The earth is indeed a dynamic system. Volcanic and tectonic activities (that result in the deformation of the earth’s crust) are elevating parts of the earth’s surface, opposing processes are continually removing materials from higher elevations and moving them to lower elevations.The latter processes include:
1. Weathering - disintegration and decomposition of rock at or near the surface of the earth.
2. Erosion – incorporation and transportation of material by a mobile agent (river, wind, ice)
3. Mass wasting – transfer of rock material down-slope under the influence of gravity.
What is
weathering?
All materials are susceptible to weathering. As you know, things tend to break down when left outside. Metal rusts, paint peels, wood cracks, newspapers turn yellow, and so on. Basically, things tend to fall apart when they exposed to air. The same sorts of things happen to rock. Why does rock weather? Simply, weathering is the response of the earth materials to a changing environment. Weathering is a general term for the processes which break rock and other materials down. CO2 in atmosphere forms carbonic acid in water, attacks minerals. Cations in minerals are replaced by H+ and OH- radicals. Cations and Si (Silicate: abundant mineral in rocks) go into solution, leaving insoluble aluminum silicate - "clay ". Clay structure is "sheet-like" - makes small platy grains.
The distribution of rainfall is important for weathering, because driven by water delivery to the land. Streams are main agent for moving materials by either solution or particles. Streams discharge roughly equal to surface area for world's continents, except Australia and Antarctica: low because of especially low rainfall. Chemical weathering per unit area is 20-40 metric tons/km2/year and is pretty constant for continents. Mechanical weathering has more variation (20-300 metric tons/km2 /year), because of large variation in slopes. Asia is largest, because of large mountain ranges (e. g. Himalayas). Continents with high mean elevations have high mechanical weathering. Very high erosion where there are mountains and high rainfall (e. g. Indus from Himalayas and Amazon from Andes: both are in areas of high rainfall and have high sediment load). Mountains are being worn down very quickly. Implies recent geologic process that makes mountains.
Types
of Weathering
1. Chemical:Minerals making up a rock are chemically altered. They either transform to other minerals or dissolve.
2. Physical/Mechanical:Rocks are fragmented through generally inorganic mechanisms, while the chemical composition of the rocks' minerals does not change.
3. Biological:Living organisms can accelerate either of the previous two mechanisms.
Chemical
Weathering
Rainwater is always slightly acidic (and in some places, very acidic!). Carbon dioxide dissolves in water and becomes carbonic acid, which is the stuff that makes sodas acidic (the concentration of carbon dioxide is much lower in regular water, which is why it doesn't feel or taste acidic). This acid reacts chemically with minerals and either dissolves them or turns them into other minerals. Here are three types of chemical weathering which I want you to know about:
Dissolution/Leaching:Some rocks dissolve completely when exposed to rainwater; two important ones are rock salt and limestone. When these rocks dissolve, the materials which make them up become ions in solution in the water, and are carried away with it.
Oxidation/Rusting:In general, oxidation is when an atom or ion loses one or more electrons. A prime example is when iron rusts - rusting is when iron ions change from one form to another, and they lose one electron along the way. Iron-bearing minerals also ``rust'' as the iron contained in their structures changes form as above.
Hydration:Some minerals react with water and acid to take up hydrogen and ``kick out'' other cations; this process is called hydration. Feldspar minerals tend to hydrate and change to clay; this is discussed well in your book.
Physical
Weathering
Rocks are naturally fractured at several levels. Mineral grains have boundaries, which can be areas of weakness in the rock. Sedimentary rocks (next time) often are layered and the layers sometimes are not bound together well. More massive rocks can have joints (cracks which have no relative motion across them, only spreading) which will open as the rocks are exposed due to erosion. Physical weathering acts to widen these fractures. Some examples of physical weathering mechanisms:
Frost wedging :Frost wedging happens when water filling a crack freezes and expands. The expanding ice presses against the rock and wedges open the crack.
Heat/Cold Cycles:As rocks are alternately heated and cooled, they expand and contract; minerals expand and contract different amounts, and this differential expansion and contraction may stress rocks and crack them. This is mostly found in deserts; the actual mechanisms by which it happens are still under study.
Unloading:Plutonic igneous rocks form at depth in the crust, and may have zones of weakness in them. When these rocks are exposed (through erosion of overlying rock) they expand, and the zones of weakness open up as joints. Granite tends to open and peel away sheets of rock (think of peeling an onion): called exfoliation.
Abrasion:Wind, waves, rain, glaciers, and so on abrade rocks' surfaces, wearing them down.
Biological
Weathering
Biological weathering is, of course, weathering done by living things. I suppose it could really be called a special case of either physical or chemical weathering, but it is kind of neat that life on the planet can weather rocks. Some examples:
Tree roots:Tree roots grow into cracks and widen them, which helps physical weathering.
Bacteria:Some bacteria and other organisms secrete acidic solutions, which helps chemical weathering.
Controls
on Weathering Rate
Weathering doesn't proceed at the same rate everywhere. For example, rocks in arid regions may be only very slightly weathered, while similar rocks in the tropics are so weak you can kick them apart. Some general statements can be made about the factors which influence the rate of weathering:
The more massive the rock, the slower it will weather physically. Heavy rainfall and high temperatures promote chemical weathering. Thus desert rocks show less weathering than rocks in tropical rain forests. Soil is both a weathering product and an agent of weathering. Since soil holds moisture and organisms, rocks will weather more quickly when in contact with soil than when they are not. The longer a rock has been exposed at the surface, the more weathered it will be, all other things being equal.
Chemical weathering rates are controlled by the environment but also by the minerals involved. The more soluble a mineral is, the more weathered it will be. Calcite and halite are highly soluble, while quartz is not. The more quickly a mineral dissolves (the higher the rate of dissolution), the more weathered it will be. This is related to how hard it is to break bonds in the crystal structure of the mineral. Quartz dissolves very slowly, since it is a tough framework structure. Olivine, on the other hand, dissolves more rapidly, since it is made of isolated silica tetrahedra which are not as tightly bound to one another. The earlier a mineral crystallizes from melt, the less stable it is at surface conditions. Feldspars will weather more quickly that quartz, despite the fact that they are both framework silicates. This is because feldspars freeze at higher temps than quartz and are thus more unstable at surface conditions. In particular, know the weathering order for: halite, olivine, pyroxene, potassium feldspar, clay minerals, quartz, and hematite. This list is in order of increasing resistance to weathering (i. e. , hematite weathers most slowly, while halite weathers most quickly).
Different
Weathering Modes Help Each Other
Keep in mind that you really can't separate physical from chemical or biological weathering, because all three proceed at the same time (though not at the same rates, necessarily). In fact, the three are bound up with each other:
Physical weathering helps chemical weathering by breaking rocks up into smaller chunks, thus exposing more surface area. With more surface area exposed, chemical reactions happen faster. Think of dissolving sugar in water. Sugar lumps will dissolve more slowly than the same amount of sugar which is granulated --- because the surface area is higher for the granulated sugar. Try it if you don't believe me. Also, if slopes are flattened by physical weathering, water and soil can build up and thus speed chemical weathering.
Chemical weathering helps physical weathering by weakening the minerals which make up rocks. This makes the rocks easier to break up by mechanical means.
Biological weathering helps both, as I described above. Trees fracture rock with their roots, which makes them easier to break up physically and exposes more surface area for chemical weathering. Bacteria which secrete acid solutions speed chemical weathering. And so on and so on and.
Summary
From lecture, your notes, these notes, and your reading I want you to know the following:
What is weathering?
Why does it happen?
Major types of weathering
How they work
Examples of each
Controls on weathering rates
How the types of weathering help each other out
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Nilgün Okay
Last modified on Tuesi Dec. 4 11:06:10 PST 2000