Soils, Weathering, and Erosion

Question 1

Soils are formed by a series of processes commonly referred to as

Answer 1

Weathering

Question 2

The composition of a soil is controlled by

Answer 2

The composition of bedrock on which to forms, climate, and the extent of chemical weathering. (all true)

Question 3

One of the key components of chemical weathering is the presence of

Answer 3

Water

Question 4

Feldspars chemically weather through a reaction with water to become clay minerals. This chemical reaction is especially rapid

Answer 4

The more acidic the water

Question 5

Higher temperatures along with moister climates generally increase the rate of

Answer 5

Biological and chemical weathering.

Question 6

Which of the following types of soil would be likely to drain most readily (be most permeable)?

Answer 6

Sand

Question 7

Soil Erosion

Answer 7

Soil erosion is a natural process. It becomes a problem when human activity causes it to occur much faster than under natural conditions. Major global environment problem. (all true)

Question 8

Soil erosion is a significant contributor to this:

Answer 8

Storm water runoff pollution

Question 9

Which of the following is the most true?

Answer 9

At this time, soil is eroding in the U.S. about 10 times faster than it is forming.

Question 10

Terracing of farmland

Answer 10

Reduces surface-water runoff velocity and thus the efficiency of soil erosion and transport.

Question 11

What is soil?

Answer 11

• Unconsolidated forms on land surface.
• Minerals, organics, gases, and liquids
• Original sedimentary layers gone, new horizons form in place
• NO PLANTS NECESSARY

Question 12

What is soil made of?

Answer 12

Mineral Matter 45%
Gases 25%
Water 25%
Organic 5%

Question 13

Soil Vs. Sediment

Answer 13

Sediments are loose, unconsolidated accumulations of mineral or rock
particles that have been transported by wind, water, or ice, or shifted
under the influence of gravity, and redeposited.

Soils are the products of weathering, bioturbation, leaching. They are
typically zonated into horizons and are therefore likely to be equal to or
more complex in their interactions with the elements than sediments.

Question 14

How do soils form?

Answer 14

Soil is the unconsolidated material above bedrock and contains both mineral matter and organic matter (decaying plants, usually) along with air and water. Soil differs from sediment, which is just weathered rock that is transported or deposited.

Notice all the different zones or layers (called horizons). These layers are not the same as those formed by sedimentation, instead each horizon forms and grows in place by weathering of rock and sediment.

Question 15

What is Weathering and how does it affect rocks near the surface?

Answer 15

•Rocks are agglomerations of minerals.

Processes that break
down rock often also break down minerals (weathering):
•physical/mechanical weathering permits the elements to remain in the minerals, which are transported as small fragments to a basin in which they are ultimately deposited.
•chemical weathering (removes the elements from the original mineral lattices and either transports them in solution, i.e., dissolved in water, or in small fragments of newly formed mineral phases)

Question 16

What is Physical Weathering and how does it

affect rocks near the surface?

Answer 16

Joints are fractures, or very fine cracks, in rocks that show no significant offset.
Joints help break rocks into smaller pieces and permit water and roots to penetrate into the rock, thereby promoting weathering.

Question 17

What is Chemical Weathering and how does it

affect rocks near the surface?

Answer 17

•Chemical weathering of rocks is sometimes referred to as the decomposition of rock and it results from the combined action of chemical
attack and microbial attack
•The end result of rock decomposition is a regolith/soil profile
•The single most important factor in determining the chemical weathering of rock is water, especially when it contacts the rock as rain. Rainwater is as close to pure water as it is possible to find among the reservoirs of natural
water. Pure water is an aggressive solvent because the molecule is polar and is capable of stabilizing the charges of ions that dissolve in it.

Question 18

What happens when rocks dissolve?

Answer 18

The main agents for chemical weathering are water and weak acids formed in water,
such as carbonic acid (H2CO3). These agents dissolve some rocks, loosen mineral
grains, form clay minerals, and widen fractures.

e.g. Limestone and other rocks rich in calcium
carbonate or magnesium carbonate are soluble in water and acids. They dissolve and form pits and cavities, which widen and interconnect over time.

Question 19

Chemical Weathering

Answer 19

Some of the most noticeable results of chemical weathering come from the simple dissolution of the rock. Obviously, in the dissolution of rock, any trace elements present will be released
•The type of rocks most susceptible to this kind of weathering are those composed of salts: the
evaporites and the carbonates
•Entire landscapes are formed as a result of
bedrock dissolution. The best known such
landscape is called karst

Question 20

Ion exchange

Answer 20

dissolved ions attach themselves
to soil particles and are
selectively removed by plants

Soils with high ion exchange capacity
have a lot of electrically charged clay
and organic particles.

How old a soil is usually determines
how much clay it has. The more
rainfall a soil gets, the faster it breaks
down into clay.

Question 21

How does weathering make the ocean salty?

Answer 21

most of the ocean’s salts are derived from

weathering and the dissolution of rocks on land

Question 22

What processes occur during soil formation?

Answer 22

Soil forms gradually over thousands of years and involves the vertical
transport of dissolved material up and down through the soil profile

Question 23

How do soil profiles differ in different climates?

Answer 23

Tropical Climates: Abundant rainfall and abundant soil moisture cause severe chemical leaching, leaving behind a soil rich in iron (Fe) and aluminum (Al) oxides, giving the soil a
deep red color. This extremely leached
type of soil is a laterite

Arid Climates: Arid climates have minimal precipitation. There is limited vegetation, so there is no O horizon, only a thin A horizon.

Question 24

How do soil profiles differ over different rock types?

Answer 24

Because rocks contain assemblages of minerals, the weathering of
different rock types can produce parent material with varying proportions
of clay and quartz minerals. This, in turn, causes residual soils to vary in their drainage and water storage properties, ultimately leading to the
preferential growth of different plant types.

Question 25

WHAT FACTORS INFLUENCE

SOIL FORMATION?

Answer 25

• climate
• organisms
• parent material
• topography
• time

Question 26

WHY IS SOIL IMPORTANT?

Answer 26

The nutrients in soil are necessary to support plant life (most of the nutrients that a plant needs comes from the underlying soil)
• Weathering processes are VERY SLOW, so it takes a long time
to create rich soils (~100 years for 1 mm of soil)
• There are more than 6 billion people in the world relying on just
11% of the Earth’s land surface today, and only 3% has naturally fertile soil.

Question 27

Soil Erosion vs. Soil Formation

Answer 27

•Soil losses in U.S. amount to billions of tons
per year – about 0.04 cm per year
•Human activities, including farming, accelerate
the loss of soil
•On a global basis, soil degradation is
caused primarily by overgrazing (35%),
agricultural activities (28%), deforestation
(30%), overexploitation of land to produce
fuelwood (7%), and industrialization (4%)

Question 28

How can we reduce soil erosion rates?

Answer 28

•Protect the soil from fast moving wind!
•Plant wind breaks perpendicular to 
dominate wind direction!
•Protect the soil from fast moving water!
•Reduce the slope so runoff is slowed!
•Plow fields parallel to contours!
•Terrace fields!
•Grow plants with extensive root systems 
to hold the soil in place