The Earth Contents

The Earth - Weathering - Page 13

Question 1 answer

Weathering is the adjustment of rocks to the new surface conditions which are very different to the conditions under which they formed such as reduced temperature, and pressure, and exposure to air and water.

Question 2 answer

Heat (thermal expansion and contraction), pressure (expansion by pressure release), and crystallisation (expansion by crystallisation).

Question 3 answer

Thermal expansion and contraction occurs in climates with extreme differences between day and night temperatures such as many desert regions. Expansion and contraction can quickly induce stresses within rocks which result in their fracture.

Question 4 answer

Stresses are induced because rocks which formed at depth, under high pressures, expand when brought to the Earth's surface and subsequently fracture.

Question 5 answer

Crystallization of ice starts with the penetration of moisture into small cracks in the rock surface, which allows water to build up during warmer spells of weather, and then to freeze in the cracks in colder weather. The freezing causes expansion of ice crystals which is sufficient to prise open the crack, widening it and extending it deeper into the rock. Ultimately the cracks become extensive enough to shear away a block from the rock face, which falls to the cliff base to create a talus cone.

Question 6 answer

Once blocks have been separated from a cliff, they become exposed to gravity-induced collisions. Other blocks may fall on them, and running water may also cause movement and collisions. Each collision may fragment the blocks still further and may abrade grains from exposed edges. In this way, physical weathering can ultimately reduce even the most substantial rock mass to a collection of small particles.

Question 7 answer

The three different mechanisms in chemical weathering are; dissolution, acid hydrolysis and oxidation.

Question 8 answer

Dissolution is the simplest of all weathering reactions. The polar nature of water makes it very effective at dissolving ionic-bonded solids because it enables water to envelope both positive and negative ions. Hence oppositely charged species can be kept apart in solution, with the water layers shielding differently charged ions from each other. However, most rock-forming minerals are silicates, which are mainly covalently bonded and which therefore are only slightly reactive in the presence of pure water. Simple dissolution is important for only a few minerals such as gypsum and sodium chloride.

Question 9 answer

Chemical weathering by acid hydrolysis neutralises the acids naturally present in water. Carbonate rocks such as limestone are reactive towards acids as is demonstrated by a comparison of the waters draining from igneous rocks with those derived from limestone areas. On igneous rocks the silicate minerals containing calcium, magnesium,
sodium and potassium are attacked only slowly by acid hydrolysis,
with the result that the waters have only low concentrations of dissolved
solids and their natural acidity is not neutralised. By contrast, the rapidly reacting carbonate rocks produce waters with high concentrations of dissolved solids whose acidity has been effectively consumed by weathering.

Question 10 answer

The process of chemical weathering by acid hydrolysis neutralises the acid. Carbonate rock minerals, such as limestone, are particularly reactive towards acids, especially when compared to silicate rock minerals. This is demonstrated by a comparison of the waters draining from igneous rocks with those derived from carbonate (e.g. limestone) areas, with the waters found reflecting the characteristics of each rock type. On igneous rocks the silicate minerals containing calcium, magnesium, sodium and potassium are attacked only slowly by acid hydrolysis, with the result that the waters have only low concentrations of dissolved solids and their natural acidity is not neutralised. By contrast the rapidly reacting carbonate rocks produce waters with high concentrations of dissolved solids whose acidity has been effectively consumed by weathering.

Question 11 answer

Rocks formed below the Earth's surface, in the absence of air, may often contain reduced iron and sulphur. Elements in their reduced states are unstable in contact with air, and are easily oxidised. The breakdown and alteration of minerals containing reduced elements is an effective weathering mechanism.