3. Comprehension check.

1. What are the most important processes of chemical weathering?

2. Is water found in nature pure? What makes it acidic?

3. What helps dissolve rocks?

4. What is oxidation? Give an example.

5. What is hydrolysis?

6. Most solution reactions are reversible. What does it mean?

4. Lexical exercises.

a) Match the words and the meanings:

intact clean, unmixed

common numerous, plentiful

pure loss, damage

remote simple, ordinary

abundant far, distant

harm hard, strong, firm

tough unbroken, undamaged

durable lasting, existing for a long time

b) Find English equivalents in the text:

выветривание; делювиальные склоны; дневной цикл нагревания – охлаждения; морозное расклинивание; гидролиз; окисление; кислотные дождь; декомпрессия (снятие нагрузки (давления) с образованием трещин); механическое (химическое) выветривание; острые края; валун; образование ржавчины

5. Summary.

Weathering is the decomposition and disintegration of rocks and minerals at the Earth’s surface. Erosion is the removal of weathered rock or soil by moving water, wind, glaciers, or gravity. After a rock or soil has bee eroded from the immediate environment, it may be transported large distances and deposited.

Mechanical weathering can occur by frost wedging, abrasion, organic activity thermal expansion and contraction, and pressure-release fracturing.

Chemical weathering occurs when chemical reactions decompose minerals. A few minerals dissolve readily in water. Acids and bases often markedly enhance the solubility of minerals. The hydrolysis is a form of chemical weathering. Oxidation is the reaction with oxygen to decompose minerals.

Unit IV GROUND WATER

1. Read and translate the following word combinations.

To fill with water; subterranean ground water; to saturate the Earth’s crust; to dig wells; to pump the water to the surface; to provide drinking water; porosity of rock or soil; permeability; to porous and permeable; the density of fractures in the rock; to soak into the ground; to descend into the crust; the zone of saturation; the water table; the zone of aeration; capillary action; a capillary fringe; the soil moisture belt; to recharge the ground water; to be poor aquifers; to create landforms; an intricate stalactite; high humidity; the splashing water

2. Read the text.

CHARACTERISTICS OF GROUND WATER

POROSITY AND PERMEABILITY

In the upper few kilometers of the Earth, bedrock and soil contain small cracks and voids that are filled with air or ground water. The proportional volume of these open spaces is called the porosity of rock or soil. The porosity of sand and gravel is typically high – 40 percent or higher. Mud can have a porosity of 90 percent or more because the tine clay particles are electrically attracted to water. Most rocks have lower porosities than loose sediment. Sandstone and conglomerate can have 5 to 30 percent porosity. Shale typically has porosity less than 10 percent. Igneous and metamorphic rocks have very low porosities unless they are fractured.

Porosity indicates the amount of water that rock or soil can hold; in contrast, permeability is the ability of rock or soil to transmit water (or any other fluid). Water can flow rapidly through material with high permeability. Most materials with high porosity also have high permeability, buy permeability also depends in how well the pores are connected and on pore size.

The connections between pores affect permeability because the pores, no matter how large, must be connected for water to flow through rock or soil. Uncemented sand and gravel are both porous and permeable because their pores are large and well connected. Thus, water flows easily through them. Sedimentary rocks such as sandstone and conglomerate can have high permeabilities if cement has not filled the pores and channels. The permeability of many other rocks depends on the density of fractures in the rock.

If the pores are very small, electrical attractions between water and soil particles slow the passage of water. Clay typically has a high porosity, but because its pores are so small it commonly has a very low permeability and transmits water slowly.

THE WATER TABLE

When rain falls, it usually soaks into the ground. Water does not descend into the crust indefinitely, however. Below a depth of a few kilometers, the pressure from overlying rock closes the pore, making bedrock both nonporous and impermeable. Water accumulates on this impermeable barrier, filling pores in the rock and soil above it. This completely wet layer of soil and bedrock above the barrier is called the zone of saturation. The water table is the top of the zone of saturation. Above the water table lies the unsaturated zone, or zone of aeration. In this layer, the rock or soil may be moist but not saturated.

Gravity pulls ground water downward. However, electrical forces can pull water upward through small channels, just as water rises in a paper towel dipped in water. This upward movement of water is called capillary action. Thus, a capillary fringe 30 to 60 centimeters thick rises from the water table.

Topsoil usually contains abundant litter and humus, which retain moisture. Thus, in most humid environments, topsoil is wetter than the unsaturated zone beneath it. This moist surface layer is called the soil moisture belt, and it supplies much of the water needed by plants.

If you dig into the unsaturated zone, the hole does not fill with water. However, if you dig below the water table into the zone of saturation, you have dug a well, and the water level in a well is at the level of the water table. During a wet season, rain seeps into the ground to recharge the ground water, and the water table rises. During a dry season, the water table falls. Thus, the water level in most wells fluctuates with the seasons.

An aquifer is any body of rock or soil that can yield economically significant quantities of water. An aquifer must by both porous and permeable so that water flows into a well to replenish water that is pumped out. Sand and gravel, sandstone, limestone, and highly fractured bedrock of any kind make excellent aquifers. Shale, clay, and unfractured igneous and metamorphic rocks are poor aquifers.

CAVERNS AND KARST TOPOGRAPHY

Just as streams erode valleys and form flood plains, ground water also creates landforms. Rainwater reacts with atmospheric carbon dioxide to produce a slightly acidic solution that is capable of dissolving limestone. This reaction is reversible. The dissolved ions can precipitate to form calcite again. (Recall that limestone is composed of calcite.)

CAVERNS

Most caverns, also called caves, form where slightly acidic water seeps through a crack in limestone, dissolving the rock and enlarging the crack. Most caverns form at or below the water table. If the water table drops, the chambers are opened to air. While caves form when limestone dissolves, most caves also contain features formed by deposition of calcite. Collectively, all mineral deposits formed by water in caves are called speleothems. Some are long, pointed structures hanging from the ceilings (stalactites); others rise from the floors (stalagmites).

Stalagmites build from the floor upward to complete stalactites. Because stalagmites are formed by splashing water, they tend to be broader that stalactites. As the two features continue to grow, they may eventually join to form a column.

SINKHOLES

If the roof of a cavern collapses, a sinkhole forms on the Earth’s surface. A sinkhole can also form as limestone dissolve from the surface downward.

Although sinkhole formation is a natural even, the problem can be intensified by human activities. The Winter Park sinkhole (May 1981, Florida) formed when the water table dropped, removing support for the ceiling of the cavern. The water table fell as a result of a severe drought augmented by excessive removal of ground water by humans.

KARST TOPOGRAPHY

Karst topography forms in broad regions underlain by limestone and other readily soluble rocks. Caverns and sinkholes are common features of karst topography. Surface streams often pour into sinkholes and disappear into caverns. In the area around Mammoth Caves in Kentucky, streams are given names such as Sinking Creek, an indication of their fate. The word karst is derived from a region in Croatia where this type of topography is well developed. Karst landscapes are found in many parts of the world.