Geomorphological features and Classification (Classification of landforms): Landforms are the individual topographic features exposed on the Earth s surface. Landforms vary in size and shape and include features such as small sand dunes, or large features such as the River or Mountains. Landforms develop over a range of different timescales. Some landforms develop rather quickly (over a few seconds, minutes, or hours), such as a landslide, while others may involve many millions of years to form, such as a mountain range. Landform development can be relatively simple and involve only a few processes, or very complex and involve a combination of multiple processes and agents. Landforms are dynamic features that are continually affected by a variety of earth-surface processes including weathering, erosion, and deposition. The landforms are a product of either constructive and destructive processes or a combination of both. Landforms are also influenced by other agents or processes including time, climate, and human activity. Landforms can be classified into following types: 1. Tectonic landforms 2. Extrusive Igneous Landforms 3. Intrusive Igneous Landforms 4. Fluvial Landforms 5. Karst Landforms 6. Aeolian Landforms 7. Coastal Landforms 8. Glacial Landforms 1. Tectonic Landforms: i. Anticlines: Anticlines are defined as folds in which strata are uparched. The older rocks are in interior and younger rocks form outermost bend. The limbs dip away from each other. Symbologically an anticline is indicated by two arrows diverging away from the central point. 1
ii. Synclines: Synclines are defined as folds in which strata are downarched. The younger rocks are in interior and older rocks form outermost bend. The limbs dip away from each other. Symbologically an anticline is indicated by two arrows converging towards the central point. iii. Horst and graben: These are elongated fault blocks of the Earth s crust that have been raised and lowered, respectively, relative to their surrounding areas as a direct effect of faulting. Horsts and grabens may range in size from blocks a few centimetres wide to tens of kilometres wide, the vertical movement may be up to several thousand feet. 2
2. Volcanic Landforms: Extrusive Igneous: The landforms formed due to cooling and solidification of molten lava at the earth's surface are called extrusive igneous/volcanic landforms. i. Cinder cones: Cinder cones are relatively small cone shaped hills formed by the accumulation of cinders (cinders are walnut and pea sized solid volcanic materials) and ash during volcanic eruptions. Cinder cone ii. iii. iv. v. 3
ii. Shield volcanoes: Shield volcanoes are broad shaped mountain landforms built by the accumulation of low viscosity lava that goes to long distances. Their slopes are often very gentle and peaks are relatively flat. They received their name because their gently domed form resembles the exterior of a warrior s shield. iii. Strato volcanoes: Strato volcanoes, also referred to as composite cones, are tall and conical, nearly symmetrical mountainous landforms having steep slopes, formed by a combination of hardened lava flows and intense pyroclastic eruptions (solid eruptions) arranged in layers (strata). 4
iv. Caldera: A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. ETM+ image of Deriba Caldera, Sudan 5
v. Lava Domes: Lava domes are basically what the name describes, domes of lava. They are formed through accumulations of thick, highly viscous (thick) lava that soon cools and hardens into a shell of rock. If conditions are right, say more lava rises up or the environment is too dry, the rock shatters and tumbles down the sides. The process repeats itself and after a while the dome builds itself up. 3. Intrusive Igneous Landforms: The landforms formed due to cooling and solidification of magma inside the earth surface is called intrusive igneous/volcanic landforms. i. Dikes: Dikes are small tabular intrusions that are mostly perpendicular to the layers of pre-existing rock. The thickness of dikes ranges from few centimetres to several metres but the length extends from a few metres to several kilometres. ii. iii. iv. Sills: These are also tabular intrusions that are intruded parallel to layers of preexisting rock. The thickness of sills ranges from few centimetres to several metres. Laccoliths: Similar to a sill, except the layers above are bent upward by the intrusion. Batholiths: 6
These are large intrusions with irregular shape that cover hundreds or thousands of square kilometers. v. Stocks /Plutons: These are small irregular intrusions. vi. Lopolith: These are mega-sills that may cover hundreds of square kilometers. 4. Fluvial Landforms: Landforms formed due to the action of rivers and streams are known as fluvial landforms. i. Flood plain: A river may overflow its banks during times of heavy rain. When the river recedes, the river deposits the material it is carrying, leaving behind silt. Floodplains are the wide and flat plains formed by the deposited material on either side of the river with successive floods over the years. The heavier and coarser sediments are first deposited at the river s edge. The finer particles are being deposited a little further away from the river. 7
Satellite image of Mississippi river flood plain 8
ii. Waterfall: Waterfall is a steep descent of water from a height. Waterfalls occur due to changes in bedrock resistance, or along fractures or faults in the bedrock. Less resistant materials are weathered more quickly than resistant rocks, thus creating waterfalls. Faults and fractures often provide natural pathways for the downslope movement of water Ikonos image of Niagara falls, Canada iii. Alluvial fan and Bajada: An alluvial fan is a fan shaped deposit of silt, sand, gravel, and boulders (alluvium) on land where a river emerges from a mountainous area and flows out onto a more gently sloping plain. Alluvial fans generally form in arid environments with a high sediment load and where there is minimal vegetation to disrupt the fan formation. Alluvial fans may form from a single high-flow event or from the accumulation of multiple events. Bajada is a compound alluvial fan formed by the merging or convergence of many alluvial fans (minimum two alluvial fans). 9
Alluvial fan Google earth image of bajada near Red sea, Jordan 10
5. Karst Landforms: Karst is a term used to describe landscapes that are formed by chemical weathering process controlled by groundwater activity. Karst landscapes are predominantly composed of limestone rock that contains > 70 percent calcium carbonate. i. Caverns: Limestone caverns/caves are large sub-surface voids where the rocks have been dissolved by carbonation. Cavern ii. Sinkholes: Sinkholes are oval or circular depressions that range in diameter from a meter or so to tens of meter. Sinkholes are depressions formed when the limestone bedrock is chemically weathered by naturally occurring chemicals in the ground water. 11
Satellite image of Sinkhole iii. Disappearing streams: Disappearing streams are streams that flow on the surface and then seemingly disappear below ground. Disappearing streams disappear into a sinkhole or other karst solution features such as caves. They may also disappear into factures or faults in the bedrock near the stream. Disappearing streams are also referred to as losing streams, sinks, or sieves. Disappearing streams will often continue flowing underground and may resurface at another location downstream from where they disappeared. 12
Google earth image of disappearing stream in Malaysia iv. Solution valley: Several sinkholes merging to create a larger depression called a solution valley. 13
Karst landforms 6. Aeolian Landforms: Landforms formed by the action of windblown sediments are known as aeolian landforms. i. Sand dunes: The hill or ridge of sand formed by deposition of sand in a desert area is called sand dune. Types of sand dunes: a) Barchan dunes are solitary, crescent or half moon shaped dunes. They form where sand source is limited, wind direction is constant, and the ground is void of vegetation. Their windward slope is gentle and leeward slope is steep. Barchans have two horns pointing downwind. They can reach heights of 30 meters and spread nearly 300 meters. 14
b) Transverse dunes are a series of long ridges that are parallel to one another, and are perpendicular to the prevailing wind. They form in areas where the prevailing winds are steady, there is an abundant supply of sand, and vegetation is sparse. They can reach heights of 200 meters and may extend for 100 s of kilometers. Wind c) Longitudinal dunes also referred to as Seifs are long ridges of sand that form parallel to the prevailing wind. They form in areas where there is moderate supply of sand, and they range in size 5-10 meters tall to 100 meters in height and width. Wind 15
d) Parabolic dunes are similar in form to barchan dunes except their tips point into the wind. Parabolic dunes form inland from coastal shorelines from sands on the beach. ii. Loess: Loess deposits are regionally extensive accumulations of windblown silt resulting from thousands of dust storms. Loess is yellowish material rich in lime and is very fertile. The silt particles are smaller than sand but bigger than clay, diameter is 0.05 mm. 16
iii. Yardang: A yardang is an elongated ridge or remnant rock feature formed by wind erosion. Yardangs occur in arid environments where prevailing winds come from a single direction. The winds must be strong, steady, and carry a coarse-sediment load that weathers the exposed face of the yardang. Yardangs can appear into a variety of forms, and some may resemble common objects or human-like forms. iv. Mushroom or pedestal rock: The shape of this rock is like umbrella or mushroom with broad top and narrow base. It is formed when pebbles strike a huge rock in their way.at that time as the rock is huge its upper part is least affected and the base is most affected. Thus the upper part remains broader than the base. 17
6. Coastal landforms: Landforms formed due to erosional and depositional processes at coasts, mainly due to action of sea waves are known as coastal landforms. i. Sea cliff: When a coast facing the sea has wall like steep slope, than it is known as sea cliff. Cliffs are formed due to constant erosion of highland near sea shore. ii. Wave cut platforms: The wave-cut platform is a relatively flat surface that is formed due to erosion and break down of cliff on shoreline. The sea water may rush temporarily and periodically on platform. 18
iii. Beaches: Beaches are depositional landforms along the coastal area where sediment is transported and deposited by waves and currents. Although the sediment along the beach is continually being mobilized there is an overall net accretion of deposition. The width of the beaches varies from one location to another and from one shoreline to another. In some locations a shoreline might even lack a beach altogether. Beaches are developed along the gently sloping shorelines and absent completely along an upland coast. iv. Sand bars and spits: The ridges of sand lying more or less parallel to shoreline but not attached to land are known as offshore bars or sand bars. If the sandbars are formed in such a way that their one end is attached to the land while the other end is towards the sea than they are called as spits. Spits are elongated depositional landforms that are attached at one end to the coast and extend outward from the coast. 19
Spit Sand bar 7. Glacial landforms: The landforms formed due to action of the glaciers are known as glacial landforms. Glaciers are large masses of flowing ice formed by the accumulation and compaction of melted snow. i. U shaped valley: When the glacier moves through the pre existing V shaped stream valley it causes erosion of bottom as well as sides of the valley. Due to such continuous erosion the valleys become deep and wide. The valleys have wide floor and wall like steep sides appearing like English alphabet U, hence the name U shaped valley. 20
ii. Hanging valley: Hanging valleys are formed at the confluence where smaller tributary glaciers meet the larger valley glaciers. The valley glaciers occupy the main valley and they deepen the valley intensively. But the small tributary glaciers undergo little erosion and develop smaller valleys. When the glacier melts these tributary valleys are left hanging above the main valley. Hanging valleys are only visible after the glacier melts and reveals the underlying topography. Hanging valleys are provide ideal spots for development of waterfalls. iii. Moraines: Like running water glacier also carries large quantity of boulders, clay, pebbles etc. This is known as load of glacier. This load is eventually dropped by glacier at certain places. This dropped material is known as moraine. The four most common moraine types are lateral, medial, ground and terminal moraines. Lateral moraines are long linear ridges that are deposited to the side of the valleys. Medial moraines are long linear ridges that are deposited at inner margins of two meeting glaciers. 21
Ground moraines are long linear ridges formed when the load is deposited on the floor of valley. These form as the ice margin rapidly retreats, not pausing long enough to form end moraines. Terminal/ End moraine are long linear or arc shaped ridges formed when the load is deposited at the end of glacier. 22
iv. Eskers: Eskers are long and narrow ridges composed of stratified sand and gravel deposited by a glacial meltwater stream. Esker v. Fjords (pronounced as fiord): Fjords are over deepened U-shaped under sea glacial valleys along which sea water has entered the land. These are formed due to intense glaciations. It can also be defined as a glacial valley in which sea water has entered and which is surrounded by steep land on three sides. 23
Fjords 24