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Weathering and Soil
The Earth's Soil Cycle
What Are Earth's Materials?
The next time you take a walk, think about what you are walking on. What is the Earth made of? The outermost layer of the Earth is known as the crust, and this layer is responsible for the majority of life on Earth. It supports the growth of plants, the survival of animals, the structure of our land, and the development of the human civilization. The Earth's crust has four main components, which are referred to as Earth's materials. These materials include minerals, rocks, soil and water. It is the combination of these materials that makes life on earth possible.
All About Soil
Soil is the mixture of minerals (rock, sand, clay, silt), organic matter, gases, liquids, and the countless organisms (matter from dead plants and animals) that together support life on Earth. Soil is a natural body known as the pedosphere and which performs four important functions: it is a medium for plant growth; it is a means of water storage, supply and purification; it is a modifier of Earth's atmosphere; it is a habitat for organisms; all of which, in turn, modify the soil.
Soil is considered to be the "skin of the Earth" and interfaces with its lithosphere, hydrosphere, atmosphere, and biosphere. Soil consists of a solid phase (minerals and organic matter) as well as a porous phase that holds gases and water. Soil continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with erosion. Soil science has two basic branches of study: edaphology and pedology. Pedology is focused on the formation, description (morphology), and classification of soils in their natural environment, whereas edaphology is concerned with the influence of soils on organisms. In engineering terms, soil is referred to as regolith, or loose rock material that lies above the 'solid geology'. Soil is commonly referred to as "earth" or "dirt".
Soil is a major component of the Earth's ecosystem. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, from ozone depletion and global warming, to rain forest destruction and water pollution. Soil is the largest surficial global carbon reservoir on Earth, and it is potentially one of the most reactive to human disturbance and climate change. As the planet warms, soils will add carbon dioxide to the atmosphere due to its increased biological activity at higher temperatures. Thus, soil carbon losses likely have a large positive feedback response to global warming. Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, and a medium for plant growth. Since soil has a tremendous range of available niches and habitats, it contains most of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species.
The carbon content stored in soil is eventually returned to the atmosphere through the process of respiration, which is carried out by heterotrophic organisms that feed upon the carbonaceous material in the soil. Since plant roots need oxygen, ventilation is an important characteristic of soil. This ventilation can be accomplished via networks of soil pores, which also absorb and hold rainwater making it readily available for plant uptake. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival.
Soils can effectively remove impurities, kill disease agents, and degrade contaminants. Typically, soils maintain a net absorption of oxygen and methane, and undergo a net release of carbon dioxide and nitrous oxide. Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins. Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms. Soil provides a substrate for plants (roots anchor in soil), a source of food for plants, and a home for many animals (insects, spiders, centipedes, worms, burrowing animals, bacteria, and many others).
Soils supply plants with mineral nutrients held in place by the clay and humus content of the soil. For optimum plant growth, the generalized content of soil components by volume should be roughly 50% solids (45% mineral and 5% organic matter), and 50% voids of which half is occupied by water and half by gas. The percent soil mineral and organic content is typically treated as a constant, while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other. The pore space allows for the infiltration and movement of air and water, both of which are critical for life in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching the plant roots and soil organisms. The soil texture is determined by the relative proportions of sand, silt, and clay in the soil. The addition of organic matter, water, gases and time causes the soil of a certain texture to develop into a larger soil structure called an aggregate. Of all the factors influencing the evolution of soil, water is the most powerful due to its involvement in the solution, erosion, transportation, and deposition of the materials of which a soil is composed. The mixture of water and dissolved and suspended materials is called the soil solution. Since soil water is never pure water, but contains hundreds of dissolved organic and inorganic substances, it may be more accurately called the soil solution.
Types of Soil, Formation and Horizons
Types of Soil: There are many different types of soils, and each one has unique characteristics, like color, texture, structure, and mineral content. The depth of the soil also varies. The kind of soil in an area helps determines what type of plants can grow. There are 12 orders (types) of soil: Alfisols, Aridisols, Entisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, Gelisols, Andisols, and Vertisols.
Soil Formation: Soil is formed slowly as rock (the parent material) erodes into tiny pieces near the Earth's surface. Organic matter decays and mixes with inorganic material (rock particles, minerals and water) to form soil.
Soil Horizons (layers): Soil is made up of distinct horizontal layers; these layers are called horizons. They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers ( subsoil, regolith and bedrock).
Weathering, Erosion and Decomposition
Mountains, and valleys form as tectonic plates crash into one another. These mountains do not last forever. As they age, they are torn down by the very slow, but persistent forces of weathering, mass wasting, and erosion. The atmosphere is comprised of many different types of gases. These gases are able to penetrate into the openings of rocks. Plant roots, microscopic animals and plants, and digging animals also help to break down rocks. To better understand the different forces that cause weathering, geologists separate them into three categories. These categories are mechanical, chemical, and biotic.
Weathering
Weathering and erosion slowly chisel, polish, and buff Earth's rock into ever evolving works of art—and then wash the remains into the sea. Weathering is the process of decomposing, breaking up, or changing the color of rocks. Weathering may be caused by the action of water, air, chemicals, plants, or animals. Weathering takes place as rocks are broken down into progressively smaller pieces by the effects of weather. These pieces do not move to a new location, they simply break down, but remain next to one another. The processes are definitively independent, but not exclusive. Weathering is the mechanical and chemical hammer that breaks down and sculpts the rocks. Weathering takes place as rocks are broken down into progressively smaller pieces by the effects of weather. These pieces do not move to a new location, they simply break down, but remain next to one another.
Microscopic spaces between the grains of a rock are numerous. These spaces act like microscopic caves, allowing water to flow freely through them. As water travels through these spaces, it causing weathering, as chemicals are carried with it that can loosen the bonds between grains, and help to break the rock into smaller pieces. In some cases, the rocks become so weak, that they can be crumbled with the force of a bare hand. Almost all rocks contain numerable joints. Joints are tiny hairline cracks that are not all the way broken through. Both sides of the rock are still held together with enough strength that they move together. However, they are broken apart sufficiently as to allow small amounts of water to easily travel between them. A fault is a crack in the bedrock where neither side is held together. As a result, both sides are free to move independently of one another. Faults are typically much larger than joints, and can span in some cases several hundred miles. Faults are responsible for significant weathering. This is because of their size. They allow significant amounts of water to enter the bedrock. As lava cools, bubbles of hot gas often form. These bubbles become suspended in the thickening lava and are unable to escape. As a result, the cooling lava is very porous, leaving very large spaces though which water can travel.
Mechanical Weathering
Mechanical weathering takes place when rocks are broken down without any change in the chemical nature of the rocks. The rocks are essentially torn apart by physical force, rather than by chemical breakdown. Mechanical weathering is the process of breaking a large rock into smaller pieces without changing the minerals in the rock. Mechanical weathering may be caused by frost, ice, plant roots, running water, or heat from the sun. The forces that break rocks down can be numerous, and include such things as pent up energy as the Earth’s crust slowly moves. When great amounts of pressure build up, the resulting mechanical effect can be that very large joints, or faults are created. The most common type of mechanical weathering is the constant freezing, and thawing of water. In liquid form, water is able to penetrate the many holes, joints, and fissures within a rock. As the temperature drops below 32 ° F, this water freezes. As water freezes, it expands, becoming about 10% larger than it was in liquid form. The result is that the holes and cracks in rocks are pushed outward. Even the strongest rocks are no match for this force.As the water thaws, it is then able to penetrate further into the widened space, where it later freezes yet again. The expansion of holes and cracks is very slow. However, water does not mind. It is very patient. Month after month, year after year, water freezes and thaws over and over, creating larger and larger holes and cracks in the rocks.Another important type of mechanical weathering is salt wedging. As water enters the holes and cracks in the surface of rocks, it often carries salt with it. As the water later evaporates, the salt is left behind. Over time, these salt deposits build up, creating pressure that can cause rocks to split and weaken.Temperature changes also effect mechanical weathering. As temperatures heat up, the rocks themselves expand somewhat. As the temperatures cool down, rocks contract slightly. The effect can be the weakening of the rock itself. It's important to keep in mind that weathering is a surface or near-surface process. As you know, metamorphism also produces chemical changes in rocks, but metamorphic chemical changes occur at depth where either the temperature and/or pressure are significantly higher than conditions found on the Earth's surface.
Chemical Weathering
Chemical weathering takes place in almost all types of rocks. Smaller rocks are more susceptible, however, because they have a greater amount of surface area. Chemical weathering involves chemical changes in the minerals of the rock, or on the surface of the rock, that make the rock change its shape or color. Carbon dioxide, oxygen, water, and acids may all cause chemical weathering. Chemical reactions break down the bonds holding the rocks together, causing them to fall apart, forming smaller and smaller pieces. Chemical weathering is much more common in locations where there is a lot of water. This is because water is important to many of the chemical reactions that can take place. Warmer temperatures are also more friendly to chemical weathering. The most common types of chemical weathering are oxidation, hydrolysis and carbonation. Oxidation takes place when oxygen combines with other elements in rocks to form new types of rock. These new substances are usually much softer, and thus easier for other forces to break apart.Hydrolysis occurs when water combines with the substances in rocks to form new types of substances, which are softer than the original rock types. This allows other forces, such as mechanical weathering, to more easily break them apart.Carbonation takes place when carbon dioxide reacts with certain types of rocks forming a solution that can easily be carried away by water.
Biotic Weathering
The word ‘bio’ means life. Thus biotic weathering is any type of weathering that is caused by living organisms. Most often the culprit of biotic weathering are plant roots. These roots can extend downward, deep into rock cracks in search of water, and nutrients. In the process they act as a wedge, widening and extending the cracks. Other causes of biotic weathering are digging animals, microscopic plants and animals, algae and fungi.
Mass Wasting
The power of gravity on Earth is inescapable. Hold your hand high above your head, and then relax your muscles. What happens? Now, hold a book in the air and release it. Again, what happens? The force of gravity tugs at both your arm and the book, pulling them down towards the center of the Earth. The same force that pulls your arm down is also ever at work pulling rocks, boulders, dirt and dust downward. Whenever the opportunity presents itself, gravity pulls a rock lower and lower towards the lowest surface possible. Rocks, dirt and soil lie on the side of a mountain or hill, apparently unmovable. For many hundreds or even thousands of years the rocks and dirt change very little. Over time, however, as small amounts of dirt and additional rocks are added to the pile, the weight and mass of the pile build up. In one sudden and grand event, the entire pile might move several hundred feet within only a couple of minutes or seconds, only to once again come to rest on the side of the mountain or hill, waiting for the next event.
The most common type of mass wasting is falling. Rocks, builders, pebbles, and dirt loosened by freezing, weathering, and other forces, simply fall downward, until they hit something that stops their descent. Often a pile of rocks forms at the bottom of a cliff or mountain. We call a pile of rocks, boulders, and dirt a talus. Very often, these taluses form a cone shape, as they ascend up the side of the mountain.
Landslides take place when dirt, pebbles, rocks and boulders slide down a slope together. Sometimes these landslides are small, and hardly noticeable. Other times however, they can be substantial, involving the entire side of a mountain. These destructive slides can be triggered by a number of different causes. Often rain, which adds additional weight to the side of a slope can cause slides. Other times they might be caused by erosion, as the base of a slope is slowly removed by a stream, weakening the entire side of the mountain.As a slide progresses down a mountain slope, it can pick up tremendous speed, and energy. Some slides have been reported to travel at speeds approaching 200 miles per hour. The resulting winds can be so forceful, that they are known to strip the leaves off of surrounding trees. The momentum of falling material has been known to cause some of the materials to roll several hundred feet back up the other side of a valley.The amount of material moved in a landslide can be tremendous. In some cases this material is so substantial, that it is measured in cubic miles. This much material falling across a stream, can be the cause for the formation or a new natural lake.
Flows take place much more slowly than do slides, and usually involve great amounts of water. After a heavy rainstorm the ground can become too wet to absorb any additional water. The result is that the water is forced to run off on the surface, gathering dust, dirt, rocks, and in some cases even boulders as it builds up. The leading edge of a flow gathers the most debris, causing it to be thicker and slower moving. This acts as a slow moving dam. Eventually, such as in a wide area on a slope, the more liquid mud from behind breaks through the dam and rushes outward creating a muddy plain.
The slowest type of mass wasting is referred to by geologists as a creep. These types of movements are so slow that they require special equipment just to measure them. A creep takes place when the entire side of a hill or mountain moves downward under the weight of gravity, very slowly, usually much less than one inch per year.
Erosion
Once the small pieces of rocks are changed or broken apart by weathering, they may start to be moved by wind, water, or ice. When the smaller rock pieces (now pebbles, sand or soil) are moved by these natural forces, it is called erosion.
Erosion takes place when materials in the landscape are moved from one location to another. This might happen as dust is blown off the side of a cliff face by wind, or as silt is carried downstream by a river. Once rocks have been broken down by weathering and by mass wasting, the final fate of all materials is to be carried away to another location by erosion. The most influential force in erosion is water. Water’s ability to move materials from one location to another, along with the fact that it is found everywhere along the surface of the earth, make it a superb tool for erosion. Working together they create and reveal marvels of nature from tumbling boulders high in the mountains to sandstone arches in the parched desert to polished cliffs braced against violent seas.
Water is nature's most versatile tool. For example, take rain on a frigid day. The water pools in cracks and crevices. Then, at night, the temperature drops and the water expands as it turns to ice, splitting the rock like a sledgehammer to a wedge. The next day, under the beating sun, the ice melts and trickles the cracked fragments away. Repeated swings in temperature can also weaken and eventually fragment rock, which expands when hot and shrinks when cold. Such pulsing slowly turns stones in the arid desert to sand. Likewise, constant cycles from wet to dry will crumble clay. Bits of sand are picked up and carried off by the wind, which can then blast the sides of nearby rocks, buffing and polishing them smooth. On the seashore, the action of waves chips away at cliffs and rakes the fragments back and forth into fine sand.
Plants and animals also take a heavy toll on Earth's hardened minerals. Lichens and mosses can squeeze into cracks and crevices, where they take root. As they grow, so do the cracks, eventually splitting into bits and pieces. Critters big and small trample, crush, and plow rocks as they scurry across the surface and burrow underground. Plants and animals also produce acids that mix with rainwater, a combination that eats away at rocks.
Rainwater also mixes with chemicals as it falls from the sky, forming an acidic concoction that dissolves rock. For example, acid rain dissolves limestone to form karst, a type of terrain filled with fissures, underground streams, and caves like the cenotes of Mexico's Yucatán Peninsula.
Back up on the mountains, snow and ice build up into glaciers that weigh on the rocks beneath and slowly push them downhill under the force of gravity. Together with advancing ice, the rocks carve out a path as the glacier slumps down the mountain. When the glacier begins to melt, it deposits its cargo of soil and rock, transporting the rocky debris toward the sea. Every year, rivers deposit millions of tons of sediment into the oceans.
Without the erosive forces of water, wind, and ice, rock debris would simply pile up where it forms and obscure from view nature's weathered sculptures. Although erosion is a natural process, abusive land-use practices such asdeforestation and overgrazing can expedite erosion and strip the land of soils needed for food to grow.
The Grand Canyon is the eleventh largest national park, but is the fourth largest in the continental US. It is 277 miles long. The width of the park varies from one end to the other, but in some places it is 18 miles wide. All of the acreage totals 1904 square miles! But the Grand Canyon is not just big around. It is also deep. It averages one mile in depth, but parts of it are much deeper than that. In fact, it is so big and so deep that it can be seen from space!
Water is responsible for most of the erosion. Lots of water flows through the Colorado River, and has for a long time. But when it rains in the desert-country of the Grand Canyon, the baked dirt quite often cannot take the moisture in. The rainwater begins to run down toward the river, making flash floods common. The flood water moves so fast that it topples rocks and boulders in its path. Dirt is swept along, leaving behind only hard rock formations. During the harsh winters of the region, water seeps in the tiny cracks and crevices of the rocks. When it freezes, it gets bigger and cracks the rocks even more. Wind also adds to the erosion process of the Grand Canyon, which is still changing every year.
Drainage Basin / Watershed
The precipitation that falls into a valley, and on surrounding interfluves flows downward usually creating a stream or river. The area of land that contributes water to a stream or river is called a watershed, or drainage basin. Small drainage basins generally contribute to streams, while the water from larger drainage basins come together to form large rivers. Often, small drainage basins or watersheds combine with one another, creating larger and larger networks of drainage basins. All of these combined drainage basins are together referred to simply as a drainage basin, or as one watershed.The area between two drainage basins is known as a drainage divide. In North America a massive drainage divide known as the Continental Divide separates the water that flows towards the Pacific Ocean, with water that drains towards the Gulf of Mexico, and towards the Atlantic Ocean.
Erosion By Overland Flow
Between valley’s at the top of interfluves water runs across the landscape in flat sheets known as overland flow. As rain drops begin falling in a rain storm they are first absorbed by the landscape. As the ground becomes saturated, the drops begin moving across the landscape above the surface. As this happens, small amounts of dust and dirt are carried with the water. This is known as splash erosion. As more and more water falls, the sheet of moving water becomes larger and larger. Eventually the water forms rills. Rills are small channels of water running across the surface of the landscape. The creation of rills happens much more quickly in areas where there is little vegetation. Plant roots help to hold dirt and rocks in place, retarding the formation of rills.Eventually, many rills come together, forming larger gullies. Gullies can get quite large, and help to feed large amounts of water into streams and rivers.
Erosion By Streamflow
As water enters the floor of a valley, it typically is dumped into a river or stream. Rivers and streams are moving bodies of draining water, that have a tremendous amount of force. Because of their strength, streams and rivers can cause a great amount of erosion. Dirt and dust is carried away in the water of the river, leaving only pebbles and rocks. The rocks are constantly smacking into one another, as the force of the river moves them about. This causes them to be continually breaking into smaller and smaller pieces.Rivers have been known to carve deep canyons in the bedrock in only a few hundred thousand years.
Valleys And Interfluves
All terrain can be classified in one of two groups. These groups are valleys and interfluves. Valleys are areas of lower terrain, while interfluves are areas of high terrain located between valleys.
Often the difference between valleys and interfluves is obvious. This is especially the case where high mountains surround a low valley. In other cases, the difference is difficult to detect. The interfluve may be just a few feet higher than the valley floor.
Why are valleys and interfluves important? Water always flows from high terrain downward, towards lower terrain. Thus, water is not found in abundance in interfluves, while it gathers in the form of streams and lakes in valleys.
Deposits
As rivers carry dust, pebbles, and rocks downstream, this material is eventually deposited at some location further down. These deposits form at bends in a river, as well as in locations where rivers dump water into lakes, seas, and oceans. The effect of deposits is that new land is created using materials from other locations upstream.
Floodplains
Floodplains form alongside shallow meandering rivers. As the rivers move back and forth across the landscape they form an area around the river where the elevation of the land is lower than other areas. This lower land around the river is known as a floodplain. During times of excess precipitation, water leaves the banks of the river, but remains confined to the floodplain.