Earth Processes

The Earth's Dynamics

The earth is a dynamic (changing) planet. The processes that alter the earth’s surface can be divided into several categories. The first categorization is; internal processes(diastrophism) and external processes(surficial). The other categorization, groups these processes into those forces that wear away the land including weathering and erosion (destructive processes), and constructive processes including volcanism and mountain building which increase the elevation of the land and opposition to gravity (These forces depend on the earth’s internal source of energy). All earth processes are manifestations of energy and these processes are responsible for sculpturing the land surface. External processes occur at or near the Earth’s surface and are powered by the energy from the sun whereas internal processes are due to heat energy which keeps rocks in the mantle below the earth’s crust in a molten state. Rocks and minerals form in response to Earth’s internal and external processes. There are six different types of energy and their contribution to earth’s dynamic processes which vary from significant to insignificant. The six types of energy are Potential energy, Kinetic energy, Heat energy, Chemical energy, Electrical energy, and Nuclear energy. 

Constructive Forces build up features on the surface of the Earth whereas Destructive Forces break down features on the Earth’s surface. Examples of destructive processes include weathering (chemical or mechanical), erosion (water - rivers and oceans, wind), the impact of organisms, and Earthquakes. Examples of external processes include weathering, mass wasting, and erosion. Internal processes include mountain building, volcanism, Earthquakes, and tectonic activities. The constant interaction between these two processes (internal, and external) determines the configuration of the earth’s surface.

 Not only do Earth processes have an impact on people, but we humans can dramatically influence Earth processes as well. River flooding is natural, but the magnitude and frequency of flooding can be changed significantly by human activities such as clearing forests, building cities, and constructing dams.

External Processes

The external processes are a result of solar energy and gravitational forces. They shape the relief created by internal processes. External agents carry out this process: water, ice, wind, atmosphere, and human beings. They include all the changes that alter or wear down the rocks and deposit materials resulting from erosion. 

Weathering

This is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks at or near Earth’s surface. Without weathering and erosion, the face of our planet might more closely resemble the lunar surface, which has not changed appreciably in nearly 3 billion years. There are two types of weathering mechanical, and chemical. Mechanical weathering is accomplished by physical forces that break the rock into smaller and smaller pieces without changing the rock’s mineral composition. Chemical weathering involves a chemical transformation of rock into one or more new compounds. Mechanical and chemical weathering usually occur simultaneously in nature and reinforce each other. Four physical processes are important in breaking rocks into small fragments; frost wedging, salt crystal growth, expansion from unloading (sheeting), and biological activity.

 The rate of weathering depends on; rock characteristics, climate, the total surface area of the rock, and anthropogenic activities

Sheeting

Image Credit: Courtesy

 Frost wedging: when water finds its way into cracks in rocks, it expands, breaking the rock into small fragments.

Frost wedging

Image credit: Courtesy

Salt Crystal Growth: It happens mostly at the coast, along the shorelines. The growth of salt crystals can split the rocks. 

Sheeting: the exposure of large masses of igneous rocks, particularly granite, by erosion leads to the breaking loose of concentric slabs, in a process known as sheeting. This occurs because of the great reduction in pressure when overlying rock is eroded away, in a process known as unloading.

Exfoliation/sheeting

Image credit: Courtesy

Biological Activity: Both mechanical and chemical weathering are accomplished by the activities or organisms. Plant roots in search of minerals and water grow into fractures, and as the roots grow, they wedge the rock apart. Burrowing animals further break down the rock by moving fresh material to the surface, where physical and chemical processes can more effectively attack it. Human beings, through blasting also play a significant role in enhancing weathering.

Biological Activity, Image Credit: Courtesy

 In chemical weathering, there is an interaction of rock with mineral solutions (chemicals) to change the composition of rocks. In this process, the water interacts with minerals to create various chemical reactions and transform the rocks. It contributes to mechanical weathering by weakening the outer portions of some rocks which, in turn, makes them more susceptible to being broken by mechanical weathering processes. Chemical weathering involves the following processes; hydrolysis, hydration, carbonation, solution, oxidation, and reduction.

 Soil is a product of weathering process. It is produced from rocks (parent material) through the processes of weathering and natural erosion. Earth’s land surface is covered by regolith, the layer of rock and mineral fragments produced by weathering. Soil is a combination of mineral and organic matter, water, and air—that portion of the regolith that supports the growth of plants.

 Weathering and Ore Deposits

Weathering creates many important mineral deposits by concentrating minor amounts of metals that are scattered through the unweathered rock into economically valuable concentrations.

Such a transformation is often termed secondary enrichment and takes place in one of two ways;

In one situation, chemical weathering coupled with downward-percolating water removes undesired materials from decomposing rock, leaving the desired elements enriched in the upper zones of the soil.

The second way is basically the reverse of the first. That is, the desirable elements that are found in low concentrations near the surface are removed and carried to lower zones, where they are redeposited and become more concentrated. 

Examples of ores created as a result of enrichment by weathering processes include Bauxite, copper, and silver.

 Erosion

Soil erosion is a natural process; it is part of the constant recycling of Earth materials that we call the rock cycle.

Once soil forms, erosional forces, especially water and wind, move soil components from one place to another. Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water. water Erosion; Raindrops strike the land with surprising force. Each drop acts like a tiny bomb, blasting movable soil particles out of their positions in the soil mass. Water flowing across the surface carries away the dislodged soil particles. Because the soil is moved by thin sheets of water, this process is termed sheet erosion. After this thin, unconfined sheet flows for a relatively short distance, threads of current typically develop, and tiny channels called rills begin to form. Still deeper cuts in the soil, known as gullies, are created as rills enlarge.

 

image credit: courtesy

The three types of wind erosion are; Surface creeps, Saltation, and Suspension. Surface creep in a wind erosion event, involves rolling across the surface of large particles ranging from 0.5 mm to 2 mm in diameter which causes them to collide with, and dislodges other particles. Saltation occurs among middle-sized soil particles that range from 0.05 mm to 0.5 mm in diameter. Such particles are light enough to be lifted off the surface but are too large to become suspended. Suspension involves tiny particles less than 0.1 mm in diameter being moved into the air by saltation, forming dust storms when taken further upwards by turbulence. These particles include very fine grains of sand, clay particles, and organic matter.

Mass Wasting

Mass wasting refers to the downslope movement of rock, regolith, and soil under the direct influence of gravity. It is distinct from the erosional processes because mass wasting does not require a transporting medium such as water, wind, or glacial ice.

Rock falls, slumps and debris flows are all examples of mass wasting. These events may occur very rapidly and move as a flow.

Gravity is the controlling force of mass wasting, but several factors play important roles in overcoming inertia and creating downslope movements.

Long before a landslide occurs, various processes work to weaken slope material, gradually making it more and more susceptible to the pull of gravity.
During this span, the slope remains stable but gets closer and closer to being unstable. Eventually, the strength of the slope is weakened to the point that something causes it to cross the threshold from stability to instability. Such an event that initiates downslope movement is called a trigger.

Landslide triggers may include: Intense rainfall, Rapid snowmelt, over-steepened slopes, removal of vegetation, Earthquake, Volcanic eruption, and Stream or coastal erosion

In the evolution of most landforms, mass wasting is the step that follows weathering. Once weathering weakens rock and breaks it apart, mass wasting transfers the debris downslope, where a stream, acting as a conveyor belt, usually carries it away.

Classification of Mass Wasting

 Geologists include several processes under the umbrella of mass wasting. Each process is defined by the type of material involved, the kind of motion, and the velocity of the movement. If soil and regolith dominate, terms such as debris, mud, and earth are used. In contrast, when a mass of bedrock breaks loose and moves downslope, the term rock may be part of the description. The kind of motion is described as either a fall, a slide, or a flow. When the movement involves the free falling of detached individual pieces of any size, it is termed a fall. Falls are common on slopes that are too steep for the loose material to remain on the surface. Many falls result when freeze-thaw cycles and/or the action of plant roots loosen rock and then gravity takes over. Sometimes falls may trigger other forms of downslope movement. Many mass-wasting processes are slides, which occur whenever material remains fairly coherent and moves along a well-defined surface. Sometimes the surface is a joint, a fault, or a bedding plane that is roughly parallel to the slope. However, in the movement called a slump, the descending material moves en masse along a curved surface of rupture. The third type of movement common to mass wasting is termed flow. Flow occurs when material moves downslope as a viscous fluid. Most flows are saturated with water and typically move as lobes or tongues. During events called rock avalanches, rock and debris can hurtle downslope at speeds exceeding 200 kilometers (125 miles) per hour. The common rapid mass-wasting processes include slump, rockslide, debris flow, and earthflow. Slump refers to the downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface. They are a common form of mass wasting, especially in thick accumulations of cohesive materials such as clay. As the movement occurs, a crescent-shaped scarp (cliff) is created at the head, and the block’s upper surface is sometimes tilted backward. Rockslides occur when blocks of bedrock break loose and slide down a slope. If the material is mostly soil and regolith, the term debris slide is used instead. They usually take place in a geologic setting where the rock strata are inclined or where joints and fractures exist parallel to the slope. Rockslides can be triggered by an earthquake, or when rain or melting snow lubricates the underlying to the point that friction is no longer sufficient to hold the rock unit in place. Debris flow is a relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water. They are sometimes called mudflows when the material is primarily fine-grained. They tend to occur most frequently in semiarid mountainous regions, though they can also occur in many other climate settings. Debris flows called lahars are also common on the steep slopes of some volcanoes. Because of their fluid properties, debris flows frequently follow canyons and stream channels. Debris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars. Earthflows most often form on hillsides in humid areas during times of heavy precipitation or snowmelt. When water saturates the soil and regolith on a hillside, the material may break away, leaving a scar on the slope and forming a tongue- or teardrop-shaped mass that flows downslope. They range in size from bodies a few meters long, a few meters wide, and less than 1 meter deep to masses more than 1 kilometer long, and several meters wide. They generally move at slower rates than the more fluid debris flows, with the velocities ranging from 1 mm per day to several meters per day. This, however, is dependent on the material's consistency, and how steep the slope is.

Earthflow (image credit: courtesy)

Earth’s Internal Processes

They derive their energy from the Earth’s interior. They are driven mostly by temperature differences within the Earth’s mantle. They cause the Earth’s crust to move, thereby elevating the earth’s surface. They include Earthquakes, tectonic activities, the Earth’s mountain-building, and volcanic processes that produce slopes through sporadic changes in the elevations of landmasses and the ocean floor. There are climate–geology connections involving the impact of internal processes on the atmosphere.

For example, the particles and gases emitted by volcanoes can change the composition of the atmosphere, and mountain building can have a significant impact on regional temperature, precipitation, and wind patterns.

Magma can be confined underground or it may erupt onto the surface. Intrusion is an internal process. As hot magma squeezes into cracks and zones of weakness, the cooling magma passes its heat energy to the nearby rock, which could result in the change of rocks, through contact metamorphism. 

Plate Tectonics

Earth is big enough and generates enough heat to allow plate tectonics to operate and form the large-scale features observed today. The existence of tectonic activity on Earth, especially plate tectonics, is largely responsible for Earth’s uniqueness. Tectonic-related volcanoes, like in the steam-emitting, divergent rift shown below release water vapor and other gases that are an essential part of our atmosphere.  

In 1912, Alfred Wegener, a German explorer, astronomer, and meteorologist proposed that the continents we see are the broken fragments of a single landmass that he called Pangaea, which means all Earth. 

 He supported his theory by pointing out that a particular fossil leaf (Glossopteris) is found in South America, Africa, Australia, Antarctica, and India. Bringing the continents together would put the areas where these fossil organisms are found next to each other. The same placement would also line up distinctive rock types and mountain areas. On the basis of this evidence, Wegener proposed his hypothesis which came to be known as continental drift. Without plate tectonics, Earth could have a thick, stifling, CO₂-rich atmosphere like that of Venus and might be lifeless. Without tectonics to dewater and degas Earth's interior, Earth might not have oceans, lakes, streams, or other parts of the hydrosphere. 

The basic idea of plate tectonics is that Earth’s surface is divided into a few large, thick plates that move slowly and change in size. Intense geologic activity occurs at plate boundaries where plates move away from one another, past one another, or toward one another. Plates are bounded by three distinct types of boundaries, which are differentiated by the type of movement they exhibit. Divergent plates (constructive), convergent plates (destructive), and transform plate boundaries (conservative). Divergent and convergent plate boundaries each account for about 40 percent of all plate boundaries. Transform faults account for the remaining 20 percent. 

Earthquakes

An earthquake occurs when energy stored in rocks is suddenly released. They form when the forces acting on rock exceed the rock’s strength. Most earthquakes are the result of the movement of Earth’s crust produced by plate tectonics. As a whole, tectonic plates tend to move gradually. Along the boundaries between two plates, rocks in the crust often resist movement. Over time, stress builds up. When stress overcomes the strength of the rocks involved, movement occurs along fractures in the rocks. 

Description

The vibrations caused by this sudden movement are felt as an earthquake. When an earthquake occurs, it releases mechanical energy, some of which is transmitted through rocks as vibrations called seismic waves. These waves spread out from the site of the disturbance and travel through the interior or along the surface of Earth. Scientists record the waves using scientific instruments (seismometers) at seismic stations. The record produced by a seismometer is called a seismogram. The place where the earthquake is generated is called the hypocenter or focus. Earthquakes are described using a richer scale, and modified Mercalli scale.

Mountain Building

A mountain is a large terrain feature that rises more or less abruptly from surrounding levels. Volcanoes are mountains; so are erosional remnants of plateaus (mesas). Mountains form through dynamic processes which crumple, fold, and create faults in Earth’s crust. The height of mountains is controlled primarily by the density and thickness of the crust. Convergence causes the crust to thicken and form mountain belts Mountains on the ocean floor and some mountains on continents form through processes other than convergence. The layers of a mountain record the vast geologic history of the region. Looking at a globe or a map of Earth’s surface, you immediately notice the oceans and continents. It can be estimated that about 71 percent of Earth’s surface is below sea level, and about 29 percent lies above sea level. The term for the processes that collectively produce a mountain belt is orogenesis. Most major mountain belts display striking visual evidence of great horizontal forces that have shortened and thickened the crust. 

These compressional mountains contain large quantities of preexisting sedimentary and crystalline rocks that have been faulted and contorted into a series of folds. Although folding and thrust faulting are often the most conspicuous signs of orogenesis, varying degrees of metamorphism and igneous activity are always present. Mountains form at the subduction zones, where two plates converge. At the subduction zones, the subduction of the oceanic lithosphere triggers partial melting of mantle rock, providing a source of magma that intrudes the crustal rocks that form the margin of the overlying plate. In addition, colliding plates provide the tectonic forces that fold, fault, and metamorphose the thick accumulations of sediments that have been deposited along the flanks of landmasses. Together, these processes thicken and shorten the continental crust, thereby elevating rocks that may have formed near the ocean floor to lofty heights. Where oceanic lithosphere subducts beneath an oceanic plate, a volcanic island arc and related tectonic features develop. Island arcs result from the steady subduction of the oceanic lithosphere, which may last for 200 million years or more. Periodic volcanic activity, the emplacement of igneous plutons at depth, and the accumulation of sediment that is scraped from the subducting plate gradually increase the volume of crustal material capping the upper plate. The continued growth of a volcanic island arc can result in the formation of mountainous topography consisting of belts of igneous and metamorphic rocks.

Collisional mountain belts are formed when one or more buoyant crustal fragments collide with a continental margin as a result of subduction. Oceanic lithosphere, which is relatively dense, readily subducts. The continental lithosphere, which contains significant amounts of low-density crustal rocks, is too buoyant to undergo subduction. Consequently, the arrival of a crustal fragment at a trench results in a collision with the margin of the adjacent continental block and an end to subduction. Continental collisions result in the development of mountains characterized by shortened and thickened crust achieved through folding and faulting, for example, the Himalayas, and the Appalachians.

Fault-block mountains, also known as topographic mountains are bounded by high-angle normal faults that gradually flatten with depth. Most fault-block mountains form in response to broad uplifting, which causes elongation and faulting. They can also be produced by continental drifting. 

Volcanic mountains are created when volcanism piles volcanic materials on a preexisting surface. Such mountains vary in size from small scoria (cinder) cones to large shields and composite volcanoes. 

Mountains are also formed by differential erosion. A granite pluton intruded into softer rocks commonly resists erosion and is left higher than its surroundings. A mountain or hill that remains when other rocks have been eroded down is an erosional remnant.

 

Fold mountains, and hills are formed when the land surface and near-surface rocks are deformed via folding. Folding can warp and uplift Earth’s surface as well as the underlying rock layers.  Uplift and erosion of a folded, hard layer create a topographical high that remains long after the folding stops. For more information on fold mountains, visit: fold mountains

          Read more on mountain building here mountains

Summary

Major mountain belts are made up of a number of mountain ranges. The major factors that control the growth and development of mountain ranges are intense deformation (during an orogeny), isostasy, and weathering and erosion. An orogeny involves the folding and faulting of sedimentary and volcanic rock, regional metamorphism, and igneous activity. 

Mountain belts can develop as a result of the collision of one or more crustal fragments including island arcs and oceanic plates to a continental plate (oceanic-continental convergence), or the merger of one or more crustal fragments including island arcs, and oceanic plates to a continental plate, collision of continental plates e.g Ural mountains which were as a result of the collision of Asia and Europe, the convergence of the African and European plates created the Alps, continental rifting, producing topographic mountains or fault-block mountains, vertical movement of the crust, volcanism, folding, and differential erosion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 .