mechanical and chemical weathering
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases Figure 1.5. With more surfaces exposed, there are more surfaces on which chemical weathering can occur. Mechanical weathering may increase the rate of chemical weathering. Click image to the left or use the URL below. URL:
chemical weathering by water
A water molecule has a very simple chemical formula, H2 O, two hydrogen atoms bonded to one oxygen atom. But water is pretty remarkable in terms of all the things it can do. Remember that water is a polar molecule. The positive side of the molecule attracts negative ions and the negative side attracts positive ions. So water molecules separate the ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt. Weathered rock in Walnut Canyon near Flagstaff, Arizona. Hydrolysis is the name of the chemical reaction between a chemical compound and water. When this reaction takes place, water dissolves ions from the mineral and carries them away. These elements have been leached. Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Clay minerals are more stable at the Earths surface.
chemical weathering by carbonic acid
Carbon dioxide (CO2 ) combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature, where it works to dissolve rock. Pollutants, such as sulfur and nitrogen from fossil fuel burning, create sulfuric and nitric acid. Sulfuric and nitric acids are the two main components of acid rain, which accelerates chemical weathering (Figure 1.3). Acid rain is discussed in the chapter Human Impacts on Earths Systems. This statue at Washington Square Arch in New York City exhibits damage from acid rain.
chemical weathering by oxygen
Oxidation is a chemical reaction that takes place when oxygen reacts with another element. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust (Figure 1.4). Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils.
plants and animals
Now that you know what chemical weathering is, can you think of some other ways chemical weathering might occur? Chemical weathering can also be contributed to by plants and animals. As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration.
Chemical weathering is the other important type of weathering. Chemical weathering may change the size of pieces of rock materials, but definitely changes the composition. So one type of mineral changes into a different mineral. Chemical weathering works through chemical reactions that cause changes in the minerals.
no longer stable
Most minerals form at high pressure or high temperatures deep in the crust, or sometimes in the mantle. When these rocks are uplifed onto Earths surface, they are at very low temperatures and pressures. This is a very different environment from the one in which they formed and the minerals are no longer stable. In chemical weathering, minerals that were stable inside the crust must change to minerals that are stable at Earths surface.
Remember that the most common minerals in Earths crust are the silicate minerals. Many silicate minerals form in igneous or metamorphic rocks. The minerals that form at the highest temperatures and pressures are the least stable at the surface. Clay is stable at the surface and chemical weathering converts many minerals to clay (Figure 1.1). There are many types of chemical weathering because there are many agents of chemical weathering. Deforestation in Brazil reveals the under- lying clay-rich soil.
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a) may change the composition of a piece of rock, but definitely changes the size. b) is no different from mechanical weathering in changing rocks. c) is much less important than mechanical weathering because it acts under very few conditions. --> d) may change the size of a piece of rock, but definitely changes the composition.
the most common minerals in earths crust are the carbonate minerals.
a) true --> b) false
why do silicate minerals turn to clay at the surface?
--> a) unlike silicates, clay is stable at surface temperatures and pressures. b) silicate minerals are unstable in the acid rain found at the surface. c) clay is what all minerals at the surface eventually turn to, silicates are just quicker. d) none of the above
a) acid rain b) the process of removing dissolved minerals as they are carried to lower layers in soil --> c) hydrogen or hydroxide ions replace the cations in a mineral to change the mineral d) oxygen reacts with another element to create a metal oxide
which of these is a metal oxide?
a) iron --> b) rust c) carbonic acid d) all of the above
the acid in normal (non-polluted) rainwater comes from
--> a) carbon dioxide combining with water in the atmosphere. b) photosynthesis c) hydrochloric acid combining with rainwater in the atmosphere d) hydrolysis
oxidation is common because
a) oxygen is a polar molecule that attaches to ions in the rock. b) oxygen is the most common component of the atmosphere. --> c) oxygen is highly chemically reactive. d) both a and b
the more surface area of a rock is exposed, the more chemical weathering can occur.
--> a) true b) false
how does a water molecule dissolve rock?
a) the positive side of the water molecule attracts negative ions in the rock. b) the negative side of the water molecule attracts positive ions in the rock. c) the positive ions attract positive ions and negative ions attract negative ions. --> d) a & b are correct.
how can plants cause chemical weathering?
a) plant roots break apart the rock as they reach downward into it. --> b) plants take in ions from minerals as nutrients and so alter the rock. c) plants release carbonic acid that chemically alters the rock. d) a & b are correct.
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