Weathering rates depend on several factors. These include the composition of the rock and the minerals it contains as well as the climate of a region. Different rock types weather at different rates. Certain types of rock are very resistant to weathering. Igneous rocks, especially intrusive igneous rocks such as granite, weather slowly because it is hard for water to penetrate them. Other types of rock, such as limestone, are easily weathered because they dissolve in weak acids. Rocks that resist weathering remain at the surface and form ridges or hills.
Different minerals weather at different rates. Mafic silicates like olivine and pyroxene tend to weather much faster than felsic minerals like quartz and feldspar. Different minerals show different degrees of solubility in water in that some minerals dissolve much more readily than others. Water dissolves calcite more readily than it does feldspar, so calcite is considered to be more soluble than feldspar. Massive rocks like granite generally to not contain planes of weakness whereas layered sedimentary rocks have bedding planes that can be easily pulled apart and infiltrated by water.
Weathering therefore occurs more slowly in granite than in layered sedimentary rocks. Rainfall and temperature can affect the rate in which rocks weather. High temperatures and greater rainfall increase the rate of chemical weathering. Rocks in tropical regions exposed to abundant rainfall and hot temperatures weather much faster than similar rocks residing in cold, dry regions. Soils affect the rate in which a rock weathers. Soils retain rainwater so that rocks covered by soil are subjected to chemical reactions with water much longer than rocks not covered by soil.
Soils are also host to a variety of vegetation, bacteria and organisms that produce an acidic environment which also promotes chemical weathering. Minerals in a rock buried in soil will therefore break down more rapidly than minerals in a rock that is exposed to air. The longer a rock is exposed to the agents of weathering, the greater the degree of alteration, dissolution and physical breakup.
Lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow which remains exposed to the elements for long periods of time. Chemical weathering is a process where minerals in a rock may be converted into clays, oxidized or simply dissolved. Silicates comprise almost all minerals in igneous rocks and are also important components in metamorphic rocks. Not all silicates, however, survive weathering processes to become incorporated into sedimentary rocks.
Moisture speeds up chemical weathering. Weathering occurs fastest in hot, wet climates. It occurs very slowly in hot and dry climates.
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases figure 5. With more surfaces exposed, there are more surfaces on which chemical weathering can occur. Mechanical weathering may increase the rate of chemical weathering. Plant and animal life, atmosphere and water are the major causes of weathering. Tree roots, for example, can fracture rocks in the same way they buckle pavement.
Warm, humid climates are most favorable to life. Contrast the rich diversity of life in a rainforest, for example, with the scarcity of life in the dry Sahara or the frigid Antarctic. Consequently, rates of biological weathering are most rapid in warm humid climates like those in tropical regions. Some types of rocks weather more rapidly in humid climates, while dry climates make other rocks more susceptible to attack. Limestone weathers rapidly in areas with wet climates, where rainwater mixed with carbon dioxide in soil or creates a weak acid that dissolves the limestone to form crevices and valleys.
Sandstone, by contrast, weathers more rapidly in dry climates, because the quartz in the sandstone is largely invulnerable to chemical weathering but can fall prey to fracturing caused by ice formed when water freezes and expands in cracks in the stone. Wet climates accelerate the rates of chemical weathering, caused when C0 2 in dirt mixes with air and water to form a weak acid. The weak acid breaks down rocks more rapidly in wet climates compared with dry ones.
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