Minerals that form in igneous and metamorphic rocks at high temperatures and pressures may be unstable at temperatures and pressures at the Earth's surface, so they react with the water and atmosphere to produce new minerals. This process is known as chemical weathering. The most effective chemical agents are weakly acidic solutions in water. Therefore, chemical weathering is most effective in hot and wet climates.
Rainwater mixes with Co2 from the atmosphere and from decaying organic matter, including smog, to produce carbonic acid according to the following reaction:
H2O + Co2 ^ H2CO3 Water + carbon dioxide ^ carbonic acid
Carbonic acid ionizes to produce the hydrogen ion (H+), which readily combines with rock forming minerals to produce alteration products. These alteration products may then rest in place and become soils, or be eroded and accumulate somewhere else.
Hydrolysis is a process that occurs when the hydrogen ion from carbonic acid combines with potassium feldspar to produce kaolinite, a clay mineral, according to the following reaction:
2KAlsi3o8 + 2H2TO3 + H2O ^ Al2si2o5(oH) 4 + 4sio2 + 2K+1 + 2HCO3
feldspar + carbonic acid + water ^ kaolinite + silica + potassium + bicarbonate ion
This reaction is one of the most important reactions in chemical weathering. The product, kaolinite, is common in soils and is virtually insoluble in water. The other products, silica, potassium, and bicarbonate are typically dissolved in water and carried away during weathering.
Much of the material produced during chemical weathering is carried away in solution and deposited elsewhere, such as in the sea. The highest-temperature minerals are leached the most easily. Many minerals combine with oxygen in the atmosphere to form another mineral by oxidation. Iron is very easily oxidized from the Fe+2 state to the Fe+3 state, forming goethite or with the release of water, hematite.
Different types of rock weather in distinct ways. For instance, granite contains potassium feldspar and weathers to form clays. Building stones are selected to resist weathering in different climates, but now, increasing acidic pollution is destroying many old landmarks. Chemical weathering results in the removal of unstable minerals and a consequent concentration of stable minerals. Included in the remains are quartz, clay, and other rare minerals such as gold and diamonds, which may be physically concentrated in placer deposits.
on many boulders, weathering penetrates only a fraction of the diameter of the boulder, resulting in a rind of the altered products of the core. The thickness of the rind itself is useful for knowing the age of the boulder, if rates of weathering are known. These types of weathering rinds are useful for determining the age of rockslides and rockfalls and the time interval between rockfalls in any specific area.
Exfoliation is a weathering process where rocks spall off in successive shells, like the skin of an onion. Exfoliation is caused by differential stresses within a rock formed during chemical weathering processes. For instance, feldspar weathers to clay minerals, which take up a larger volume than the original feldspar. When the feldspar minerals turn to clay, they exert considerable outward stress on the surrounding rock, which is able to form fractures parallel to the rock's surface. This need for increased space is accommodated by the minerals through the formation of these fractures, and the rocks on the hillslope or mountain are then detached from their base and more susceptible to sliding or falling in a mass-wasting event.
If weathering proceeds along two or more sets of joints in the subsurface, it may result in shells of weathered rock which surround unaltered rocks, looking like boulders. This is known as spheroidal weathering. The presence of the several sets of joint surfaces increases the effectiveness of chemical weathering, because the joints increase the available surface area to be acted on by chemical processes. The more subdivisions within a given volume, the greater the surface area.
Biological weathering is the least important of the different categories of weathering. In some places plants and microorganisms may derive nutrition from dissolving minerals in rocks and soil, thus contributing to their breakdown and weathering. There are enormous numbers of microorganisms and insects living in the soil horizon, and these contribute to the breakdown of organic material in the soils and also contribute their tests or bodies when they die. Biological weathering may also include some of the effects of roots pushing rocks apart or expanding cracks in the weathered rock horizon. These effects also move rock fragments, so they are discussed under the topic of mechanical weathering.
FACTORS THAT INFLUENCE WEATHERING
The effectiveness of weathering processes is dependent upon several different factors, explaining why some rocks weather one way at one location and a different way in another location. Rock type is an important factor in determining the weathering characteristics of a hillslope, because different minerals react differently to the same weathering conditions. For instance, quartz is resistant to weathering, and quartz-rich rocks typically form large mountain ridges. Conversely, shales readily weather to clay minerals, which are easily washed away by water, so shale-rich rocks often occupy the bottoms of valleys. Examples of topography being closely related to the underlying geology in this manner are abundant in the Appalachians, Rocky Mountains, and most other mountain belts of the world.
Rock texture and structure is important in determining the weathering characteristics of a rock mass. Joints and other weaknesses promote weathering by increasing the surface area for chemical reactions to take place on, as described above. They also allow water, roots, and mineral precipitates to penetrate deeply into a rock mass, exerting outward pressures that can break off pieces of the rock mass in catastrophic rockfalls and rockslides.
Wegener, Alfred Lothar
The slope of a hillside is important for determining what types of weathering and mass-wasting processes occur on that slope. Steep slopes let the products of weathering get washed away, whereas gentle slopes promote stagnation and the formation of deep weathered horizons.
Climate is one of the most important factors in determining how a site weathers. Moisture and heat promote chemical reactions, so chemical weathering processes are strong and fast and dominate over mechanical processes in hot, wet climates. In cold climates, chemical weathering is much less important. Mechanical weathering is very active during freezing and thawing, so mechanical processes such as ice wedging tend to dominate over chemical processes in cold climates. These differences are exemplified by two examples of weathering. In much of New England, a hike over mountain ridges will reveal fine, millimeter-thick striations that were formed by glaciers moving over the region more than 10,000 years ago. Chemical weathering has not removed even these one-millimeter-thick marks in 10,000 years. In contrast, new construction sites in the Tropics, such as roads cut through mountains, often expose fresh bedrock. In a matter of 10 years these road cuts will be so deeply eroded to a red soil-like material, called gruse, that the original rock will not be recognizable.
As in most things, time is important. It takes tens of thousands of years to wash away glacial grooves in cold climates, but in the Tropics, weathered horizons that extend to hundreds of meters may form over a few million years.
See also atmosphere; hydrosphere; soils.
Birkland, P. W. Soils and Geomorphology. New York:
Oxford University Press, 1984.
Wegener, Alfred Lothar (1880-1930) German
Meteorologist, Geologist Alfred Lothar Wegener was born on November 1, 1880, in Berlin, Germany, and obtained a Ph.D. in astronomy from the University of Berlin in 1904. He is well known for his studies in meteorology and geophysics and is considered by many to be the father of the theory of continental drift. His is also known for his work on dynamics and thermodynamics of the atmosphere, atmospheric refraction and mirages, optical phenomena in clouds, acoustical waves, and the design of geophysical instruments. Wegener was an avid balloonist, and pioneered the use of weather balloons to monitor weather and air masses while he was working at the Royal Prussian Aeronautical Observatory near Berlin. Alfred and his brother, Kurt, broke a world endurance record for hot-air balloons in 1906, staying aloft for more than 52 hours. Alfred married the daughter of famous Russian climatologist Vlad-dimir Koppen. On his fourth and last expedition to Greenland, Alfred and his companion Rasmus Vil-lumsen became lost in a blizzard on November 2, 1930, and Wegener's body was not discovered until May 12, 1931.
Alfred Wegener's interest in meteorology and geology led him on a Danish expedition to the unmapped northeastern Greenland coast in 1906-08, mainly to study the circulation of polar air masses. This was the first of four Greenland expeditions he would make, and this area remained one of his dominant interests. In 1909 he took a position at the University of Marburg in Germany, where he lectured on meteorology, astronomy, and mapping. He authored a textbook in meteorology called The Thermodynamics of the Atmosphere, based on a series of lectures he gave at the university, and published it in 1911, when he was just 30 years old.
Wegener is most famous for being the first person to come up with the idea for continental drift. This interest was initially sparked while he was teaching at the University of Marburg in 1911 and noted the striking similarity of fossils from continents now separated by large oceans. The accepted theories for this similarity at the time were that land bridges between the continents occasionally rose up, allowing plants and animals to move from continent to continent. However, Wegener studied the apparent correspondence between the shapes of the coastlines of western Africa and eastern south America, and he hypothesized that the continents had themselves moved apart and that land bridges did not rise between stationary continents. soon, he came up with a model in which most of the world's continents had drifted away from a former supercontinent starting around 180 million years ago. He continued to study the paleontological and geologic evidence, concluding that these similarities demanded a detailed explanation.
In 1912 Wegener returned to Greenland, on a perilous journey in which the team "narrowly escaped death" while climbing a tidewater glacier on the coast that suddenly began calving, and then he became the first person to spend the entire winter on the ice cap. In the spring the team broke another record, making the longest crossing of the ice sheet ever, walking 750 miles (1,200 km) across barren ice at elevations up to 10,000 feet (3,000 m). During this trip Wegener collected many scientific samples and data on glaciers and climate and became the first person to track storms over the polar ice cap.
Wegener continued to study many different features on the continents when he returned to Marburg and found that mountain ranges in south America
Wegener, Alfred Lothar
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