Hello, and welcome. I'm glad you could join us today for our program on weathering,but before we actually get into the program,I want to note that we know there are some of you out there watching the course who are not actually enrolled for credit. That's good; in fact, you can still learn a lot even if you're not enrolled for credit,but we'd like to know who you are, and what you think, so why don't you give us a call or drop us a postcard. Give me a call at 845-9488 or drop a postcard to:
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Well, today's program begins our study of the destructive forces of nature. Recall that everything that happens on the Earth's surface is an interplay between constructive forces and destructive forces.
"Constructive" forces build things up, and "destructive" forces wear them down. We've learned quite a bit about the constructive forces such as mountain building, and volcanism, and even earthquakes, and how these features relate to the global movements of tectonic plates.
We've also learned about the nature of Earth materials although we will study sedimentary and metamorphic rocks in later lessons, so now we can turn our attention to the interactions of these materials with the surface environment of Earth.
You see, Earth materials can be altered and transformed into new materials at or near the Earth's surface. The model we use for understanding how this material is recycled at or near the Earth's surface is called the "rock cycle," so we can think of the weathering process as preparation for the cycle of erosion. These materials can be picked up, moved, and laid down somewhere else, or they may be deeply buried and subjected to increase heat and pressure, which alters them further.
All of these are important processes, which provide the familiar landscapes which we see every day as we look around the Earth, so I'd like to clear up before we even get into things here a distinction between several words that are used in this part of the rock cycle. Those words are "weathering," "erosion," "transportation" and "deposition."
"Weathering," the topic of today's program is the alteration of rocks and minerals at or near Earth's surface in response to new conditions of temperature, pressure, and moisture.
"Erosion," on the other hand, is the picking up of these materials by agents of erosion, such as wind, water, waves, and ice.
"Transportation" is just what it sounds like. It's the movement of these materials to some different location by these same agents. Streams, for example, carry large amounts of material as sediments. Wind picks up dust and so forth from the surface. Ocean waves erode at the coast line, and even ice and glaciers pick up material,scrapes materials along in front of it, all of these transport materials from one place to another.
Once the material is in transportation, it
eventually gets dropped, so "deposition" is the laying down
of these weathered materials in some new location. The new location
may be very close to where they were, or it may be very far away.
REVIEW OF LESSON ASSIGNMENT
So let me review with you the lesson assignment. We are now in Chapter 12, pages 267 to 283. You might also want to
Okay, I also want to take a few minutes to review
the objectives with you.
In this lesson, we want to:
Now, as usual, sometimes these objectives will be covered here in the video program; sometimes they're in the textbook; and you may have to synthesize the material to cover all of the objectives.
Before we watch the video today, let's do just a brief introduction. We want to look at the types of weathering and a general overview of soils, and we'll come back after the video and discuss some of this in detail after we've had a chance to see some of the things that we're talking about, so let's look at chemical weathering first. The two types are "chemical" and "mechanical" or "physical" weathering, so "chemical" weathering is basically the rearrangement of atoms to adjust to new conditions at the surface.
See, the stability of a particular mineral, remember that a "mineral" is basically a collection of atoms arranged in a certain crystal structure, so the stability of a particular mineral depends upon the conditions under which it was formed, and you may remember that the same atoms can form different crystal structures, so remember now that the Earth's surface, the Earth's crust, is a complicated assemblage of atoms of different types, which have arranged themselves into a variety of crystal structures. Many of these atoms have arranged themselves in these crystal structures during the process of igneous rock formation; that is, that the rocks have formed, and the minerals have crystallized at relatively high temperatures, as much as 1,100 degrees Celsius. Even the later cooling minerals in the Bowen Reaction Series form at temperatures of maybe 800 degrees Celsius, so those minerals then are stable under those conditions under which they were formed. You take those same minerals and put them near the Earth's surface where they're in contact with gases in the air and water but also at a lower temperature, those atoms are basically unstable in their arrangement. In that arrangement, since the atoms are unstable, they will try to rearrange themselves into new crystal structures forming new minerals, so we can say here that minerals that form at high temperatures are less resistant to surface conditions and so weather much faster; on the other hand, those minerals which are formed at the surface, especially those minerals which are products of weathering, are very stable at the Earth's surface and tend to stick around under those conditions at a long time.
We'll see as we get into sedimentary and metamorphic rocks that these weathering products once they're buried and subjected to higher temperatures and pressures may undergo another form of alteration to form yet another group of minerals, so we can generally say that the stability of a particular mineral to weathering conditions at the surface is related to the Bowen Reaction Series, and you would expect that those minerals which are formed at high temperatures, such as olivine, and pyroxene, and calcium-rich feldspar are, in fact, easily weathered because they're most far from their equilibrium condition.
Another thing that enters in here is that quartz is a very stable mineral for several reasons. Number one, quartz is the last mineral to form in the Bowen Reaction Series; that means that of all the minerals of the silicate rock forming minerals, quartz is at surface temperatures most close to its equilibrium state. Not only that, but once the quartz forms, quartz is a very hard mineral, it has a hardness of seven on the Moh's scale, and it has no cleavage, so quartz is also resistant to abrasion; that is, when a rock containing quartz crystals falls off a cliff, the quartz doesn't break easily.
Feldspar, on the other hand, which you may remember is a very common mineral in igneous rocks, is also not very stable because it forms at a higher temperature than quartz, but feldspar also has two very good directions of cleavage, which means that it breaks much easier under the processes of mechanical weathering.
Okay, so mechanical weathering, also sometimes called physical weathering is actually in many ways much simpler to understand than chemical weathering. Physical weathering is simply the breaking of material into smaller pieces. Anything that breaks the mineral into smaller pieces is actually a form of mechanical or physical weathering. It's important to note here that although we break these types of weathering into these two categories of chemical and physical, neither process operates to the exclusion of the other.
Everywhere on Earth, both processes are taking place. Perhaps on the moon, where there is no water, there's very little chemical weathering, and physical weathering is the only type of weathering that happens, but here on Earth anywhere on the surface, both of these processes operate together. In some environments, for example, in a moist tropical climate, chemical weathering may predominate. In another environment, in a hot dry desert, mechanical weathering may predominate, but they act it unison.
Not only that, but each of these processes helps the other. For example, if you take a rock and break it into smaller pieces, those smaller pieces although they have the same volume, have a larger surface area, A larger surface area means that more chemicals can come in contact with the mineral and, therefore, accelerate the chemical weathering process, so the mechanical process of breaking helps the chemical process of alteration chemically.
On the other hand, chemical weathering products usually have less mechanical strength, so they break easier, so if you have a piece of granite, for example, around which some of the feldspar has weathered to clay, the clay is less resistant to breaking and abrasion than the other minerals, so it breaks apart into small fragments easier, so these two processes operate in unison, and it's kind of an artificial classification to break them into two separate processes, that's what we do in science is to try to classify things this way.
Okay, just very briefly, "soil" is the residual or transported material, which is mixed with organic products. The word "residual" basically means that the soil is formed in place, so we can say residual soils are the result of weathering alone. "Transported" soils mean that the material that makes up the soil is brought in from some place else; it's been transported and deposited at the location where it now occurs.
Most soils are some combination of the two, residual and transported. The soil itself contains several different kinds of products, but it contains sand, and clay, and humus, as well as small amounts of other minerals and mineraloids, and, of course, a little bit water, so we will come back after the video to get a little closer look at some of these processes, but I think this time is good to watch the video first, and I'll come back afterwards first and summarize, so let's watch the video.
Funding for this program was provided by the Annenberg C.P.B. Project.
Even after 35,000 years the hieroglyphics on this ancient Egyptian obelisk stand out with impressive clarity. But this obelisk known as Cleopatra's Needle, is in far less pristine condition. After lying half buried in mud for years, it was shipped from Egypt to the United States in the late 1800's. And placed in a very humid atmosphere of New York City.
Here, the damp air reacted with natural salts absorbed from Egyptian mud to cause a great deal of damage to its surface. The reason for the contrast between the two monuments can be summed up in one word, "weathering." Like the granitic rocks of the Egyptian monuments all the rocks on the earth's surface are affected by weathering. Which is really the way rocks adapt themselves to the environment of earth's surface.
Most rocks are formed deep beneath the earth's surface or on the ocean floor. Tectonic activity such as uplift mountain building or falling sea level, brings these rocks from their environment of formation up to the earth's surface. There, the rocks are changed into a form that is stable under these new conditions by a process called, "weather."
Weathering occurs everywhere on earth, be it a cold dry polar region or the hot humid tropics. This particular outcrop is made of granite. Now, granite, in its fresh un- weathered state is quite hard, because the individual mineral grains are tightly interlocking together. This granite, however, is heavily weathered. It's quite soft, crumbles easily because the grains are falling apart, and they're also decomposing chemically.
Weathering occurs in two fundamentally different ways. The first of these involves the breaking of rock into fragments in individual mineral crystals. The physical fragmentation of rock is known as mechanical weathering. The rocks we see are usually cracked to start with because of tectonic activity. Systems of natural cracks in rocks are called "joints." One common kind of joint results from a process known as "pressure release."
The rock forming this great granitic dome in Yosemite National Park was originally buried kilometers beneath the surface. While a rock is below the surface, the layers of earth's crust above pressed down on it. With tectonic uplift, surface erosion reduces the thickness of the overlying rock. The downward pressure is reduced, and the rock expands primarily in an upward direction. At shallow depth, the expansion causes the rock to crack into sheets separated by joints parallel to the overlying surface. Exposed by erosion, plates of jointed granite then detach themselves and slide down slowly unpealing the face of the rock, layer by layer, like the stripping of skins from an onion. This is known as "exfoliation." Literally the stripping away of leaves. This opens the way for other types of mechanical weathering.
For example, ice wedging. When water freezes, its volume expands as much as 9%. This expansion exerts enomorous pressure as those acquainted with bursting water pipes in winter well know. We also see the effect of this pressure on roads in cold climates. After a particularly frigid winter, the water that has seeped under the pavement freezes and expands, heaving up the road surface. The same thing happens to rocks. When water gets into the cracks and freezes, it expands, wedging the rock apart, extending the crack and even breaking the rock into pieces. Which is what happened to form these jaggered spires of granite.
The term, "mechanical weathering," or "disintegration," describes the natural break up or fragmentation of rock at the earth's surface. A chunk of granite, for example, may be broken up into smaller pieces by ice wedging or tectonic activity. But this doesn't change the composition of the rock. It's still granite, it still consists of its original crystals of quartz, feldspar, and ferromagnesium minerals. Contrast this with the other main form of weathering.
Chemical weathering, which has a much more drastic effect on rock, although the two types of weathering are interrelated. By breaking rocks into smaller pieces, mechanical weathering increases the amount of exposed surface. This hastens the chemical weathering of the entire rock. The principal agents of chemical weathering are water in the ground and moisture in the air.
Usually these solutions are weakly acidic, and are capable of causing the slow chemical decomposition of most rock-making minerals. As rain falls, for example, it dissolves small qualitities of carbon dioxide in the atmosphere producing weak carbonic acid. As this solution percolates through decaying vegetation in soil, it picks up more acids and thus becomes more effective in decomposing underlying rocks, especially limestone. This time lapse sequence shows how rainfall gradually dissolves limestone. Thus, unlike mechanical weathering, chemical weathering actually destroys or changes the composition of the rock. The rusting of metal is a good example. The red rust on this nail is a different substance from the metallic iron at it's core. One substance has changed into another. The iron of the nail has combined with oxygen from the atmosphere to form rust, iron oxides, the minerals, limonite and geothite or hematite.
Because water is an important agent in chemical weathering, the degree of weathering is related to the amount of water existing in the natural environment. The rate that a rock will weather is primarily controlled by the climate. If you have a very wet, moist climate, you have a lot more chemical weathering. The rocks will literally decompose at an accelerated rate.
If it's a dry climate, only mechanical weathering will be able to operate, and that's a much more slower process to create soils. In addition to water, temperature is important, because warmer temperatures mean faster chemical reactions. As a result, weathering in the tropics is very rapid. Most deserts are also warm places, but here, the absence of water slows chemical weathering. Eventually most types of minerals chemically weather, or decompose, when they are at or near earth's surface. Only a few minerals are really stable at the earth's surface. The mineral, quartz, for example; hematite, iron oxide, is stable; that's rust, it's stable the earth's surface; but most minerals are not.
Most minerals form at temperatures very different than 25 degrees centigrade in one atmosphere of pressure. The minerals that are the most unstable at the earth's surface and weather the fastest are those that are formed at the highest temperatures. Minerals that are the most stable and will weather the least are those that formed initially at the lowest temperatures. Granite is composed of various minerals formed over a range of temperatures. Each mineral type responds to weathering differently. Here's an example of a granite that's fresh, it's unweathered, and as try as I might, I can't break this rock apart. You can see that dark grey quartz and the pink orthoclase feldspar, they're in the rock but they haven't weathered.
We'll now turn to another granite that is already begun chemical weathering. It's starting to fall apart. And we find that the different minerals are proceeding differently. Plagioclase are weathering the fastest, and has already turned to clay. Orthoclase feldspars is starting to weather as well, but the quartz has not weathered at all.
But the basic framework of the rock is beginning to fall apart and now this rock, why you know, I can begin to break this rock apart with my hands. It is starting to decompose. And this is how a rock changes to become sediment. The grains are breaking apart are dislodged and now will fall into the sedimentary system. These grains accumulate in piles called, "gruss."
As sediment is washed away by rivers and streams, sand-sized mineral grains may be sorted out, deposited, and cemented to eventually form sandstone, and the silt may accumulate separately and harden as siltstone. Weathering is essentially a process of distruction involving the physical and chemical breakdown of the parent rock. But it's a natural process, one that's been active throughout geologic time.
With the advent of the industrial age, man has upset this natural balance by accelerating the weathering process. The billions of tons of carbon dioxide, nitrogen oxides, and sulfur dioxide that we add to the air each year from the burning of fossil fuels and forests, creates far more problems for the atmosphere than sometimes turning it brown and making it unpleasant to breathe. These gases combine chemically with the rain water. In some cases, creating solutions that are hundreds of times more acidic than natural rain alone. The effect of this caustic precipitation falling on the earth's surface creates a serious global scale environmental problem, commonly known as "acid rain."
The most important source of acid rain is the sulfur dioxide admitted by automobiles and factories burning high sulfur coal. This combines with moisture in the atmosphere to form sulfuric acid. Additional acids can form from carbon dioxide, carbon monoxide and nitrogen oxides from auto exhausts and from burning of almost all kinds. The excess acids in rain fall accelerate chemical weathering of masonry and stone downwind, and can wreak havoc on the leaves of plants and trees.
A striking effect of acid rain is the historical deterioration of some forests and lakes. Over the last 40 or 50 years the woodlands of central Europe and eastern North America had been noticeably affected by acid precipitation and related dry pollutants. And with continued industrial pollution, the process compounds year by year.
Although weathering is fundamentally a process of destruction, it has the beneficial effect of breaking rocks down into soil, which is essential to all land-based life. Humans could probably live without oil or coal or wood to burn, but we wouldn't survive without soil.
Soil, has many functions. It's the habitat for land plants, it harbors bacteria which produce nutrients essential to these plants, and it's a storehouse for water and for trace elements, which by the way are also produced by chemical weathering. In fact, soil, is the foundation upon which the entire terrestrial food chain depends.
Consider, for example, calcium and phosphorous, two of the essential elements in bone. Calcium is abundant throughout nature, but animals obtain phosphorous primarily by eating plants which absorb the phosphorous from the soil in which they grow. The most important original source of the phosphate in soils is the relatively rare mineral, apatite. The apatite in a chunk of granite the size of a fist would probably fit onto the head of a pin. But when granite is weathered into soil, the tiny apatite grains decompose releasing soluble phosphate, which is subsequently taken up by plants. And then through the food chain, by people. Apatite is found in many varieties of rocks, though usually only in minute traces. Still, it is the major source of all the phosphates so vital to life. If it were not released by the weathering, no vertebrate land animals could exist.
Soils are formed as a result of the mechanical and chemical breakdown of rock. They are enriched by the decomposition of plant life which produces the rich organic matter known as "humus." It's this combination of weathered fragments of rock and humus that creates the most fertile soils. In essence, soil is a thin layer that rests on bed rock, like a coating of rust on iron. As the soil matures, it forms into several different layers or "horizons."
The "A" horizon is the loamy top soil the gardeners know so well. This is where most plant humus is derived. Rain waters seeps down through this layer and washes or leaches some of the soil materials down to the "B" horizon. The next layer, where the leached materials accumulate. The "C" horizon, the bottom most layer is the zone where the underlying bed rock is partly disintegrated and decomposed, but still recognizable.
The nature of a soil is determined partly by what sort of parent rock it develops from, because this is its prime material. If the parent rock is granite, it decomposes into a sandy soil made up of quartz grains. Clay in the soil comes from the weathering of feldspars. Such soil is usually permeable and can easily nourish vegetation. A fine grained rock, such as basalt, decomposes more rapidly to clay minerals than iron compounds. Basalt forms soil that is generally darker and less permeable than granitic soil, but still fertile.
Climate and time also play important roles in the evolution of soil. As time passes and the soil matures, it progressively loses the signature of the rock it was formed from and takes on characteristics determined by the climate. Fertile soil doesn't come into existence quickly. It took about 15,000 years for this layer of transported soil less than a meter thick to develop in the American Midwest. Soils in the tropics can develop faster, in a few thousand years.
These millennia of soil evolution can be destroyed in a few short years when certain types of human activity disturb the surface. For example, soil can be destroyed by poor soil management; such as, over grazing, or overworking the land. Or, simply cutting down too many trees. One region that has been adversely affected by this is the tropics. Where the jungle floor would seem to be the most fertile environment possible. Clearing the land for agriculture has, in many places, been a disastrous waste. Because the floor of the rain forest is actually a rather unproductive soil. Where, then, does all the luxuriant vegetation come from. The fact is, the tropical rain forest creates its own nutrients in the form of humus, a thick layer of decaying trees and plants. But because of the intense rainfall in the tropical jungles, nutrients in the soil are quickly leached and drained away. So, when a forest is cut down, the humus is exposed to erosion. The loss of this nutrient source ultimately results in a barren wasteland.
Soil erosion is not limited to tropical rain forests. It is, in fact, a worldwide problem that can take many different forms. In the 1930's, the American Midwest experienced a catastrophe of major proportions--the dust bowl. Draught conditions combined with high winds to blow the region's soil into massive swirling clouds of dust. The famous dust bowl of the American Midwest occurred between 1934 and 1938, and the natural circumstances that led to that catastrophe really were not unusual. High seasonal winds are common in the midwest and draught occurs cyclicly. It's a common part of that environment. What was unusual was the fact that the grassland there had been removed by agriculture, by grazing, and the loose soil underneath, a lot of which is dust, blown in from past glacial events, was free, and exposed to the wind, so away it blew. And this was a catastrophe that didn't slow down, that continued because of a sustained draught. Plowed fields were stripped bare of top soil, planted crops were ruined, buried in piles of dust. For many farmers the result was bankruptcy.
Well, in retrospect, a lot could have been done to prevent the disaster of the dust bowl, but, of course, in those days, we didn't know as much as we do now about these problems, and so it's not a surprise that it happened and no one can really be blamed. As a result of the dust bowl, the United Stated Soil Conservation Service was established in 1935. Its goal was to develop farming techniques that would help to protect this precious natural resource. And to prevent similar disasters from ever occurring again.
For farmers in the Mohave Desert the issue of soil management is a matter of ongoing concern. There are multiple factors that threaten soil and crops in this region ranging from erosion to limited water resources. Because of the complexity of these soil management issues, farmer, Wayne Soppeland, turned to the Soil Conservation Service. Rick Aguayo who works for the Service then made the first of several trips to Soppeland's farm. His immediate goal, to analyze the overall situation and come up with a soil conservation plan. We discussed what was needed in this area and what conservation practices they already had in place, and he made certain suggestions on how things could be improved, and we went on to creating a plan out of those suggestions.
Once we get them to make decision, "Yeah I want to have a conservation plan," we need to determine what his objectives are, what's he plan on doing with the property now, 5, 10 years down the road. Once we do that, we start evaluating what the resources are, what the soil resources that he has to work with, what are his farming operation, his irrigation system.
At the heart of the conservation plan is an in-depth soil survey to determine the precise kind of soil on Soppeland's farm. Once Aguayo analyzes this, he can recommend a very detailed set of conservation practices. Many of Aguayo's specific recommendations are designed to overcome erosion, one of the most troublesome threats to Soppeland's soil. Our major problem was soil erosion, results from the wind. And that can happen at any time of the year, winter, summer, fall, doesn't matter, and we have fairly heavy winds for long periods of time, and we have to take a number of precautions related to that.
One of these precautions, a barrier known as a windbreak, is composed of very tall trees. Wayne Soppeland has relied on this technique to protect his precious soil from the ravages of erosion for a long time. The windbreaks also protect Soppeland's farm from being smothered by drifting sand. As Soppeland has learned through the years, the windbreak is extremely effective, provided it's sufficiently dense and tall. When there is an opening in the protection, or no windbreak at all, the damage can be severe. But windbreaks are not the total answer to erosion problems.
A technique called, conservation tillage, has also proved to be especially useful. Tilling, or plowing the fields, is routinely done to prevent the formation of residues which block the percolation of water through the soil. But if too much tilling is done, the soil is so frequently loosened that much of it can be blown or washed away. In other words, instead of plowing the soil and disking it a number of times leaving it completely exposed, we do as little as possible. We will disk once leaving as much stubble on the ground so that if a wind storm comes up during that time, very little dirt is moving.
And then we get back in and try to plant the next crop as soon as possible and begin the irrigation. So that the period of time that the soil is exposed is very minimal. Another concern for both Soppeland and Aguayo is irrigation management. They recognize that the right amount of water is critical, not only for the immediate needs of the crop, but for the long-term health of the soil, as well. Too little water would parch the fields, while too much, would result in the formation of saline residues and cause root problems.
In this case Wayne has sprinkler irrigation systems that a lot of the farmers are using either real lines or center pivots, and best we try and design them so that they provide the optimum moisture for the crop while we do some erosion and ground water contamination. It's just that we don't want to over water.
In addition to his suggestions concerning tillage and water management, Aguayo has set up a crop rotation sequence designed to maximize the productivity of the soil. Along with the use of windbreaks, these soil conservation techniques have already made a significant different on Wayne Soppeland's farm. What's especially gratifying for Soppeland is that these improvements are not just quick fixes, but significant changes that will keep his soil productive for years to come. Thinking of soil as just another pile of dirt is an extremely short sighted notion. Without soil, without dirt, life on land could never have developed and flourished as it has. It is our responsibility to protect and preserve this valuable resource. But, beyond that, it's important that we respect and understand the entire weathering process. Otherwise, we run the risk of upsetting a delicate natural balance of physical and chemical factors that have combined to support terrestrial life for over 400 million years.
Funding for this program was provided by the Annenberg CPB Project.
Well, this video shows some nice examples of weathered materials, and it shows some nice pictures of weathered structures, but let's summarize what we've seen about weathering. Let's look, first of all, at chemical weathering agents and processes.
Now, in the process of chemical weathering, there are several distinct processes which may act in unison and to varying degrees, and their combined effects produce the variety of weathering products. The easiest of these to visualize is simple solution.
Some minerals are more soluble than others; for example, halite, the common mineral we refer to as table salt is quite soluble in water, but most minerals are for all practical purposes insoluble in pure water. The problem is, of course, that natural water is seldom pure; the water always has other things dissolved in it, and specifically the water contains a small amount of acid. Acid in water drastically increases the solubility of most minerals. The acid comes from mostly carbon dioxide.
If you dissolve carbon dioxide in water, it forms a type of acid called "carbonic acid," and we know that carbon dioxide is a small but significant fracture of the atmospheric gases. This combination of carbonic acid and water is especially corrosive to certain kinds of minerals, most notably to the mineral calcite.
Calcite forms the type of rock called "limestone," and there are several examples of limestone artifacts, which once they've been removed from the desert, are quickly weathered away. For example, an obelisk called Cleopatra's Needle went almost unaltered for 3,500 years in the Egyptian Desert. After being removed from the desert and put in New York City, it's almost completely defaced; in other words, the hieroglyphic symbols are almost completely gone after only a hundred years. The reason why, of course, is that in the atmosphere of New York City there's a lot more rainfall, and, of course, some of the acid rain, which is a form of man-made pollutant, also contributes to this effect.
Not only that, but because of this effect of acid rich waters on a weathering of minerals, it provides a very useful way to calculate or to measure the weathering rates of various types of minerals by looking at tombstones. Tombstones, of course, have the dates on them in which they were made, and so by looking at the amount of weathering on tombstones of various compositions in various places in various climates, we can get a pretty good sense of how the rate of weathering is affected by these various things.
Okay, I also want to note here that the acid in the water is corrosive to other minerals as well, not only to calcite , and the presence of the acid aids in the processes of oxidation and hydrolysis, which we'll come to in a minute, especially in the silicate minerals and actually makes them more susceptible to weathering. So let's look at "oxidation."
"Oxidation" is a process where atoms combine with the element oxygen like the rusting of iron, and, of course, as we know for iron, this process is aided by the presence of moisture and may be aided also by the presence of other atoms; for example, we know that salt water is more corrosive to iron than fresh water. This particular process of oxidation is most important in the weathering of the ferromagnesian minerals because they contain large amounts of iron. In this case, the oxygen combines with the iron to form a mineral called "hematite" or a mineral called "limonite," which is sort of an orange colored minerals which often forms stains on the surfaces of other minerals.
The oxidation of the iron bearing mineral "pyrite" also releases sulfuric acid, and this sulfuric acid, which in itself is a weathering product, may create a hazard in many mining areas where pieces of pyrite are left sitting around in piles.
The chemical process we call "hydrolysis" refers to the reaction of a substance with water. Think of the word "hydro" means water, so "hydrolysis" is a reaction of a substance with water. It turns out that the presence of acid accentuates this process, and this process is especially active on the silicate minerals.
I want to turn our attention here for a second to clay minerals. I have to interject this here because I'm going to be talking about the clay minerals in a second. There are certain types of minerals which are formed by chemical weathering which all together fall into a category called "clay minerals." These minerals almost always occur in microscopic sized particulars, and, of course, everybody knows what clay feels like. It's sort of a soft squishy material that has a high moisture content. All of the clay minerals of various types incorporate water into their structure, and all of them are sheet silicates.
Remember sheet silicates are those silicate minerals which are made from sheets of tetrahedra joined at the corner. The sheets are held together by individual metallic ions. The exact type of clay mineral that's formed depends upon the parent material; in other words, the mineral, and also the temperature and humidity. The various types I'll just read for you: kaolinite, often called china clay; it's used to make prime porcelain; illite, which is actually a fine grained version of the mineral muscovite; another type called "smectite , or also known as "montmorillonite has the feature that when it absorbs water, it expands to many times its size, and there have been many examples of buildings which were built on smectite rich clay soils, where as the soil gained moisture and dried out, the building would move up and down and crack the foundations.
Another type of clay mineral is called "chlorite ," which is actually a ferromagnesian clay, which is common here in Hawaii, especially in weathered caldera rocks; in fact, the sediments in Kaneohe Bay here on Oahu are containing a significant portion of chlorite from the weathering of the Koolau caldera, which, of course, is rich in ferromagnesian minerals because of the basaltic rock content.
As far as the products of chemical weathering now, generally the products can be easily illustrated, and if you look at the table in the text, which shows you these chemical equations, the equations may be somewhat complicated but remember that the chemical weathering process is really nothing more than combinations of atoms to form new minerals, and the products can actually quite easily illustrated, so let's take a look at a simplified diagram which illustrates this variety.
In general, the minerals that we're concerned with are silicate minerals, so if we combine silicate minerals with carbon dioxide, which forms carbonic acid and water, one of the products that we get.
Well, in general the products are clay minerals of one type or the other, those types that I just mentioned plus bicarbonate ions. "Bicarbonate" is like the ion found in common baking soda, sodium bicarbonate, and it's a residual product from the carbon dioxide that goes into the carbonic acid. The silicate part of the silica minerals simply becomes silica dissolved in solution; in other words, SiO2 molecules like quartz simply carried along in solution. What's left over? Well, we've dealt with the silicate part. We've dealt with the carbon dioxide. The water is incorporated in the clay structures. What's left over is the metal ions. Remember those eight common materials in the Earth's crust. Six of those are metals. Those metal ions are released during the weathering process.
When I say released, I mean they are released from their crystal structures of the silicate minerals. Some of them are immediately incorporated; for example, the iron combines rapidly with oxygen to form iron oxides. The aluminum also combines rapidly with oxygen, so iron and aluminum ions are generally left behind as part of the soil. The other ions, most of these are soluble in water. The calcium, the sodium, the magnesium, and potassium are quite soluble in water, and, in fact, are carried along as part of the dissolved load of streams. We'll talk about the dissolved load of streams also in future programs, but these metal ions as they're dissolved in the streams are carried along with the water.
Some of them are dumped into the ocean, and there they are removed by various processes and contribute to the salinity of the ocean; in fact, if you take a look at the materials that are in the salt of the ocean, you'll find exactly those things: potassium, sodium, calcium, and magnesium. So although this diagram is relatively simplified, it does give us a good sense of what those products are, and as we study sedimentary rocks, we'll see that these weathering products indeed show up in sedimentary rocks in one form or another.
Okay, let's turn our attention quickly to the mechanical weathering process. Remember that mechanical weathering is any process which breaks the rock into smaller fragments. That includes several different types of processes. The most common of these, although not so common here at least at low elevations in Hawaii is "frost action." You may know that water expands upon freezing if you've ever mistakenly left a closed bottle of water in the freezer overnight so that it freezes, you know that ice expands as the water freezes, so cracks in rocks into which water stands as the water freezes at night can wedge the rocks apart. Not only that, but heating and cooling is especially important in desert areas which have a wide temperature range. As the rocks heat, they expand, and as they cool, they contract, and this continual contraction and expansion can actually crack the rocks. This process of heating and cooling is especially important even here in Hawaii, especially at high elevations because here in the tropics we have intense sunshine, and at high elevations we also have low night time temperatures and little rainfall, so in many cases this may be the only type of weathering that's significant in these Hawaiian rocks.
Another type of mechanical weathering is simply "abrasion." "Abrasion" is simply the scraping off of pieces of the mineral, and, of course, the harder minerals are more resistant. Quartz is a very common mineral in sedimentary rocks because it's very hard and has no cleavage. Feldspar is less common in sedimentary rocks although it's a more common mineral in igneous rocks, but it has a good cleavage, and it breaks easier.
I want to turn our attention to a couple of special types of weathering, which actually combine both mechanical and chemical processes. One of these is a process called "spheroidal" welding. The word "spheroidal" means, well, "sphere" is a ball shaped piece. "Spheroidal" means almost a ball shaped piece. This is actually a combination of chemical and physical weathering. You see, when rocks fracture, corners usually result. The rocks that simply break usually have angular sides and corners. These corners are more susceptible both to abrasion; in other words, grounding, and also chemical weathering. Not only that, but the chemical weathering process itself proceeds inward. It starts in the minerals on the outside of the rock and works its way inward, and the outer layers of the rock often peel off because of expansion, expansion because the clay minerals that are formed as part of the weathering process have greater volume, so the rock expands, the outer layers break off like onions, and, not only that, but those layers of clay then are much more susceptible to mechanical weathering.
We shouldn't confuse this with another type of expansion, which is called "exfoliation." "Exfoliation" results from the release of internal pressure. Remember that granite is formed at great depth in batholiths, so as the rock cools, it's under tremendous pressure. When that batholith is lifted to the surface, and the pressure is released, the rock simply expands outward and often peels off again like the layers of an onion in concentric cells.
I might also note here that plant roots also exert tremendous force on rocks. A root of a plant that's growing into a crack in a rock can produce tons of force to split that rock apart.
Okay, I want to briefly now summarize what we've just noted here about weathering products. Weathering products fall into two categories. Okay, there are solid materials and there are materials in solution. The solid materials we might classify as either sediments or soils. The sediments can fall into various sizes: boulders, cobbles, sand, silt, or microscopic clay sized grains.
The composition of these materials: quartz, as the result of quartz's stability; clay, which is formed from the chemical weathering process; iron oxides, which are formed also from the chemical weathering process; aluminum oxides; carbonates; and a few other things.
In solution we find silica. We find carbonate ions, and we find the metal ions, and, believe me, all of these things will show up again when we study sedimentary rocks. As far as soils go, soils generally have a certain type of structure, which relates also to the process of weathering.
A typical soil profiles includes three distinct layers of soil. The topmost layer, typically called the "A" horizon, sometimes called the zone of leeching. In this layer of soils, the dissolved material migrates downward. It's removed from the upper layer. It's dissolved in the water and migrates downward into the lower layers.
The next layer down, the "B" layer is often called the "zone of accumulation. Here the dissolved material that's leeched out from the upper layers accumulates, and this is where many of the nutrients for plants are found, which is why the plants have deep roots that go down below the same horizon.
The third layer down, the "C" horizon is called the "zone of weathering." Here you find partly altered and unaltered bedrock and many pieces of unaltered rocks sort of incorporated into the soil. It's worthy of note here that tropical soils usually have very poorly developed horizons because of the high temperatures and high rainfall; in other words, the zone of accumulation simply is missing. There's so much rainfall that the zone of leeching extends all the way down to the sea layer.
Okay, the soil itself contains several different materials, and I'll go over these very briefly with you. Sand from mechanical weathering. The sand is usually small grains of quartz or calcite. Soils that contain clay from chemical weathering, and again the type of clay depends upon the climate materials and the climate and the type of parent rock. The soil also contains humus, which is a general term for organic material, which may have an acid component and also moisture, which is held to the clay by cohesion. The clay holds onto the water. There are several factors that influence the presence of soil. I'll list these for you, and you might want to go back and check out these factors in the textbook.
Obviously, the type of soil depends upon the rock type from which it's formed. Those soils which are formed on the bedrock themselves are called "residual" soils. Time, of course, influences the degree of weathering. Even the slope of the land can affect the soil. The slope affects the erosion and transport of materials. The drainage affects the soil type, and even the orientation of the slope, whether it faces north or south, affects the amount of sunlight and soil moisture. Obviously, the climate influences the soil. The climate influences the rates and also determines the predominance of chemical versus mechanical weathering. Plants and animals also play an important role in soil information. They add nutrient. They help the soil hold water, and microorganisms have an active role in the decomposition of organic material, and, of course, earthworms and other burrowing creatures mix and enrich the soil as well.
Well, I think that's about it for the material for this lesson. Next time we'll study Lesson 16 on mass wasting. "Mass wasting" is a general downslope movement of materials under the influence of gravity, so, as usual, be sure to follow the plan and the study guide and don't forget to look at the objectives, and after you're done with the lesson, go back and make sure you've learned all those objectives, so as usual, enjoy yourself, learn about the Earth, study hard, but I'll see you next time.