GES 101 - LECTURE EIGHT

GEOMORPHIC WORK

Geomorphic work is the broad category that defines the processes that create landforms.

The ways in which the development of landforms have been viewed have changed over time.

THE EROSION CYCLE

Proposed by William Morris Davis (1850-1934).

He proposed a paradigm which explained the development of landforms.

A paradigm is a model with general power to explain the nature of some large-scale phenomena on the Earths surface.

Davis was influenced by the ideas of Darwin and modeled his ideas after the theory of evolution.

He felt that a landscape evolved through an orderly progression of stages which he termed youth, maturity, and old age.

Initial uplift of a flat region would create gravitational energy that would cause streams to downcut.

In youth there are still many flat uplands with more valley slopes developing by late youth.

Maturity is marked by valley slopes and narrow divides between river valleys.

In late maturity the valley slopes have been worn down and the valley floors have become broad and have little change in elevation.

Old age is achieved by the continued erosion of the landscape.

An old age landscape is composed of a peneplain with some monadnocks.

Peneplains are a low nearly featureless plain produced by erosional processes in humid fluvial (river) environments.

Monadnocks are isolated remnants of rock rising above a peneplain. Named after Mount Monadnock in New Hampshire.

The peneplain may be uplifted or rejuvenated to begin a new cycle of erosion.

The whole process is driven by gravity and fluvial erosion.

Davis argued that the processes would take the same course in every case.

By 1960 Davis theory was abandoned in favor of dynamic equilibrium as a paradigm.

The development of landforms is related to inputs, throughputs, and outputs of energy and material.

GEOMORPHIC WORK

Any geomorphic system is defined by how energy and material flows through that system.

The forms that some of the material takes are called landforms and are related to the way in which energy and material flows through the system.

INTERNAL INPUTS

Generally internal or endogenic inputs refer to changes in the Earth's surface as a result of processes like folding, faulting, or volcanics.

Internal inputs can affect the Earths surface, altering gravitational energy.

Uplift can steepen slopes which can accelerate erosion.

EXTERNAL INPUTS

External or exogenic energy inputs include solar and chemical energy.

External or exogenic material inputs include sediments and water that flow through the system.

GEOMORPHIC SYSTEMS

A geomorphic system is an open system in which a portion of the system receives inputs.

For example a river receives water from the atmosphere in the form of precipitation. Under the influence of gravity the collected water flows downhill and carries with it broken pieces of rock (sediment).

In a geomorphic system the inputs are transported through the system as throughputs.

In a river, sediments are carried by the water flowing through the channel. Gravitational energy drives the system.

Eventually throughputs are no longer transported by the system and are deposited.

At the mouth of a river in the ocean sediments are deposited on the ocean floor and are no longer transported by flowing river water. At sea level there is no gravitational energy causing the water to flow downhill.

Systems are very simply composed of areas of erosion (inputs), transportation (throughputs), and deposition (outputs).

Systems are complex in that inputs, throughputs, and outputs are occurring on several scales, both temporal and spatial.

Systems are driven by energy inputs.

TYPES OF ENERGY

Solar energy- energy transferred from the sun by electromagnetic radiation.

Heat energy- energy associated with the internal random motion of molecules in matter.

Gravitational energy- potential energy related to the elevation and mass of an object.

Kinetic energy- energy arising from the motion of objects.

Chemical energy- energy stored in bonds that join molecules of chemical compounds.

GEOMORPHIC AGENTS

Commonly landforms are identified by the geomorphic agent that forms the landform.

A geomorphic agent can be a mobile material such as the atmosphere which is moved by solar energy or water which is moved by gravity.

Gravity can act as a geomorphic agent when it influences rock material directly.

Geomorphic agents include gravity, flowing water, wind, waves, and ice.

These geomorphic agents produce distinctive landforms.

UNDERSTANDING LANDFORMS

Landforms can be understood from the standpoint of the type of geomorphic agent and whether the landform is erosional or depositional.

For example the geomorphic agent that forms a sand dune is the wind which has had inputs of solar energy to move the atmosphere.

Sand may be blown out from one area (an erosional feature) and deposited in dunes at another area (a depositional feature).

Each of the components have characteristics that are related to the unique combination of energy and material forming a dynamic equilibrium.