924. He had been accompanied in Burma by his three-year-old son, who contracted dysentery and died shortly before Holmes’s departure.

In 1924 he was appointed to the newly-created post of reader in geology at Durham University. Eighteen years later his achievements were recognised, when he became a Fellow of the Royal Society in 1942. In the following year he was appointed to the chair of geology at Edinburgh University, which he held until retirement in 1956.

Continental drift[link]

Spreading at a mid-ocean ridge

Holmes championed the theory of continental drift promoted by Alfred Wegener at a time when it was deeply unfashionable with his more conservative peers. One problem with the theory lay in the mechanism of movement, and Holmes proposed that Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface. His second famous book Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.^[3] Part of the model was the origin of the seafloor spreading concept.^[4][5] His later measurements of the age of the Earth (4,500±100 Ma) were based on measurements of the relative abundance of uranium isotopes by Alfred O. C. Nier.

He was awarded both the Wollaston Medal and the Penrose Medal in 1956. The Arthur Holmes Medal of the European Geosciences Union is named after him.

A crater on Mars has been named in his honour.

The Durham University Department of Earth Sciences' Arthur Holmes "Isotope Geology Laboratory". http://www.dur.ac.uk/geochem.www/group/arthurholmes.htm Arthur Holmes.  is named after him, as is the students' Geology Society.

Notes[link]

1. ^ "Commemorative Plaques in Gateshead Borough". http://www.bpears.org.uk/Misc/Gateshead_Plaques/#HolmesA. 
2. ^ Holmes, Arthur (9 June 1911). The association of lead with uranium in rock-minerals and its application to the measurement of geological time. Proceedings of the Royal Society, Series A 85: 248–256. 
3. ^ Holmes, Arthur (1944). Principles of Physical Geology (1 ed.). Edinburgh: Thomas Nelson & Sons. ISBN 0-17-448020-2. 
4. ^ Wessel, P.; Müller, R. D. (2007). "Plate Tectonics". Treatise on Geophysics. 6. Elsevier. pp. 49–98. 
5. ^ Vine, F. J. (16 December 1966). "Spreading of the Ocean Floor: New Evidence". Science 154 (3755): 1405–1415. DOI:10.1126/science.154.3755.1405. PMID 17821553. http://www.es.ucsc.edu/~rcoe/eart206/Vine_SFspreading_Science06.pdf. 

References[link]

• Lewis, Cherry (2000). The Dating Game: One Man's Search for the Age of the Earth. Cambridge University Press. ISBN 0-521-89312-7. 

External links[link]

• Geological Society of America: short biography
• Another short biography
• Arthur Holmes Geological Society