Geomorphic processes and agent -Physical weathering in detail

GEOMORPHOLOGYLect. No. 2

19 / 10 / 2014

Concept Description AlternativesUnusual processes without modern analogs occurred in the geologic past.

Similar geomorphic processes that operate today operated throughout geologic time.

Uniformitarianism

Cataclysmic processes and changes are most important in landscape evolution.

Relatively frequent, modest-sized processes dominate in the progressive changing of the landscapes over long time periods.

Gradualism

Some landscapes arise from disorderly processes.

Erosional agents on a planetary surface produce an orderly sequence of landforms as a function of time.

Orderliness

Geologic structure predominates over climate in influencing landform assemblages.

Landscapes reflect the influence of certain climatic processes by developing a characteristic assemblage of landforms.

MorphoclimaticZonation

In stable tectonic areas, extensive landscapes of Tertiary, or even Mesozoic, age are preserved

Most of the Earth´s topography is no older than Pleistocene.

Youthfulness ofTopography

Relict landscape elements persist despite Pleistocene changes and comprise palimpsests

The geologic and climatic changes of the Pleistocene were critical in the shaping of most landscapes.

Legacy of the Pleistocene

Geomorphology attains its maximum usefulness by historical extension, analyzing ancient conditions.

Geomorphology is primarily concerned with present- day processes that shape present-day landscapes.

Modernism

Complexity of geomorphic systems is more common than Simplicity.

It is necessary to search for simplicity in geomorphological systems.

Simplicity

◙ Objectives of the lecture: ●Identification of the geomorphic processes and agents which

modify the earth surface. ●Distinguishing between different geomorphic processes and

agents. ●Identification of the physical weathering, and its effects on the disintegration of rocks.

◙ Lecture contents: ●Definition of the geomorphic agents.. ●Definition of the geomorphic processes ●Classification of geomorphic processes: -Exogenetic processes (Epigene): *Degradation: +Weathering: Physical weath. & Chemical weath. +Mass-wasting. +Erosion. *Aggradation.

●Explanation about weathering. ●Factors affecting the type and rate of weathering in general. ●Types of weathering: a) Physical weathering. -Definition of physical weathering. -Types of physical weathering : *Expansion due to unloading. * Expansion due to crystal growth. * Expansion due to thermal effects. *Action of organisms. b) Chemical weathering.

-Endogenetic processes (Hypogene):

*Diastrophism. +Orogenic movements. +Epierogenic movements. *Volcanism: Extrusion. & Intrusion.

-Extraterrestrial processes.

● Geomorphic agent: it is any natural medium which is capable on eroding and transporting earth material.

Example for geomorphic agents:Running water., Ground water., Wind., Glacier., Currents and waves (movements within standing water).Note: Gravity is not a geomorphic agent because it can not erode and transport earth materials, so it is only a directional force.

●Geomorphic processes: are all those physical and chemical changes which affect on the modification of the earth surfacial forms.

Geomorphic Process is the dominant internal or external geologic force that has interacted with the existing geologic structural framework to shape the Earth’s surface.

Geomorphic Process has two hierarchical elements within this classification: Geomorphic Process Type and Geomorphic Subprocess

Geomorphic Process Type - A general description of the dominant geomorphic process responsible for the nature, origin and development of the landforms.

Geomorphic Process Types are Fluvial, Glacial, Periglacial, Lacustrine, Tectonic, Volcanic, Mass Wasting, Coastal Marine, Solution, and Eolian.

Geomorphic Subprocess - A subdivision of Geomorphic Process which groups related landforms.

For example, the Glacial Geomorphic Process type is subdivided into Ice Erosion, Meltwater Erosion, Water Deposition (in close proximity to ice), Ice Deposition, Active Ice and Snow Features, and Proglacial Deposition.

Not all Geomorphic Process types have Subprocess.

Classification of geomorphic processes

A/ Exogenetic processes (Epigene): are those processes which originate and act outside the earth crust. They include:1/ Degradation:” the wearing down of rock by disintegration.” ● Weathering ● Mass-wasting ● Erosion (including transportation by):Running water, Ground water, Currents and waves, Wind, Glaciers.2/ Aggradations by:” increase in land elevation due to the deposition of sediment”. Running water, Ground water, Currents and waves, Wind Glaciers , Work of organisms, including human.B/ Endogenetic processes (Hypogene): are those processes which originate and act within the earth crust and including: ●Diastrophism” refers to deformation of the Earth's crust, and more especially to folding and faulting”.●Volcanism

C/ Extraterrestrial processes: are these processes origntate form the impact of meteorites (Infall of meteorites)..

Exogenetic Endogenetic Extraterrestrial

A/ Exogenetic processes1/ Degradation: *It includes all those processes which remove the

earth surface down. * It is composed of three distinct processes which are:

weathering, mass-wasting and erosion.

In geology, degradation refers to the lowering of a fluvial surface, such as a stream bed or floodplain, through erosional processes.

It is the opposite of aggradation

Weathering is the breaking down of rocks, soil and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters.Weathering occurs in situ, roughly translated to: "with no movement" , and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations. Weathering is the first step for a number of other geomorphic and biogeochemical processes.

●Weathering:-It may be defined as the disintegration or decomposition of rocks in place. -It occurs at or near the earth surface. -It reduce the solid rock masses to the clastic state .

●Factors influencing the rate and type of weathering:

1-Structure:

2-Topography:

It includes many physical and chemical features of rocks. The physical features are: joints, faults, bedding planes, minute fractures and voids. The chemical features include the mineralogical composition which cause determination of the weathering type.

* directly effects the amount of rock exposure to weathering and affects

the amount of precipitation and temperature.

•indirectly topography affects the type and

amount of vegetation.

3-Climate: *It includes the temperature and humidity especially. *It determines the type and rate of the weathering. 4-Vegetation: *the abundance and type of vegetation influence the rate and type of weathering. *It determines the amount of decaying organic matter from which CO2 and humic acid may be derived.

Two important classifications of weathering processes exist – physical and chemical weathering; each sometimes involves a biological component.

Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure.

physical weathering is accentuated in very cold or very dry environments, chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together, and each tends to accelerate the other.

Products of Weathering:The process of weathering can result in the following three outcomes on rocks and minerals:(1). The complete loss of particular atoms or compounds from the weathered surface.(2). The addition of specific atoms or compounds to the weathered surface.(3). A breakdown of one mass into two or more masses, with no chemical change in the mineral or rock.

The processes that may cause mechanical rupture are abrasion, crystallization, thermal insolation, wetting and drying, and pressure release.

●Types of physical weathering :-

1.Expansion due to unloading: Pressure release of rock can cause physical weathering due to unloading.

The majority of igneous rocks were created deep under the Earth's surface at much higher pressures and temperatures. As erosion brings these rock formations to the surface, they become subjected to less and less pressure.

This unloading of pressure causes the rocks to fracture horizontally with an increasing number of fractures as the rock approaches the Earth's surface. As a result, greater areas of rock are exposed to mechanical and chemical weathering.

A number of structural features are formed due to unloading:A-Large scale fractures:- they are roughly concentric (parallel) with the earth topography

B-Sheety structure:-in granitic rocks the individual sheets become more closely spaced when approach from the earth surface.

The fracture in vertical direction form exfoliation. If the fracture is in horizontal direction, it calls sheeting.

C-Exfoliation domes: they are large monoliths, formed at considerable depth. The horizontal extent of the exfoliation shells may be hundreds or even thousands of feet.

2.Expansion due to crystal growth:

- occurs as a result of the new crystal growth in rock fractures.-The new crystal growth is of ice and other crystals such as halite.

-The freezing and thawing are most active in the highly fractured rocks due to the formation of ice crystal.

3.Expansion due to thermal effects: results from the expansion and contraction of rock, caused by temperature changes.

For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals.

As some minerals expand more than others, temperature changes set up differential stresses that eventually cause the rock to crack apart.

In arid and semiarid regions the expansion which occurs during the day and contraction at night, constantly repeated, weaken the texture of the rock, then the outer heated layer tend to Pool away from the cooler rock underneath and Flaks this process is known as ‘exfoliation’.

4. Action of organism:

●it is of minor importance.●the growth of plant roots causes widening the rock fractures. ●organism bring fresh rock materials to or near the earth surface, and then attacking them by a chemical weathering.

5- Abrasion occurs when some force causes two rock surfaces to come together causing mechanical wearing or grinding of their surfaces. Collision between rock surfaces normally occurs through the erosional transport of material by wind, water, or ice.

Abrasive weathering often occurs to rocks in rivers, beaches, or desert areas. Water and wind can easily remove the upper layer of a rock with friction. The rock will not only reduce in size, it will also become invariably spherical or oblong in shape and extremely smooth.

Next Lecture CHEMICAL WEATHERING

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