Environmental Science Review

The Earth’s Components and Characteristics

A study of Environmental Science should begin with a basic overview of the systems that are at work on Earth. This chapter provides a brief overview of several of these topics. They include: basic earth science phenomena, including earthquakes, volcanoes, plate tectonics, the seasons, and geologic time. The next section will explore the atmosphere, the role of water on our planet, and the roll of soil on our planet.


Basic Earth Science Phenomena [A head]

The movement of the plates across the surface of Earth as well as the relationship between the Sun and Earth play major roles in the landscape on Earth as well as in our daily lives here on Earth.

Plate Tectonics [B head]

Look at a map of the world. Notice how the continents seem to fit together like pieces in a giant jigsaw puzzle? Early scientists, like Alfred Wegener, noticed the same thing. Alfred Wegener was a German meteorologist and geophysist who in the early 1900s noticed that the continents fit this pattern. He ultimately suggested that at one point in Earth’s history all the continents were together in one supercontinent which he called Pangaea. He said that at some point in time the continent broke from Pangaea and drifted to their current positions. He called his idea continental drift.

[insert Figure 3.1 breakup of Pangaea, here]

Wegener had other evidence for continental drift. He learned of fossil organisms that had been found on different continents but could not have crossed the oceans that currently exist between those continents. One example of such a fossil is Mesosaurs. Mesosaurus was an aquatic reptile whose fossilized remains have been found in South America and southern Africa. Wegener contended that it was if Mesosaurs was distributed on those two continents, then it should have been more widely distributed around the world. It isn’t, so Wegener explained the fossil evidence by suggesting that South America and Africa were once connected.

Other evidence supporting Wegener’s ideas included the fact that certain rock types and structures seemed to match o n the continents that would fit together as part of the jigsaw puzzle. For example, the Appalachian Mountains in the eastern United States travel up off the coast of Newfoundland. There is a band of mountains in western Europe that are similar and the same age as the Appalachians. This suggests that the mountains formed at the same time, when the continents were connected in the past.

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Continental drift had a fundamental flaw – it could not explain how the continents move. Wegener’s idea did not catch on. Many scientists did not accept his ideas about the continents moving. They had their own explanations for the features and phenomena that were occurring o n the surface of Earth.

However, as scientists began to find evidence that the plates do move, others began to revisit Wegener’s theory of continental drift. A huge mountain chain, or ridge, in the middle of the Atlantic Ocean was studied. It is part of a huge mountain chain that runs through Earth’s ocean basins. Scientists find evidence of sea-floor spreading at these ridges. Sea-floor spreading is the process where new oceanic crust forms as magma rises to t he surface and hardens. As plates move apart, the sea floor spreads too and new crust is formed.

How do scientists know for sure this is happening? They use what is called magnetic reversals to prove it. The north and south poles on Earth have switched several times in Earth’s history. Right now, if you look at a compass, the north arrow actually is pointing north, near the North Pole. But at different times in Earth’s history, the north end of the arrow o n a compass would have been pointing toward the South Pole. No one really knows why this happens but it has occurred many times. Some minerals contain magnetic material that aligns itself with the current north Pole. And once the minerals have hardened into rock, this can not be erased.

[Insert Figure 3.2, seafloor spreading and magnetic pole reversals, here]

As the sea floor spreads and grows at a ridge, the magnetic minerals align themselves with the magnetic field. If the magnetic poles reverse, the new rocks that form will align themselves the new way. This creates bands of rocks on the ocean floor that are equidistance from the ridge. This is seen as evidence that the plates are moving apart.

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With this new knowledge about sea floor spreading, magnetic pole reversals, and plate boundaries, scientists formed a new theory to explain how the plates on Earth move. They called this theory Plate Tectonics. Plate tectonics is the theory that earth’s lithosphere is divided into tectonic plates that move on top of the asthenosphere (the upper layer of the mantle). Plates can be composed primarily of continental crust or can be composed primarily of oceanic crust. Continental plates have a composition very similar to granite and tend to be thicker and older than oceanic plates. Oceanic plates are composed primarily of basalt and are denser than continental plates.

There are three basic types of plate boundaries that you need to remember: convergent, divergent and transform. A plate boundary is a place where two or more tectonic plates meet. The type of plate boundary describes how the plates move with respect to each other when they do meet.

A convergent plate boundary occurs when two plates move toward each other. If continental plates move toward each other, it is possible that mountain will form since the plates are of equal density and they will crash into each other and buckle up. If two oceanic plates collide, this may also form mountains, under the ocean, as the plates are again the same density. The collision of a continental and an ocean plate causes what is known as a subduction zone. The oceanic crust is denser than the continental crust and will sink beneath it. This is an area where many volcanoes may form, as will be discussed further later in this section. This is an area where crust is destroyed.

[insert Figure 3.3, subduction zone, here]

A divergent plate boundary is where two plates move apart. The ridges, such as the Mid Atlantic Ridge, discussed earlier are divergent plate boundary. Divergent plate boundaries are areas where new crust is created.

A transform plate boundary occurs where two plates slide past each other. Crust is neither created nor destroyed in these areas. As two plates move horizontally past each other, earthquakes may occur. The San Andreas Fault in California is one of the more infamous transform plate boundaries.

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One of the most significant pieces of information that the theory of plate tectonics provides is that it offers an explanation for how the plates move. In fact, it offers three possible causes of plate movement. These are convection, ridge push, and slab pull.

[insert Figure 3.4, movement of plates, here]

Hot magma from deep in the Earth rises. As it nears the surface, it begins to cool. This cooling magma then sinks deeper into the mantle where it warms up and begins to rise again. This cycling motion is called convection. Convection causes the oceanic plates to move sideways, out away from the ridge.

At a mid-ocean ridge, the oceanic plate is elevated compared to the rocks around it. Gravity causes the oceanic plates to slide down the ridge in a process called ridge push.

When a continental plate and an oceanic plate collide, the older, denser oceanic plate will subduct under the continental plate. As it sinks, it will pull the plate that is behind it with it, This is called slab pull.


These three forces, working together, move the plates around the surface of Earth. The plates move slowly. You typically can not feel the movement as it is on average, several centimeters each year. But new technology is tracking the movement of the plates. Global positioning systems and satellites are monitoring the movement of the plates very carefully and are beginning to predict the motion and position of the plates in the future.

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Earthquakes [B head]

Most earthquakes occur at or near the edges of the tectonic plates.

[insert Figure 3.5, map of major earthquake zones with plate boundaries, here]

As the tectonic plates move toward each other, away from each other, or past each other, faults are created. Faults are breaks in the rock along which movement can occur. The movement of the tectonic plates creates pressure on the surrounding rock. This pressure causes the rock to bend or deform. It helps to think about a rubber band during this discussion. You know how you can stretch a rubber band only so far before it breaks? This is what happens to the rock. A rock can be deformed and not break. But eventually it will reach a point where it can’t deform any more and it will break (just like the rubber band). It is this breaking, or release of energy, that results in an earthquake.


The energy released by an earthquake travels in waves. There are three different types of seismic waves: P waves, S waves, and surface waves. P waves and S waves are body waves, they travel in the crust. Surface waves travel along the surface.

A P wave, or primary wave, is the fastest moving wave in an earthquake. They are the first seismic wave to be detected during a quake. P waves can travel through solids, liquids, and gases although they travel at different speeds through each of these mediums. P waves are compressional waves. Think about a spring toy you may have used when you were younger. Say you held one end of the spring and a friend held the other end. If you gathered up several coils on your end and then let them go, the bunched coils would move “down” the spring toy. This is t he same way that P waves move. The P waves move back and forth which squeezes and stretches the material it passes through.

S waves, or secondary waves, are the second fastest wave created in an earthquake. S waves move side to side in a shearing motion. Again think of the spring toy with you and a friend holding it stretched out between you. If you were to move the spring toy from side to side, you would be creating S waves. S waves only travel through solids.

Surface waves are the slowest wave generated by an earthquake. They travel along the surface of Earth in a rolling motion. Going back to the spring toy example, imagine you and a friend are holding the spring toy and you begin to move it up and down. The result would be similar to a surface wave. The effects of surface waves do not travel far from the quake, while P and S waves can be detected throughout the globe after an earthquake.

[insert figure 3.6, earthquake waves, here]

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How can an earthquake on one side of the Earth be detected on the other? A closer look at an earthquake may help explain this.

Scientists use instruments called seismographs to detect seismic waves. Seismographs are used to determine when and where an earthquake began. The epicenter of an earthquake is the point inside the curst, along a fault, where the movement began. The focus is the location directly above the epicenter on the surface where most of the damage occurs.

[insert figure 3.7, epicenter and focus, here]
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Seismographs around the world can detect the seismic waves. Scientists use information from these seismographs to determine the epicenter of the earthquake. Each seismograph will record the time that the first P wave arrived and the arrival time of the first S wave. A seismograph creates a record called a seismogram where this information is recorded.

[insert figure 3.8, seismogram with P and S arrival times noted, here]

After determining when each wave arrived at a particular seismograph, scientists use a chart similar to this one to determine how far the seismograph was from the earthquake.

[insert figure 3.9, S-P Time Distance Graph, here]

Scientists will determine the difference in time between the arrival of the P wave and the arrival of the S wave and measure that distance along the y-axis. They will then take that distance and find out where it fits exactly between the P and the S curve on the graph. When that found, they will determine what distance that correlates to. This tells how far that seismic station is from the quake.

Then, scientists draw a circle around the seismograph station on a large map. This circle has a radius equal to the distance the seismograph is from the earthquake. Scientists now know that the earthquake occurred somewhere on that circle. This process will be repeated at least two other seismograph stations to determine where the earthquake occurred. This process, called triangulation¸ is shown in Figure 3.10.

[insert figure 3.10, finding the epicenter, here]

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You have probably heard about earthquakes on the news. And when you did, the newscaster most likely talked about the Richer Scale. The Richter Scale is just one way to discuss the size of an earthquake. There are several more.

When scientists measure an earthquake, they measure the magnitude and the intensity of the seismic waves. The magnitude of an earthquake is a measure of the amount of energy that is released. Several scales have been developed so that earthquakes around the world can be compared. The intensity of an earthquake is a local measurement of the damage done to structures and humans. this is a very subjective measurement.

Charles Richter developed the first scale to determine the magnitude of earthquakes in 1935. This is known as the Richter Scale. The Richter Scale measures the ground motion of an earthquake and then adjusts for distance to find the strength of the quake. Each time the magnitude increases by one unit, the measured ground motion becomes 10 times stronger. This means that an earthquake of magnitude 5.0 will produce 10 times as much shaking as a magnitude 4.0 quakes. The Richter Scale, as developed by Charles Richter, is really not adequate for measuring large quakes. Recently a more precise measurement, called the moment magnitude, has been used. The moment magnitude of a quake is determined by measuring the movement along a fault, rather than by looking at the ground motion at some point. The moment magnitude scale does provide accurate measurements for larger earthquake and is largely replacing the Richter Scale.

Earthquake intensity is usually measured by the Modified Mercalli Intensity Scale. This is shown in figure 3.11.

[insert figure 3.11, modified mercalli scale, here]


It is important to keep in mind that the Mercalli scale, or any intensity scale, is based on not only the severity of the earthquake but also on factors such as population density, building design, and the nature of the surface and subsurface materials.

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Volcanic Activity [b head]

Volcanoes are another result of the movement of plates on the surface of Earth. As you will see, volcanic activity can be explosive or quiet, occur deep inside Earth or at the surface, and can be a constructive and destructive force at the same time.

When discussing the volcanic eruptiosn that occur on the surface of Earth’s crust, the nature of the eruption depends on the magma’s compitioion, temparure, and the amount of dissolved gases it contains.


Seasons [B head]

The seasons on Earth are the result of interaction between Earth and the Sun. It is colder in the winter than in the summer. There are a few factors that influence this. First of all, the days are longer in the summer than they are in the winter. The longer days expose Earth to more solar radiation than in the winter, which creates warmer temperatures.

The Sun also changes place with respect to the horizon. In the summer, the Sun appears to be high above the horizon at noon. As fall approaches, the noontime Sun appears lower in the sky and the days get shorter. This is a shift in the altitude of the Sun.

This change in altitude impacts the amount of energy a spot on the surface receives from the Sun. When the Sun is high in the sky overhead, the solar rays are most concentrated. As the altitude of the Sun decreases, the solar radiation becomes more spread out and less intense. This means that the Sun’s energy is stronger and more concentrated in the summer than in the fall or winter months.

Going with this idea, it is good to recall that Earth has a spherical shape. This means that the Sun hits areas around the equator at an angle close to 90º.

The tilt of Earth on its axis contributes to the seasons also. Earth is tilted on its axis 23.5º. This tilt influences the relative altitude of the Sun over a certain part of Earth. For example, during the winter, the Northern Hemisphere is tilted away from the Sun while the Southern Hemisphere is tilted toward the Sun. The Southern Hemisphere receives more direct solar radiation. The days are longer and the temperatures are warmer. At that same time, the Northern Hemisphere is tiled away from the Sun, receiving indirect solar radiation. The result is lower temperatures and shorter days. The reverse happens during the summer months, when the Northern Hemisphere is tilted toward the Sun and the Southern Hemisphere is titled away from the Sun.

The Geologic Time Scale

The Geologic Time Scale divides Earth’s 4.6 billion year old history into subgroups that make arranging geologic events more orderly.

Eons represent the greatest span of time. There are four eons: the Hadean, Archean, Proterozoic, and Phanerozoic. The Hadean Eon represents the oldest amount of time. Scientists name the time from 4.6 billion years ago to 3.8 billion years ago the Hadean. No rocks from this eon have been found on Earth although meteorites and samples of moon rock s have provided rocks of this age. The Archean Eon was from 3.8 billion years ago to 2.5 billion years ago. The oldest rocks on Earth date back to the Archean Eon. The Proterozoic ranged from 2.5 billion years ago to 543 million years ago. The Proterozoic saw an explosion of life as the first organisms with well developed cells appeared during this eon. We now live in the Phanerozic Eon which lasted from 543 million years ago to today. The fossil record is mainly represented by organism from this eon.

Eons are subdivided into eras which are divided in to periods. The Phanerozoic Eon is the only eon with subdivisions. It is divided into the Paleozoic era, the Mesozoic Era and the Cenozoic Era. The Paleozoic era lasted from 543 million years to 248 million years. The Paleozoic saw a large increase in the number of marine organisms on the planet. By the end of the era, land plants, amphibians, reptiles and insects dominated live on Earth. The era came to an end with the largest case of mass extinction in the history of Earth. Scientists estimate that 90% of the species alive during this era became extinct.

The Mesozoic era began 248 million years ago and is called the Age of the Reptiles. Reptiles, such as dinosaurs, dominated the land. At the end of the Mesozoic, somewhere between 10 and 15% of all species on Earth became extinct. It is believed that climate change was the cause of this extinction,

The Cenozoic era began 65 million years ago and is also know as the Age of Mammals. With t he extinction of the dinosaurs and other reptiles at the end of the Mesozoic, the mammals no longer had to compete for space or resources. This allowed the mammals to flourish.

Eras are subdivided into periods. As shown in the diagram, the Paleozoic was divided into the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. The Mesozoic is split into the Triassic, Jurassic, and Cretaceous. The Cenozoic is divided into the Tertiary and the Quaternary. The periods of the Cenozoic are further subdivided into epochs such as the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene. The boundaries between these geologic time intervals coincide with times where visible changes occurred on earth. This could be times of mass extinction, evolution of species, or major geological events.


The Role of the Atmosphere

The Composition of the Atmosphere

The composition of air surrounding Earth is not constant. It changes from day to day and from place to place.

[ART: circle graph of composition of the atmosphere]

The atmosphere is composed of different gases. Nitrogen and oxygen account for 99% of the composition of the atmosphere. Nitrogen is approximately 78% of the atmosphere and oxygen is 21%. The remaining 1% is things such as argon, carbon dioxide, water vapor, and other gases. It is important to also keep in mind that particles such as dust, volcanic ash, sea salt, dirt, and smoke are also components of our atmosphere. Some areas contain more of one of these than others but these particles can have a significant influence on local weather conditions and atmospheric conditions.

There are four layers of the atmosphere: the troposphere, mesosphere, stratosphere, and thermosphere. The troposphere is the layer that is closest to Earth’s surface. The troposphere is the densest layer, containing nearly 90% of all the mass of the entire atmosphere. Almost all of the clouds, carbon dioxide, water vapor, life forms, and air pollution occur within the troposphere. The air within the stratosphere is very thin. Temperature rises as the elevation in the stratosphere increases. This is mainly due to the fact that the ozone layer is found within the stratosphere. This layer of ozone absorbs ultraviolet radiation from the Sun and serves to protect life on Earth from these harmful rays. The mesosphere is the coldest layer of the atmosphere. Temperature decreases as altitude increases in the mesosphere. The uppermost layer is the thermosphere. This layer blends in with the outer atmosphere. The temperature within the thermosphere increases with altitude.

Weather and Climate

Weather and climate are terms that are sometimes used interchangeably. But this is incorrect. Weather is the condition of the atmosphere at a certain time and place. Weather is affected by the amount of water in the atmosphere. An example of weather could be that today is a cloudy, windy day with spotty snow showers. Weather describes the temperature, humidity, atmospheric pressure, wind, and visibility at a point in time.

Climate is the average weather condition in an area over a long period of time. Climate is largely determined by temperature and precipitation. For example, the climate of northern South America is a temperate rainforest.



Atmospheric Circulation and the Coriolis Effect
The movement of air due to changes in air pressure is what we know as wind. Generally speaking, the greater the difference in pressure, the faster the wind will move. These differences in air pressure are caused by differences in the heating of the surface of Earth. Warmer air is less dense, which creates an area of low pressure in the region of the equator. The warm air from t he equator, rises and moves toward the poles. The air cools as it nears the poles and cool air is denser and will sink. This creates an area of high pressure.

Air around the surface of earth travels in convection cells which are areas of rising and sinking air. Bands of high and low pressure are created around earth every 30º or so. As showing in this diagram, as warm air rises from the equator the air begins to cool. At about 30 north and south of the equator, some of that cooler air begins to sink. This creates a high pressure area in this region. The cool air sinks back toward the equator, warming up again, so the convection cell continues. At about 60 north and south latitude, the warmer air rises, creating an area of low pressure. The movement of air in these convection cells around the surface of Earth help create the global wind patterns that affect our planet.

Another significant force influences the movement of the air along the surface of Earth. Earth is spinning on its axis. Wind that is moving along the surface is deflected by this rotation. This is known as the Coriolis Effect. The Coriolis effect causes the winds in the traveling north in the Northern Hemisphere to curve east and winds that are traveling south in the Northern Hemisphere to curve west.

All free flowing objects, including air and water, move to the right of their path of motion in the Northern Hemisphere and to the left of their motion in the Southern Hemisphere.

The Coriolis effect shifts the winds around the globe. It is important to remember that the deflection is always at right angles to the direction of air flow. The Coriolis effect only affects wind direction not wind speed and that the deflection caused by the Coriolis effect is strongest at the poles and weakest nearing the equator. The effects of the Coriolis effect are basically negligible at the equator.

This communication of convection cells every 30º of latitude and the Coriolis effect, creates patterns of air circulation called global winds. The diagram below shows the major global wind systems. Winds are named for the direction from which they blow.

The wind belts that occur at the poles to about 60º latitude are called polar easterlies. Cold air from the poles sinks to 60º north or south latitude. The belt between 30º and 60º latitude are called westerlies. Westerlies flow from the poles from west to east. These westerlies are responsible for carrying moist air across the United States, which can create rain or snow. The wind that blow from 30º almost to the equator are called tradewinds The trade winds curve to the west in the Northern Hemisphere and to the east in t eh Southern Hemisphere because of the Coriolis Effect. Early explorers and traders used these trade winds to accelerate their trip from Europe to the Americas.

The tradewinds meet in an area around the equator called the doldrums. There is very little wind in this area because the warm, rising air over the equator creates an area of low pressure.

About 30º north and south of the equator are regions called the horse latitudes. Sinking air in these regions create areas of high pressure. The winds in these areas are weak. The sinking air is dry, which explains why most of the deserts on Earth are located in the horse latitudes.

Another type of wind that is found on earth is a jet stream. A jet stream is a narrow band of high-speed wind that blows in the troposphere and lower stratosphere. Winds within a jet stream can reach speeds of 400 km/h. Jet streams do not follow regular paths. Jet streams are used by airline pilots to accelerate their trips. Jet streams are also followed closely by meteorologists who are tracing major storms or the approaching cold or warm front.

There are also local winds operating on the surface of Earth. Local winds move short distances and can blow for many direction. A local shoreline or mountain can cause dramatic temperature differences. This can in turn, create local winds during the day as the wind blows from the ocean to the shore. The air over the ocean is cooler and forms an area of high pressure. the air over the land is warmer, creating an area of low pressure. The air moves form the area of low pressure to an area of high pressure, which creates a sea breeze.
At night, the air over the ocean is warmer, creating high-pressure while the air over the land is cooler. The air then moves out to sea, creating a land breeze.

[ART: general diagram showing the unequal heading of the Earth which creates pressure belts]





The Role of Water

Freshwater and Saltwater

Water is everywhere on Earth. It exists in oceans, in glaciers, rivers, lakes, the air, soil, and in living organisms. The water on the planet constitutes the hydrosphere. 97.2% of the water on the planet is in the ocean. Ice sheets and glaciers make up another 2.15% of all the water. The remaining 0.65% is water that exists as groundwater, lakes, rivers, and the atmosphere.

Water is constantly in motion in the ocean, atmosphere, solid Earth, and the biosphere. This constant motion is called the hydrologic cycle. The hydrologic cycle is powered by energy from the Sun; Water evaporates into the atmosphere from the oceans. Winds move this moisture around the globe until conditions are right for the formation of clouds. Eventually the moisture condenses into clouds and precipitation results. When the precipitation falls to the surface water can infiltrate into t he ground, to become part of the groundwater in the area or it runs off along the surface. Some water that infiltrates into the ground is used by plants which release the water back into the atmosphere by a process called transpiration. Eventually all that water makes its way back to the ocean, and the cycle continues.

Ocean Circulation
Water in the ocean moves in currents. Ocean currents are influenced by weather, the position of the continents, and the rotation of Earth. Surface currents and deep ocean currents work together to move water Earth’s oceans.

Surface currents are movements of water that occur at or near the surface of the ocean. Surface currents can reach depths of several hundred meters and can reach lengths of several hundred kilometers. The Gulf Stream is one such current. Surface currents are controlled by global winds, the Coriolis effect, and the position of the continents. These factors working together create surface currents that move in distinct patterns. Global winds, such as westerlies, polar easterlies, and trade winds, move the water in the ocean as they blow across the surface of earth. The Coriolis effect deflects not only air, but water as well. Surface currents in the northern hemisphere turn clockwise as a result of the Coriolis effect while surface currents in the southern hemisphere turn counterclockwise. Surface currents will deflect when they meet continents.

Another factor that is important to t he movement of surface currents is the differences in water temperatures. Warm-water currents originate near the equator and move warm water round the globe. Cold-water currents originate near the poles and move cold water over the globe.

The result of these factors creates surface currents as showing in this diagram.

There are also deep currents in the ocean. Deep currents are located far below the surface of the ocean. Deep currents are influenced by differences in density. Density of ocean water is controlled by salinity and temperature.

Water Use




Water Pollution and Clean up




Ocean Circulation




The Role of Soil

The Rock Cycle

Soil is an essential resource on the planet. Soil is a loose mixture of small mineral fragments, organic material, water, and air. Soil is formed from weathered rock fragments. The best way to start a discussion about the formation of soil is to begin with the rock cycle.

Rocks change form through a continuous cycle called the rock cycle. There is no beginning or end point of the rock cycle but for the sake of discussion, we’ll start with igneous rocks. Igneous rocks form from the cooling of molten magma or lava. Igneous rocks can then break down into sediment through weathering and erosion. This sediment may find itself in a river and finally deposit in the bottom of a quiet lake. If the sediment is compacted and/or cemented together, it will form a sedimentary rock. It is possible that over time, the sedimentary rock is buried and subjected to intense heat and pressure. If this is the case, and the rock does not melt, then the rock will change into a metamorphic rock. If that metamorphic rock is subducted into the mantle and melts, it can then possibly cool into an igneous rock, starting the cycle again.

Sedimentary rocks can change into igneous rocks or metamorphic rocks. Igneous rocks may become sedimentary rock, a different igneous rock, or a metamorphic rock. Metamorphic rocks can change into other metamorphic rocks, weather into sedimentary rocks, or melt and become igneous rock. The rock cycle is continuous and does not follow a certain pattern,

Soil Formation and Composition

When a rock is weathered, it breaks down to form soil. The formation of soil depends primarily on six factors: parent material, time, climate, plants and animals, and slope.

The source of weathered mineral matter from which soil develops is called the parent material of the soil. They type of parent material affects the rate of weathering and the chemical makeup of the parent material impacts the fertility o f the resulting soil. Certain rock types produce more fertile soil than others.

If weathering has progressed over a short period of time, the parent material is a significant factor in the type of soil that forms. However, if weathering has progressed over a longer period o f time, other factors are a bigger influence over the formation of the soil. Climate in particular, comes into play more.

Climate is the most influential control over the formation of soil. Differences in temperature and precipitation dictate whether mechanical or chemical weathering will dominate the weathering process. A hot, wet climate will produce a layer of chemical weathered soil while a cold, dry climate will produce a think layer of mechanically weathered debris.

Plants and animals are the primary source of organic material to a soil. Almost all soils have some organic material, although in varying amounts.

Soil influences soil, formation and can create dramatic differences in soil type over a localized area. Steep slopes support poorly developed soils, primarily due to the fact that little water can seep into the soil and erosion is accelerated by these steep slopes. A power drained, water logged soil are in areas that are flat.



Main Soil Types

The processes that work to form soil work from the surface downward. This means that differences in composition, texture, structure, and color change gradually at increasing depths. These differences split the soil in to different layers or horizons. A vertical cross-section of soil is called a soil profile.

Soil profiles and the horizons that they contain vary from place to place. An idealized soil profile has been provided below. It is important to realize that at any one location, one or more of the horizons shown here may be missing or have different thicknesses.

This is a model soil horizon from a humid climate at a middle latitude location. The O and A horizons together make up the topsoil. This is where most of the organism material in a soil is concentrated. The O horizon is loose and partly decayed organic matter such as leaf litter while the A horizon is mineral material mixed with humus

Moving down this idealized soil profile, the next horizon is the E horizon. This layer is the zone of eluviation and leaching. Little organic material is found in this layer of light colored layer. Water percolates though the layer, washing out the finer materials. This is called eluviation. The water moving through this layer also removes soluble materials from the other layers and deposits them in lower layers. This is called leaching.

The B horizon is the layer below which is the area where the leached material is finally deposited. It is sometimes called the zone of accumulation because of that.

The C horizon sits atop of the parent material that is being weathered and contains partially altered parent material. The parent material is easily recognizable in the C horizon.

There are hundreds of soil types around t the world. But there are three very general types that can be discussed. These are pedalfers, pedocals, and laterites. Pedalfers contain iron oxides and aluminum-rich clays in the B horizon. Iron rich materials and clays are leached from the E horizon into the B horizon in areas with sufficient rainfall. This gives the B horizon a brown to reddish color in these areas. Pedalfer soils are best developed in areas with sufficient organic material, such as forest soils. Decaying organic material provides the acidic conditions that are needed for leaching.

Pedocols are characterized by large accumulations of calcium carbonate. Pedocol soils are found in areas, such as the western United States that are drier and have grassland or brush vegetation. There is less clay in a pedocol soil than in a pedalfer soil.

Laterite soils develop in wet, hot topical climates. These conditions are prefect for chemical weathering to occur which creates deep soils. Significant leaching in these areas removes materials such as calcite and silica making iron and aluminum to become concentrated in the soils. These soils are therefore, orange and red in color. Laterites lack abundant organic matter because the bacterial action is high in these tropical regions. This makes for infertile soil.

Problems and Conservation
Soil is one of the most important natural resources on the planet. It provides minerals and other nutrients to plants. In turn, all animals, directly or indirectly, get their energy from plants. Without plants, animal life would not survive. Soils also provide many animals with a place to live. And soil provides a region of water storage.

Soil loss and soil damage are very real problems in many parts of the world. Soil is damaged by overuse from poor farming practices or over grazing by animals. Soil that is overused loses its nutrients and can even become infertile. Plants can’t grow in such soil. With no plants to serve as protection, this soil becomes susceptible to soil erosion. The area may become a desert through a process called desertification.

Soil erosion is a natural process. Water and wind move soil particles to different areas. In the past, soil erosion occurred at slower rates than today. Human activity such as farming, logging, development, and construction accelerate the process of erosion by removing the natural vegetation in an area.

Natural erosion varies from place to place and depends on conditions such as soil type, climate, slope, and type of vegetation. In many areas, soil is removed at a greater pace than new soil is formed. This changes soil from a renewable resource, to a nonrenewable resource.

There are many practices and precautions humans can take so they do not accelerate the rate of soil erosion, thereby protecting this renewable resource. Farming practices such as contour plowing or terracing can protect the soil. Farmers plow their fields across the slope of hills in a practice called contour plowing. The individual rows act like a dam, preventing soil from washing away from a hillside. Terracing is a practice in which a large, steep field is converted into many smaller fields.

Other farmers may employ the practice of no till farming on their land. Old stalks and other plant material is left in the filed after harvest, to provide a cover from the rain. This serves to reduce runoff and slow soil erosion.

Nutrient depletion in soils is often as significant a problem as soil erosion. Planting only one crop in a field for years will deplete that soil of certain soils. Planting cover crops such as peanuts or soybeans between harvests, helps replace the nutrients that are being depleted. This is often the case with nitrates in t he soil. Cover crops also do double duty by providing protection from erosion.

Crop rotation or planting different crops in a field every few years also helps reduce the nutrient depletion of the soil which can occur.