FIND OUT INFORMATION ABOUT CONTINENTAL LANDFORM
Landforms are defined as specific features that appear on the Earth’s surface. Some examples are mountains, plains, plateaus, valleys and hills. Oceans and continents illustrate the largest grouping of landforms.
Landforms possess many different physical characteristics and are spread out throughout the planet. Together, landforms constitute a specific terrain and their physical arrangement in the landscape forms what is termed as topography. The physical features of landforms include slope, elevation, rock exposure, stratification and rock type.
Despite our tendency to consider Earth as static, it is actually a dynamic and ever-changing planet. Wind, water, and ice erode and shape the land. Volcanic activity and earthquakes alter the landscape in a dramatic and often violent manner. And on a much longer timescale, the movement of earth’s plates slowly reconfigures oceans and continents.
What causes these landforms are different forces that work internally and externally on the Earth’s surface and core to form some of Earth’s natural features.
Internal Forces That Cause Landform
Internal processes work on the interior layers of the Earth. Forces gradually build up and the crust will undergo movements in the Earth such as earthquakes, volcanic activity or mountain buildings. The thing that causes a change in the Earth’s landforms is plate tectonics. Tectonic plates are huge slabs of rock and underneath all of the world’s landmasses and seas. These plates move occasionally. These movements may be undetectable or may cause natural disasters such as earthquakes and volcanoes. Over the years, the amalgamation of these shifts reshapes the Earth’s surface, altering existing landforms and creating entirely new ones.
The earth’s plate responsible for causing earthquakes is the crust of the changing earth. Most earthquakes occur along the edge of the oceanic and continental plates. The earth’s crust (the outer layer of the planet) is made up of several pieces, called plates. The plates under the oceans are called as oceanic plates and the rest are continental plates.
The plates are moved around by the motion of a deeper part of the earth (the mantle) that lies underneath the crust. These plates are always bumping into each other, pulling away from each other, or past each other. The plates usually move at about the same speed that your fingernails grow. Earthquakes usually occur where two plates are running into each other or sliding past each other. Earthquakes affect the landforms of the earth.
Folding is a type of internal process on the Earth. Folding happens when forces on the Earth’s crust push toward each other from opposite directions, which bends and folds the rock layers in different ways.
Lava ejects onto the surface of the Earth through a volcano, which is a crack in the opening of the planet’s crust. Lava pushes land up and hardens when it comes out of the Earth, and the resulting mountains are also called volcanoes. Shield volcanoes can shape the land for a long distance because the lava that comes out is fluid enough to travel far. Strato volcanoes are the tallest peaks formed by volcanoes. Their smaller counterparts are called cinder cones.
- Tectonic Forces
Plate tectonic is the idea that plates carry the continents and are great slabs of solid material that make up the ocean floor. Plate tectonics comes from the Greek word,”tektonikos” meaning “builder.” It has been determined that there are about 20 rigid plates that are in slow, continuous motion. Some continents move at a rate of 1/2 to 4 inches per year which is directed by heat driven convection cells in the molten rock deep below the crust. As they move, they carry the continents and ocean floor. In the late 1800’s, Alfred Wegener, a German physical geographer, used spatial analysis to propose the continental drift hypothesis. Wegener studied the outlines of the continents and suggested that the existing land masses had been united at one point in the earths early history . He called his theory”die Verschiebung der Continent” meaning “continental displacement.” His idea stated that these stable, immovable continents were mobile with the help of the tectonic plates. With further research his theory was accepted, but not until 60 to 70 years later.
The Earth is made up of three layers: the crust, the mantle and the core. The crust is a thin (15 mile) layer covering the outside part of the earth. The second layer is the mantle which is 1,800 miles thick. The crust and the upper mantle make up what is called the lithosphere. This lithosphere is 60-90 miles below the continents and 40-50 miles below the oceans. The plates in the plate tectonic theory are the lithosphere. The continental crust is less dense or lighter than the oceanic crust and “floats” above it. The base of the lithosphere is called the asthenosphere. Here is where the lithosphere is unattached from the mantle and moves around, mostly by gravity and thermal differences in the mantle.
The core, the third layer, is 1,000 miles thick. The core and the mantle are made of hot molten rocks, but the core is much hotter than the mantle. Below the 15 mile crust there is an increased amount of heat. It is believed that this heat is ‘left over’ from the formation of the earth and decaying radioactive material is fueling the fire. In fact, it is a possibility we could use the energy from this heat to fuel our lives if we were to run out of oil. How does this heat cause the plates to move? The earth’s crust is cold, the mantle is hot and the core is even hotter, thus providing us with the explanation. In order to equalize these temperatures, convection cells are formed. Two types of rotation are produced by these convection cells.
Propelled by these heat convection cells, these plates move very slowly; one to four inches per year. (A couple of inches per year isn’t much, since the earth’s history is measured in millions of years). 225 million years ago, it is postulated that a giant continent called Pangaea (‘all-earth’) existed. This giant continent remained until about 135 million years ago, when it began to break up during the Mesozoic time. The break up consisted of India detaching itself from Africa and Antarctica and headed into the Indian Ocean. A giant mountain range is formed where the Australian-Indian Plate is pushing into the heart of Asia. Additional evidence supporting the continental drift theory is supplied by the amphibian and reptile fossils that are spread out among the widely separated continents. And the evidence of polar wandering and evidence of magnetic field reversals locked into oceanic basalt samples.
There are only three ways plates can interact while moving. This, in turn, causes there to be only three types of boundaries which are produced by different stress fields. The first boundary is called divergent. This is a tension or a stress that pulls the plates apart. Divergent boundaries cause mid oceanic ridges. Some other common characteristics are high heat flow, mild volcanic activity, and shallow earthquakes.
The second boundary is called convergent and this is a compression, or a stress that can shorten or compresses the plates. Convergent boundaries cause mountain ranges to develop. The Himalayas for example, were formed when the plate carrying India collided with the plate carrying Eurasia. This continental collision is still active and moving at a rate of 3 to 4 inches per year as the India plate is pushed under the Asian plate and the mountain continue to grow. Strong earthquake activity is very common with areas of convergent boundaries as well.
The third type of boundary is a transform boundary and this is when the plates slide past each other along faults causing mid-oceanic ridges and trenches. One plate may be forced down into the mantle under the other plate. When this occurs, a deep oceans trench forms. The largest ocean form is the Mariana Trench in the Pacific Ocean southwest of Guam. Another example is the San Andreas Fault with is between the North American plate and the Pacific plate. Here, earthquakes are common but not volcanoes, and the earthquakes tend to outline the major plates.
The earth is constantly being shaped by the dynamics of the tectonicactivity and plates motion. Some of the present day’s biggest mountains and ocean trenches are examples of the great power of these plates in motion.
External Forces That Cause Landform
External processes work on the surface of the crust through weathering, denudation (or removal of the surface), erosion and deposition (or the raising of land). Some of these works are caused by rivers, glaciers, winds and waves.
- Weathering and Erosion
Rainfall (water from the cloud) falls carrying out two major actions, firstly, it dissolves chemicals in the atmosphere and this solution causes chemical reactions on the various surfaces it falls on thereby weakening those surfaces through this action and secondly, by its impact on it wears off the surface. This second action becomes more vivid when you study the impact of a drop of water on a sandy surface. The impact dislodges the soils particles from others.
When there is significant amount of rainfall it leads to runoff (water running on the surface) and this runoff has enough force to move loose soil and rick particles to be deposited at a new location. The movement of these particles causes wear and tear on the surface on which they are moved and over time a new landscape evolves.
Water in the form of ice also causes significant landform changes especially in the temperate regions. This occurs through a process called ice or frost wedging. This essentially means water lodging in cracks and crags of rock get frozen and since frozen water expands it forces the crack to become wider and often times dislodging part of the rock surface.
Wind action follows a an almost similar pattern that is the wind carries particles into the air and those particles are not smooth but ragged in nature therefore as the wind currents drags them and often times slam them against surfaces they tend to wear down the surfaces they come in contact with yielding a new landscape over time.
The Earth periodically under goes the event of glaciation.This is a time during which global temperatures drop and environments change. Glaciers,or large masses of ice, are gigantic erosional forces. There are many different types of glaciers and most contain a large portion of the earth’s fresh water resources.
In the Arctic and sub-Arctic, glacial erosion has shaped much of the landscape. Glaciers primarily erode through plucking and abrasion. Plucking occurs as a glacier flows over bedrock, softening and lifting blocks of rock that are brought into the ice. The intense pressure at the base of the glacier causes some of the ice to melt, forming a thin layer of subglacial water. This water flows into cracks in the bedrock. As the water refreezes, the ice acts as a lever loosening the rock by lifting it. The fractured rock is thus incorporated into the glacier’s load and is carried along as the glacier slowly moves.
Abrasion happens when the glacier’s ice and rock fragments act as sandpaper, crushing the rock into finely grained rock flour and smoothing the rock below. Meltwater streams of many glaciers are grayish in color due to high amounts of rock flour.
Glacial erosion is evident through the U-shaped valleys and fjords that are located throughout the Arctic and sub-Arctic regions. Glacial moraines are formed as a glacier recedes, leaving behind large piles of rock, gravel, and even boulders. Moraines may form at the foot (terminal moraine) or sides (lateral moraine) of the glacier or in the middle of two merging glaciers (medial moraine).
Waves are formed when wind blows over calm waters and ripples are created. These ripples enlarge with time and form larger wind waves. These Ocean waves then crash into continents and are considered to be a very powerful gradational force. They wear away the sides of continents. There have been islands that were formed by volcanic action and were worn away by waves erosion in a few short years. Beaches and coastal land fronts are continually being changed by the wave action. These environment are semi fragile and can be created and destroyed quite simply by long shore drift, waves action and storms as these landmasses undergo continuous change.
Rivers (especially fast-moving) erodes the land by carrying sediment away from one location and depositing it in another. Alluvium is all the sediment that is deposited by running water. Deltas are formed where running water moves into standing water. The Missouri River Valley has been formed by this slow-moving erosional force. Tributaries are bodies of water which flow into a larger river. These form valleys within a larger valley, leaving hills on each side. Fluvial landscape is the landscape formed by rivers. “Bird’s foot” deltas get their name from their appearance on a map. Rivers move back and forth in a snake-like pattern causing erosion and deposition. This may cause the river to become so tightly curved that the river may take a short cut across the corners and create a loop or an oxbow lake. An oxbow lake is the explanation for the creation of Lake Manawa and Carter Lake.
Deposition is the process of eroding materials into sediments then depositing them elsewhere. Deltas are areas of built up from soil dumped when a river empties into a lake or ocean. Sand bars are offshore shoals of sand deposited with slower moving water.
Different Major Landforms on Earth
Major types of landforms on earth include mountains, valleys, plateaus, glaciers, hills, loess, plains and desserts.
Mountains are lands physical features protruding high beyond the hills and very high up the land surface with steep top commonly shaped up to a peak. They are created through the action of incredible forces in the earth such as volcanic eruptions. Often, mountains occur in the ocean compared to land and some are seen as mountain islands as their peaks protrude out of the water. Mountain formation result from the forces of erosion, volcanism, or uplifts in the earth’s crust. The forces of heat and pressure within the earth’s interior are the main influencing factors to these forces as stated by geologists. These forces can be summed up as the plate tectonic movements – theoretically defined as the division of the earth’s outermost layer into several plates which are in constant motion. Hence, the uplifts are cause by collision or pulling apart of the plates that also triggers other various geologic activities such as the ejection of magma onto the surface or volcanic eruptions.
The movements also contribute to horizontal compression that is the deformation of crustal strata which gives rise to folds. The Himalayas and the Europe’s Jura and Alps mountains are examples of mountains formed as a result of horizontal compression. Some mountain ranges are also formed as a result of wind, water, and ice erosion. Other mountains are created from volcanism.
Examples of volcanic mountains include Mount Fuji in Japan, Mount Vesuvius in Italy, Mount Erebus in Antarctica, and Mount Saint Helens in the United States. Majority of volcanic mountains have summit craters that still expel debris and steam.
A valley is a lowland area or surface depression of the earth between higher lands such as mountains or hills. In simple terms, it can be defined as a natural trough bounded by mountains or hills on the surface of the earth sloping down to the lake, ocean or stream, which is created because of water or ice erosion. On this basis, the rivers or streams flowing through the valley empty the land’s precipitation into the oceans.
The lowest parts of the valleys are very fertile and make very good farmlands. Majority of the valleys on land are made up of running streams and rivers and nearly all their floors slope downstream. Valleys within the mountains normally have narrow floors. The sides of a valley are termed as valley slopes or valley walls and the section of floor along riverbanks are referred to as flood plains.
Valleys physical features include U-shaped and V-shaped caused through the forces of erosion by the flowing masses that persistently widens and deepens the valley. The flowing masses are either water or glacier that carries away huge amounts of debris. Very narrow and deep valleys are known as canyons.
Plateaus are fairly flat areas higher than the land surrounding it. The surrounding areas may have very steep slopes. Some plateaus such as the Tibet are situated between mountain ranges. Plateaus cover wide land areas and together with their enclosed basins they cover approximately 45% of the entire earth’s land surface.
Some plateaus, for instance the Columbia Plateau of the United States and the Deccan of India are basaltic and were created because of lava flows spreading to thousands of square kilometers thereby building up the fairly flat land surfaces. Other plateaus form as a result of upward folding while some are due to the erosion of the nearby land that leaves them elevated. Because plateaus are elevated, they are subject to erosion.
Low plateaus make up good farming regions whereas high plateaus are considered great for grazing livestock. Most of the world’s high plateaus are deserts. Other typical examples of plateaus include the Bolivian plateau in South America, the Colorado plateau of the United States, the Laurentian Plateau and the plateaus of Iran, Arabia, and Anatolia.
Glaciers are the perennial ice sheets on the planet. They are huge masses of ice that slowly move over the land surface, predominant in high mountains and the cold Polar Regions. The very low temperatures in the regions are the enabling factor for the buildup of snow and densification into ice at depths of 15 meters or even more. Most glaciers have density thickness in the ranges of 91 to 3000 meters.
The movements begin when the compaction is so dense that it moves under the pressure of its weight. It is estimated that more than 75% of the world’s fresh water is currently locked away in these frozen reservoirs. The glaciers include the Greenland Ice Sheet and the Antarctic Ice Sheet. The Antarctic Ice sheets outlet glaciers comprise the steep and extensively long and narrow depression Beordmore Glacier, which is one of the longest outlets in the world. The gradual rice in continental temperatures has seen the glacial density grow smaller owing to melting.
Hills are raised areas on the surface of the earth with distinctive summits, but are not as high as mountains. Hills are created as a result of accumulation of rock debris or sand deposited by wind and glaciers. They can also be created by faulting when the faults go slightly upwards. Hills are generally present in low mountain valleys and plains.
The Black Hills are the most known. Deep erosions of areas previously raised by the earth’s crust disturbances carry most of the soil away leaving behind a hill. Human activities may also create hill when soils are dug and piled giant masses. Volcanic eruptions as well create hills after the eruption when the molten materials or lava cools and hardens in a pile.
Loess is a fine-grained unstratified accumulation of clay and silt deposited by the wind. It appears brown or yellowish in color and is brought about by past glacial activity in an area. In precise, it is sedimentary deposits of clay and silt mineral particles which take place on land in some parts of the world. The thickness of loess deposits are just a few meters and one of their basic feature is known as the ‘cat steps’.
It’s held together by few clay particles and is mostly composed of quartz crystals which readily slide against each other. This property makes it highly susceptible to erosion which leads to the ‘cat steps’ feature. Loess formed after the ice age when the glaciers covering a relatively large portion of the earth melted and was carried away, exposing the vast plains of mud.
Upon drying of the mud, the forces of wind blew away the mud and exposed sediments and eventually deposited them as silt in stacks on top of each other to create bold steep banks. Regions made of loess are witnessed in eastern China and the northwestern region of the United States.
Plains are broad flat areas on the earth’s surface stretching over a wide area. Plains are lower than the land in their surrounding and can be found both inland and along the coast. Coastal plains rise from the seal level up to the point they meet raised landforms such as plateaus or mountains. The Atlantic Coastal plain is a prime example of a substantially populated and fertile coastal plain.
On the other hand, inland plains are generally found at high altitudes. Thick forests normally flourish on plains in humid climates. A fairly large portion of plains are covered by grasslands, for instance, the Great Plains in the United States. Human populations prefer settling on plains because of the soil and the terrain which is good for farming and building settlements such as cities, residential areas, and transportation networks. Flood plains are also in this category and they are formed as a result of continuous accumulation of sand, silt, and mud when rivers overflow its banks.
Deserts are the hot and dry areas of the world. They are the arid and semi-arid lands with little or no vegetation. Deserts constitute approximately 20% of the earth’s total land cover and are distinguished by little or no rainfall. The deserts are divided into four major categories including the Semi-Arid Deserts, the Hot and Dry Deserts, the Cold Deserts, and the Coastal Deserts.
These deserts are located in different areas of the world. Deserts experience very high temperatures, less cloud cover, low humidity, low atmospheric pressure, and very little rain, which makes them have very little vegetation cover. The soil cover is also rocky and shallow and with very little organic matter and as such, it only supports a few plants adapted to the conditions.
Plants such as cacti and short shrubs are the ones adapted to the desert conditions because they can conserve water and tolerate the high temperatures. Animals in the deserts include insects, small carnivores, snakes, lizards, and birds adapted to survive with very little water. These animals hide during the day till nightfall to avoid the heat. An example of a desert is the Sahara of North Africa.