Earth inner forces

Crust continually affected by earth’s inner forces. The origins of such movements explain by two principle reasons. (i) Denser portion of earth tend to sink and squeeze the lighter portion between them. (ii)Liquefaction of the earth inner part allows melted materials to move towards to area of lower pressure.

The effect of these minor and major earth movements exhibits in rocks in the form of folding and faulting. If the original position of the bedrock has been changed but beds are still continuing, it refers as a folded. If the rocks break by displacement of plane of the fracture surface it will know as faulting. Due to the deforming stresses some rocks break easily than other rocks. Shale and similar rocks thin sediment layer often get these results of pressures. 

Fracture and flow

Rock flowage:

The rocks subjected to more stress it may responds in two ways. (i) They may crack and slip into lessened pressure positions, or (ii) it may flow into another form without any visible sign. Rock flowage occurs due to rearrangement of particles of the rocks. Rock flowage depends upon the nature of rock, pressure on it, the amount of pressure that applied and it distribution around the rock. 

Rock fracture:

When the rock cracks occurs it may fracture without any sign. Fracture result from shrinkage of sediments due to dry periods, compaction under load or diastrophic movements. Jointing or cracking without any movements is observe in thick shale. In limestone and sand stone not shows this evident. Generally cracks not exist in these rocks. They are tightly closed. Therefore it referred as ‘blind joints’. 

Folds and faults

Causes of folding

Folding occurs when rock layers compressed by different energetic forces. Rock layers deform from their original positions due to pressure gradients. 

Deeper in the earth, under very heavy lateral pressures and very heavy loads, strata are deformed by flowage. Folds due to this cause are complex and intricate detail than those produced by fracturing and slipping. 

All types of rocks subjected to this stresses. Igneous rocks and metamorphic rocks are subjected to folded and faulted than the sedimentary rocks. However sedimentary – rock relationship consider due to the study of fold and fault. 

The classifications of folds 

Monocline – This is a simple type of folded. It involves rocks bending. It causes downward dip in horizontal layers. The dip of layers high at folded axis. 

Anticline – An anticline arch upwards with youngest rock layers in upper part of the arch surface. In syncline fold that bends downwards and cause basin. In top of the dip contain youngest rock layers. 

Recumbent fold – It involves with parallel limps. This type of fold occurs due to highly directed pressure gradients compressed the folded on horizontal axial plane. 

When the rocks layers affected by forces from multiple directions they twists into variety of shaped. At this stage domes formed. If the beds downward away from central point that structure is dome. 

Examples:

Daxue Mountains – This Mountain formed on the eastern part of the Tibetan plateau during crustal folding in Jurassic periods. 

Verkhoyansk Mountains – This Mountain located in Russia and extend for 700 miles (1,100km). This range is anticline folded that is downward bends on its edges and upward bend on its center. It was formed in late Jurassic period and early cretaceous periods. 

Cape folded belt – This made up of 450 million years old sand stone and shale. During the building of super-continent pangea this mountain folded. 

Faults

 

Faults are unevenly distributed cracks in the crust and origins due to the movement between the opposing rock faces. Break in the rocks are relatively displacement parallel to the plane of the break. In surface slipping has taken places it known as fault plane. If the displacement area is wide and indefinite it called as fault zone. 

Faults are classified by the attitude of the fault plane, nature of the movement and direction of the motion. Movement can occur slowly and continuously or in large, sudden shifts that can leads to surface deform. 

Normal fault- Rocks one side of the fault slump below the other side. 

Reverse fault – The rocks on one side of the fault push upward above than other side. It accompanied by compression across the fault plane. When two side of a fault compressed they are squeezed together and hanging wall moves up the fault plane. 

Thrust fault – This action is similar to reverse fault and it origin at low angle. These faults stack on rock units and change the earth crust as thicker. 

Transform, or strike-slip, fault – Rocks two side of the fault slide and pass each other. 

Examples:

Anatolian fault – It’s a major transform fault crosses Turkey from east to west. 

Andaman fault – This is an Australian plate subduction-zone fault and in the strait that separate the Burma and Sumatra. 

Dead Sea fault zone – Connecting to the red sea spreading center in the south. This is a transform fault that allows sliding between the Arabian and African plates. 

References :

(1) Howard E Brown, Victor E monnett, J. Willis Stovall (1958); Introduction to Geology; Ginn    and Company; USA
(2) Michael Allaby, Dr. Robert R. Coenraads, Dr. Stephen Hutchinson, Karen McGhee, Dr. John O'Byrne, Dr. Ken Rubin (2008): The Encyclopedia of Earth, Sydney.
(3) D.M. preece, H.R.B. Wood (1946); Foundations of Geography; University tutorial press; London  
(4) http://202.116.83.77/hope/sites/geoscience2008/english/content/chapter5/content_05_01%28a%29.htm

Development of Landscapes

Erosion, transportation and depositional effects of the geographical agents are the dominant factor to shape up the earth surface and responsible for its irregularities.

Land scape is study base on three viewpoints.

• Structure – present topographic lines are depends on their structure. Examples are origin of the rocks, its components, nature of elements.

• Processes – It intend the processes that work upon these structure. These are determined by climate, elevation and nature of the rock.

• Stage – Finally, the landscapes are influenced by its stages of development.

Regions changes are result of impacts of erosion factors. Erosion factor caused changes through its ‘cycle of erosion’ or ‘geomorphic cycle’. Earth land forms are take changes through ‘youth’ to ‘maturity’ to ‘old age’. These are regarded as stages of development.

The erosion cycle in a temperate humid region

In temperate humid region moderate rainfall is the dominant factor. The effects of running water and chemical weathering are responsible for land surface modification.

In younger region there are few streams with poorly branched. Most of the area untouched by river valleys. Under these conditions, poorly drainage water accumulated in low areas, and forming lakes and swamps. These are not intermittent. Lakes are continually filling up with sediments by inflowing streams and out flowing streams cut their outlet. Vegetation grows with in them and adding organic materials with sediments to filling the basins. Lakes length disappears. Tributaries occur from head stream and they form branched and re branched.

Erosion which has contribution on this process depends on rock structure. If these rocks are horizontal, tributaries cut the intervening lands into ridges. While rocks are tiled, streams are flow through weaken rock layer. Chemical weathering dissolved the rock minerals. Therefore rock those near the surface not stand for long at high angles. Valley walls become crumbling or wash downward into stream. Steep slopes and sharp angles are starts to disappear and form mellow curves on surface land.

These stages know as maturity. At this stage extent dissection and relief occur. This relief increases the soil creep. Soil erosion starts outside of the valley and the same time side surface almost destroyed.

Land becomes lower and old stage comes on. Region becomes flat through the effect or impact of erosion and weathering. These landforms are called as peneplain. At this stage it reaches final stage.

The cycle in a dry climate

Arid and semi-arid regions elevated and flat surface will undergo with specific changes same as temperate humid region. Stream flows are irregularity because of irregularity of rainfall and lack of vegetation which has retarding effect upon runoff. Many streams not endure. 

In dry region weathering process is slow and mechanical type. This type of weathering has little impact on rocks which lie beneath the earth surface. If stream banks are steep, channels deepen at relatively rapid rate. Valleys become canyon, when elevation reaches enough height. Small streams and deep valleys taken canyon shapes. 

Small depressions or basin exists in arid region same as in humid region. In earlier stages of development, the water of intermittent streams evaporates or undergo into surface before it reach the bottom of basins. But growth and development stream systems carries the water into lower parts of the small depressions and form playa lakes. In this way the floor of the valleys raised. Best example is Death Valley. 

If basin filling up has continued, stream come from higher elevated become higher and find the way to outlet and in lower stage flow into basin. This integration drainage system intended maturity stage. 

In most places weathered materials obscured the bedrock structure and soften the outlines of topography. Horizontal layer carved into terraces. Hard layer forms higher steep, softer slopes. 

Soften rocks at considerable altitude above the main streams converted by sheet, gully and rain erosion into an intensely dissected landscapes. It known as badlands. There is no vegetation cover. Heavy rainfall washes away the weathered material before the plant life rooted. For example South Dakota & Northern Nebraska 

                                                           South Dakota badlands

At final stage dry region reduce as peneplain. In this condition wind erosion become as dominant erosive agent factor. In very arid region wind erosion reduce the land below the sea level. 

The cycle in mountainous regions

The stages of erosion cycle in mountainous region different from other level regions. Youngest shapes determined by their fundamental structure of the mountain region rather than effects of erosion. Streams are flow toward down and cut the local canyons and leave masses untouched. If glacier present, cirques excavated cavities will appear in their summits and sides. But original form of the mountain not changes. For example Southern Rockies & Bighorns. 

 

Maturity stage knows as maximum dissection by weathering, running water and some time by glaciers. Only little form of original landscape remains. Canyons and their tributaries cause inroads upon elevated or uplifted masses. If the region covered by glaciers cirques has start to work and leave steep peaks and jagged crests between them. 

Finally mountain gives place for weathering and erosion. The ruggedness and grandeur which take form in maturity stage slowly changes soften contours. Mountain stream also become tributaries because of their silt. At last mountain is brought as low.

Example: Norfolk and western railway

The cycle in a Limestone region  

In this region dominant factor is solvent action of ground water. The early stages of development, the region is similar as those under the influence of surface region. Stream valleys will visible soon and their development starts as usual way. When they cut down the layer that lie under the limestone, the water begin to find the way to outlets through underground channels.  At this point limestone starts to disappear from the surface. 

But eventually very little of the original surface is remain. In stage of maturity sinks become large. Its leads to the formation of solution valleys. These differ from stream valleys by their irregularity. Surface stream starts to disappear and subterranean drainage system has to develop. 

The stage of old age reaches its basic level. Here it intends the lowest level of solubility. This may be the level of water table or insoluble rock which lies beneath the limestone. Either, the surface becomes more flat and less rugged. Scattered monadnocks start to rise. Surface streams reappear and stream erosion takes a place to reduce the land to sea level. 

 

 The cycle under glacial conditions. 

The progress of alpine glaciers has already described in erosion cycle of mountain region. Some erosion effected by valleys deepen and widening. But these are extent of cirques. 

In southern parts of the continental land contain the evident of unfinished glacier erosion process. This caused by melting of the ice. The glacier motion is slow, ponderous and irresistible progress. In glacier land scape contains lack of sharp outlines, the angles, peaks and spurs. Landscapes appear as reluctance. 

 The shore-line cycle

It is constantly changing due to the effects of land, air and marine processes. On many coastlines dominant factor is wave action. Recently sunken coastal areas will present irregularity forms. The stage of young age, river mouth becomes as estuaries and hills have been converted into peninsulas or islands. Waves will continually attacks the islands and main land forcefully. 

Wave actions widen the weaken area and forms cave at a headland. Cliffs and wave cut terraces appear. If cave formed at headland it will be widened and deepened until the sea cuts through to form a natural arch. If waves continually erode the foot of arch its roof becomes weakens to support and it will collapse. It leaves a part of the cliff isolated as a stack. 

At the late youth shore line shows as clear changes of stage. Bars closed the mouth of bays and they extend them self or form a line from one headland to another headland. Bays are filled by sands washed from the bars or from the main land. 

A wave attacks the remaining head lands and sand – choked bays. Sea-cliff take places in borders the coast and not similar as the original shore line that remains. This may be considered as maturity stage. Older stage intends sunken coasts. 

References: 

(1) Howard E Brown, Victor E monnett, J. Willis Stovall (1958); Introduction to Geology; Ginn    and Company; USA
(2) Michael Allaby, Dr. Robert R. Coenraads, Dr. Stephen Hutchinson, Karen McGhee, Dr. John O'Byrne, Dr. Ken Rubin (2008): The Encyclopedia of Earth, Sydney.

(3) David Waugh (2003); The New Wider World; Nelson Thornes; Uk.
(4) D.M. preece, H.R.B. Wood (1946); Foundations of Geography; University tutorial press; London 

The plate boundaries

Heat that arrives from the earth mantle drives the plate’s movement. This forming three general classes of plate boundaries. They are (i) divergent boundary (ii) convergent boundary (iii) transform boundary.

Divergent boundaries are reflected by oceanic ridge. At this boundary plates are move apart from other, and new sea-floor materials accumulated at oceanic spreading center. It added addition materials to earth surface. Therefore it called as constructive boundary.  Examples are mid oceanic ridges and activated fault zones. Mild earth quakes and volcanic eruptions are often occurs at this location.  

In convergent boundary plates are collide each other. One plate is subducted into the other plate. Often denser one is sup ducted into the mantle. This activity called as subduction. We can define this activity into three categories.

• Ocean-to-ocean subduction: this kind of plate movements forms a deep trench where one plate descends or subducted.

• Continent-to-continent: When continent plate collides they cause fold and uplift the land into mountain ranges.

• Ocean-to-continent boundary: When continent plate and oceanic plates are collides, its form a deep trench where oceanic plate descends and a line of subduction volcanoes occurs on the continental plate.

This zone or subduction zones contains hydrous minerals which emissions hot water. This water melted the mantle and cause volcanic eruptions. For example Andes range in South America, Japan islands.

At transform boundary two plates are slide and past each other. This sliding boundary cause steep fracture. It’s called as transform fault. Tectonic activities are parallel with the boundary that forming a new crust or depleting the old crust. Specific earthquakes are occur result of this sliding. Best example was San Andrea’s earthquakes in California. This earthquake occurred at between the plates of pacific and north America transform boundary.  


Reference :

(1) Michael Allaby, Dr. Robert R. Coenraads, Dr. Stephen Hutchinson, Karen McGhee, Dr. John O'Byrne, Dr. Ken Rubin (2008): The Encyclopedia of Earth, Sydney. 

(2) Tom L. McKnight (1990): Physical Geography - A Landscape Appreciation: Prentice  Hall: USA.

(3) http://en.wikipedia.org/wiki/Plate_tectonics

Models of tectonic activities

 (i)                 A model of sea-floor spreading

American oceanographer Harry Hess proposed his sea-floor spreading model in 1960’s. It contains that oceanic ridges are originated by deep – seated material and risen currents. Volcanic eruptions also help to form new oceanic floor. New crusts occur at ridges and move slowly outwards. Other regions of ocean basin, in trench boundaries old crust descends into interior crust. It called as subduction. 

This model proved by two evident. They are, (i) paleomagnetism, (ii) core sampling. When a rock formed with iron it contains magnetism. Therefore iron grains move towards magnetic polar. The rocks become solid in a time, magnetic pole change as earth consistency magnetic pole. Sea-floor spread sideward since new materials accumulated at oceanic ridges. Both side of the ridges contents similar sediments. These reflected the past magnetic orientation. 

According to final research, sediments near the ridges are prolonging for long time. Sediments contiguity the ridges are known as thin, younger or recently accumulated. These materials are combined with small sediments and emissions from volcanic eruptions. 

Tectonic activities concept approved by base on these information’s and other evident.

(ii)               A model of continental drifting

Alfred Wegener developed his theory of continental drift in 1912’s. Plate tectonic theory approved by base this model. This model accepted since the sea floor spreading. 

Present continental were formed from a single mass which called as a pangea. These land mass drifted apart from each other and formed today’s continentals. These are proved through present continental positions.

• ·         Reconstructions of ancient super continental land mass shows that this continental once connected.

• ·         Many plants and animals fossils were occur in similar aged rocks and dispersed across today’s continents. 

Reference:

(i) Tom L. McKnight (1990): Physical Geography - A Landscape Appreciation: Prentice Hall: USA.
(ii)  http://www.ucmp.berkeley.edu/fosrec/Metzger3.html#

(iii)  http://en.wikipedia.org/wiki/Plate_tectonics

Active tectonic and models of landscape development

 Many geologists concern about land formation. William Morris David has great contribution on development of this concept. He revealed his famous geomorphological model. It described that land formations has a ‘cycle of erosion’ which elaborated through “youth” to “maturity” to “old age”. His model contents tectonic force as an impulsive event that occurs earlier in geomorphic cycle. Land formation process occurs earlier in cycle. Subsequently geomorphic process constitutes effect and deformation and finally it form as a peneplain.

Opposed model has elaborated by Walther Penck in 1950’s against the W.M. Davis geomorphic cycle. Penck suggested wave pattern tectonic forces occur at the beginning of the cycle or ahead of plate tectonic deformation. In this scheme deformation increase step by step towards its ultimate stage and after weaken. His model invoked rock uplifted and land formation until ultimate stage. Geomorphic process uplifted the region through orogeny forces. There for land scape are resulted from the combined action of deformation and erosion.

 

John Hack elaborated third model of land formation. If Land forms exist for long intervals from erosion and deformation it will reach dynamic equilibrium. He discovered that land forms are not increase unbounded even though forces are prolonged for long time and produced final force to rock. Supplementary slope fractures limited the land scape elevation while parent rock continually uplifted. This condition exists until land formation reach dynamic equilibrium. In this model orogeny forces and erosion are become prolong equilibrium.

Here deformation rates are not obstructed like other models. If land form reaches its final stage then it not needs supplementary growing. Fluctuations are occurs around the land form until tectonic forces become constant.