Geomorphology

Geomorphology is the scientific study of the origin and evolution of

topographic and bathymetric features created by physical, chemical or
biological processes operating at or near the Earth's surface. Geomorphologists
seek to understand why landscapes look the way they do, to understand
landform and terrain history and dynamics and to predict changes through a
combination of field observations, physical experiments and numerical
modeling. Geomorphologists work within disciplines such as physical
geography, geology, geodesy, engineering geology, archaeology, climatology
and geotechnical engineering. This broad base of interests contributes to many
research styles and interests within the field.

Overview
Earth's surface is modified by a combination of surface processes that shape
landscapes, and geologic processes that cause tectonic uplift and subsidence,
and shape the coastal geography. 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 the Earth drops
and is filled with material eroded from other parts of the landscape. The
Earth's surface and its topography therefore are an intersection of climatic,
hydrologic, and biologic action with geologic processes, or alternatively stated,
the intersection of the Earth's lithosphere with its hydrosphere, atmosphere,
and biosphere.

The broad-scale topographies of the 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. 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.

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 understands 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 hill slopes form and change. Still others investigate the relationships
between ecology and geomorphology. Because geomorphology is defined to
comprise everything related to the surface of the Earth and its modification, it
is a broad field with many facets.

Geomorphologists use a wide range of techniques in their work. These may

include fieldwork and field data collection, the interpretation of remotely
sensed data, geochemical analyses, and the numerical modeling of the physics
of landscapes. Geomorphologists may rely on geochronology, using dating
methods to measure the rate of changes to the surface. Terrain measurement
techniques are vital to quantitatively describe the form of the Earth's surface,
and include differential GPS, remotely sensed digital terrain models and laser
scanning, to quantify, study, and to generate illustrations and maps.

Practical applications of geomorphology include hazard assessment (such as

landslide prediction and mitigation), river control and stream restoration, and
coastal protection. Planetary geomorphology studies landforms on other
terrestrial planets such as Mars. Indications of effects of wind, fluvial, glacial,
mass wasting, meteor impact, tectonics and volcanic processes are studied.
This effort not only helps better understand the geologic and atmospheric
history of those planets but also extends Geomorphological study of the Earth.
Planetary geomorphologists often use Earth analogues to aid in their study of
surfaces of other planets.

History
Other than some notable exceptions in antiquity, 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
The study of landforms and the evolution of the Earth's surface can be dated
back to scholars of Classical Greece. Herodotus argued from observations of
soils that the Nile delta was actively growing into the Mediterranean Sea, and
estimated its age.[10] Aristotle speculated that due to sediment transport into
the sea, eventually those seas would fill while the land lowered. He claimed
that this would mean that land and water would eventually swap places,
whereupon the process would begin again in an endless cycle.

Another early theory of geomorphology was devised by the polymath Chinese

scientist and statesman Shen Kuo (1031–1095). 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 cliff side, he theorized that the cliff
was once the pre-historic location of 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

The term geomorphology seems to have been first used by Laumann in an
1858 work written in German. 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 during the International Geological Conference of 1891.
John Edward Marr in his The Scientific Study of Scenery considered his book as,
'an Introductory Treatise on Geomorphology, a subject which has sprung from
the union of Geology and Geography'.

An early popular geomorphic model was the geographical cycle or cycle of

erosion model of broad-scale landscape evolution developed by William Morris
Davis between 1884 and 1899. It was an elaboration of the uniformitarianism
theory that had first been proposed by James Hutton (1726–1797).[16] With
regard to valley forms, for example, uniformitarianism posited a sequence in
which a river runs through a flat terrain, gradually carving an increasingly deep
valley, until the side valleys eventually erode, flattening the terrain again,
though at a lower elevation. It was thought that tectonic uplift could then start
the cycle over. In the decades following Davis's development of this idea, many
of those studying geomorphology sought to fit their findings into this
framework, known today as "Davisian". Davis's ideas are of historical
importance, but have been largely superseded today, mainly due to their lack
of predictive power and qualitative nature.

In the 1920s, Walther Penck developed an alternative model to Davis's. Penck

thought that landform evolution was better described as an alternation
between ongoing processes of uplift and denudation, as opposed to Davis's
model of a single uplift followed by decay. He also emphasized that in many
landscapes 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' dislike
for his work, and his at-times-confusing writing style likely all contributed to
this rejection.

Both Davis and Penck were trying to place the study of the evolution of the
Earth's surface on a more generalized, globally relevant footing than it had
been previously. In the early 19th century, authors – especially in Europe – had
tended to attribute the form of landscapes to local climate, and in particular to
the specific effects of glaciations 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 the Earth's surface processes
across different landscapes under different conditions.

During the early 1900s, the study of regional-scale geomorphology was termed
"physiography". Physiography later was considered to be a contraction of
"physical" and "geography", and therefore synonymous with physical
geography and the concept became embroiled in controversy surrounding the
appropriate concerns of that discipline. Some geomorphologists held to a
geological basis for physiography and emphasized a concept of physiographic
regions while a conflicting trend among geographers was to equate
physiography with "pure morphology", separated from its geological heritage.
In the period following World War II, the emergence of process, climatic, and
quantitative studies led to a preference by many earth scientists for the term
"geomorphology" in order to suggest an analytical approach to landscapes
rather than a descriptive one.

Climatic geomorphology
During the age of New Imperialism in the late 19th century European explorers
and scientists traveled across the globe bringing descriptions of landscapes and
landforms. As geographical knowledge increased over time these observations
were systematized in a search for regional patterns. Climate emerged thus as
prime factor for explaining landform distribution at a grand scale. The rise of
climatic geomorphology was foreshadowed by the work of Wladimir Köppen,
Vasily Dokuchaev and Andreas Schimper. William Morris Davis, the leading
geomorphologist of his time, recognized the role of climate by complementing
his "normal" temperate climate cycle of erosion with arid and glacial ones.
Nevertheless, interest in climatic geomorphology was also a reaction against
Davisian geomorphology that was by the mid-20th century considered both
un-innovative and dubious. Early climatic geomorphology developed primarily
in continental Europe while in the English-speaking world the tendency was
not explicit until L.C. Peltier's 1950 publication on a periglacial cycle of erosion.

Climatic geomorphology was criticized in a 1969 review article by process

geomorphologist D.R. Stoddart. The criticism by Stoddart proved "devastating"
sparking a decline in the popularity of climatic geomorphology in the late 20th
century. Stoddart criticized climatic geomorphology for applying supposedly
"trivial" methodologies in establishing landform differences between
morphoclimatic zones, being linked to Davisian geomorphology and by
allegedly neglecting the fact that physical laws governing processes are the
same across the globe. In addition some conceptions of climatic
geomorphology, like that which holds that chemical weathering is more rapid
in tropical climates than in cold climates proved to not be straightforwardly
true.

Quantitative and process geomorphology

Geomorphology was started to be put on a solid quantitative footing in the
middle of the 20th century. Following the early work of Grove Karl Gilbert
around the turn of the 20th century, a group of mainly American natural
scientists, geologists and hydraulic engineers including 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 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 the
modern trend of a highly quantitative approach to geomorphic problems.
Many groundbreaking and widely cited early geomorphology studies appeared
in the Bulletin of the Geological Society of America, and received only few
citations prior to 2000 (they are examples of "sleeping beauties") when a
marked increase in quantitative geomorphology research occurred.

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.

In Sweden Filip Hjulström's doctoral thesis, "The River Fyris" (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 (Anders Rapp), fluvial transport (Åke Sundborg), delta
deposition (Valter Axelsson), and coastal processes (John O. Norrman). This
developed into "the Uppsala School of Physical Geography".

Contemporary geomorphology
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.
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. This in turn has indicated
the importance of chaotic determinism to landscapes, and that landscape
properties are best considered statistically. The same processes in the same
landscapes do not always lead to the same end results.
According to Karna Lidmar-Bergström, regional geography is since the 1990s no
longer accepted by mainstream scholarship as a basis for geomorphological
studies.

Albeit having its importance diminished, climatic geomorphology continues to

exist as field of study producing relevant research. More recently concerns over
global warming have led to a renewed interest in the field.

Despite considerable criticism, the cycle of erosion model has remained part of
the science of geomorphology. The model or theory has never been proved
wrong, but neither has it been proven. The inherent difficulties of the model
have instead made geomorphological research to advance along other lines. In
contrast to its disputed status in geomorphology, the cycle of erosion model is
a common approach used to establish denudation chronologies, and is thus an
important concept in the science of historical geology. While acknowledging its
shortcomings, modern geomorphologists Andrew Goudie and Karna Lidmar-
Bergström have praised it for its elegance and pedagogical value respectively.

Processes
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. 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, changes to the seabed caused
by marine currents, seepage of fluids through the seafloor or extraterrestrial
impact.

Aeolian processes
Aeolian 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, Aeolian processes are important in arid environments such
as deserts.

Biological processes
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.
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. Rivers are also capable of eroding into rock and
creating new sediment, both from their own beds and also by coupling to the
surrounding hill slopes. In this way, rivers are thought of as setting the base
level for large-scale landscape evolution in non-glacial environments. 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.
These systems take on four general patterns: dendritic, radial, rectangular, and
trellis. Dendritic happens to be the most common, occurring when the
underlying stratum is stable (without faulting). Drainage systems have four
primary components: drainage basin, alluvial valley, delta plain, and receiving
basin. Some geomorphic examples of fluvial landforms are alluvial fans, oxbow
lakes, and fluvial terraces.

Glacial processes
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.

The way glacial processes interact with other landscape elements, particularly
hill slope 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. Non-glacial geomorphic processes which
nevertheless have been conditioned by past glaciations are termed paraglacial
processes. This concept contrasts with periglacial processes, which are directly
driven by formation or melting of ice or frost.
Hill slope processes
Soil, regolith, and rock move down slope under the force of gravity via creep,
slides, and flows, topple, and fall. Such mass wasting occurs on both terrestrial
and submarine slopes, and has been observed on Earth, Mars, Venus, Titan and
Iapetus.

Ongoing hill slope processes can change the topology of the hill slope surface,
which in turn can change the rates of those processes. Hill slopes that steepen
up to certain critical thresholds are capable of shedding extremely large
volumes of material very quickly; making hill slope processes an extremely
important element of landscapes in tectonically active areas.

On the Earth, biological processes such as burrowing or tree throw may play
important roles in setting the rates of some hills lope processes.

Igneous processes
Both volcanic (eruptive) and plutonic (intrusive) igneous processes can have
important impacts on geomorphology. The action of volcanoes tends to
rejuvenate 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 and subsidence of the surface, depending on
whether the new material is denser or less dense than the rock it displaces.

Tectonic processes
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 or 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 hundreds to thousands 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 hill slope 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.

Marine processes
Marine processes are those associated with the action of waves, marine
currents and seepage of fluids through the seafloor. Mass wasting and
submarine landsliding are also important processes for some aspects of marine
geomorphology. Because ocean basins are the ultimate sinks for a large
fraction of terrestrial sediments, depositional processes and their related forms
(e.g., sediment fans, deltas) are particularly important as elements of marine
geomorphology.

Overlap with other fields

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.