Chapter 5

Weathering and Soil Formation

Weathering

• Weathering- breaking down of rocks and other materials on the Earth’s surface.

– Effects all substances exposed to the atmosphere.

• Weathered bricks on buildings are darker in color and has rounded edges.

• The piles of rock at the base of a mountain is caused by weathering.

– Effects are not always easily observed.

• Two types of weathering

• Mechanical weathering- weathering that does not involve changes in the chemical makeup of rocks.

• Chemical weathering- weathering that involves changes in the chemical makeup of rocks.

Mechanical Weathering

• Rocks are broken into different shapes and smaller pieces.– At the beginning of the process, typical rock fragments

are sharp and angular. As weathering continues, they become smooth and rounded.

• Temperature- rocks can be broken apart by changes in temperature.– Heating and cooling cycle.

• Exfoliation- the breaking off of curved sheets or slabs parallel to a rock’s surface due to weathering.

• Frost action- the breaking apart of a rock caused by the water freezing and expanding within cracks.– Occurs when water seeps into a small opening or

crack in a rock. When the temperature falls below 0o C, the freezing point of water, the water in the crack freezes and expands. The crack in the rock is made larger by the pressure of the expanding water. In time, the freezing and melting of the water cause the rock to break into pieces.

• Organic activity- plants and animals can cause mechanical weathering.

• The roots of plants sometimes loosen rock material.

• A plant growing in a crack in a rock can make the crack larger as the plant’s roots grow and spread out.

• Root-pry- is an organic activity, or an activity caused by living things.

• Gravity- sometimes pulls loosened rocks down mountain cliffs.

• Landslides- a large movement of loose rocks and soil.

– As the rocks fall, they collide with one another and break into smaller pieces. Falling rocks generally occur in areas where a road or highway has been cut through a rock formation, leaving cliffs on one or both sides of the road.

• Abrasion- is the wearing away of rocks by solid particles carried by wind, water, or other forces.

– Wind- in desert areas, the wind easily picks up and moves sand particles.

– Water- rivers carry along loose rocks and other particles. The moving rocks and particles collide, scrap against one another, and eventually break.

Chemical Weathering

• Weathering that changes the mineral composition of the rock.

• As chemical changes take place, minerals can be added to or removed from rocks.

• The minerals in rocks can be broken down in a process called decomposition.

• Water- can cause chemical weathering when combine with other things.

• Can dissolve most of the minerals that hold rocks together.

• Water can form acids when it mixes with certain gases in the atmosphere.– Acids often speed up the decomposition of rocks.

• Water can also combine with a mineral to form a completely different mineral.– When the mineral feldspar reacts with water, it forms

clay.

• Oxidation- is the process in which oxygen chemically combines with another substance.

• The result of oxidation is the formation of an entirely different substance.

• Iron in rocks combine with oxygen in the air to form iron oxide, or rust.

• If oxidation is taking place, the inner material of a rock will be a different color from the outer material.

• Carbonation- the process in which carbonic acid reacts chemically with other substances.

• Carbon dioxide dissolves in water, a weak acid called carbonic acid is formed.

• In nature carbonic acid is formed when carbon dioxide in the air dissolves in rain.

• Carbonic acid slowly decomposes feldspar and limestone.

• Sulfuric Acid- the air in certain areas is polluted with sulfur oxides. Sulfur oxides are the byproduct of the burning of coal as a source of energy.

• Sulfuric acid corrodes, or wears away, rocks, metals, and other materials very quickly.

• Plant Acids- certain types of plants can cause chemical weathering.

• Moses produce weak acids that can weather certain types of minerals in rocks.

• Moses can gradually dissolve minerals and break rock into smaller pieces.

• Lichens also produce weak acids that break down the minerals in rock.

Rate of Weathering

• Stable rock- a rock composed of minerals that resist chemical weathering.

• Factors of Weathering– Composition of rocks

• Two different types of rocks in the same climate can weather differently.

– Time the rock is exposed• A very old rock that has not been exposed to the various forces of

weathering can remain almost unchanged. But if a newly formed rock is immediately deposited on the Earth’s surface, it will begin to weather right away.

– Exposed surface area• The more surface area that is exposed the more weathering takes

place.

Soil Formation

• Soil- is formed when rocks are continuously broken down by weathering.– Formation of soil is extremely important to most living

organisms.

– Plants depend on soil directly as a source of food.• Soil supplies plants with minerals and water needed for

growth.

– Animals depend on soil indirectly for the minerals they need to live. • Some animals eat plants; other animals eat animals that eat

plants.

• Residual soil- soil that remains on top of the rock from which it was formed.– Has a composition similar to that of the parent rock it

covers.

• Transported soil- soil that is moved away from its place of origin.– Can be very different in composition from the layer of

rock it covers.– The soil can be moved away from its parent rock by

water, wind, glaciers, and waves.

• Bedrock- the layer of rock beneath the soil.

• Humus- the part of the soil formed by decaying organic material.

– Dark-colored material that is important for the growth of plants.

– Some of the chemicals produced during the process of decay speed up the breakdown of rocks into soil.

Soil Composition

• Two main ingredients of soil

– Weathered rock

• Form more than 80% of soil.

– Organic material (Humus)

• Material that was once living or was formed by the activity of living organisms.

• Pore spaces- the space between soil particles.

• Clay and quartz are the most abundant minerals in soil.

• Nitrates, air, and water are in the composition of the soil to help the growth of plants.

Soil Texture

• The type of weathering also affects soil texture.

• Texture refers to the size of individual soil particles.

• Mechanical and Chemical weathering produce:

– Gravel- between 2 and 64 millimeters in diameter.

– Sand- less than 2 millimeters in diameter.

– Silt- less than 1/16 millimeters in diameter.

– Clay- less than 1/256 millimeters in diameter.

Gravel

Sand

Silt

Clay

Soil Horizons

• Horizons- are layers of the soil.

• Soil profile- a cross section of the soil horizons.

• Soil that has developed three layers is called mature soil.

– Takes many thousands of years and the proper conditions.

• Soil that has developed two layers is called immature soil.

– Soil that has been formed more recently.

Mature Soil

• Uppermost layer is called A Horizon.

– Dark-colored soil layer in which much activity by living organisms takes place.

– Topsoil- the soil in the A horizon, or uppermost layer of mature soil.

• Humus supplies minerals essential for plant growth.

• Contains water and air which helps plant growth.

• The second layer is called B Horizon.

– Made up of leached-out minerals, clay and some humus.

• Leaching- the process in which water washes minerals from the topsoil to the subsoil.

• Subsoil- soil in the B horizon.

– Formed very slowly (more than 100,000 years).

• The third level is called C Horizon.

• Made up of partly weathered rock.

• Extends down to the top of the unweathered parent rock.

• Composition of C horizon is similar to that of the parent rock.

• The ability of soil horizons to developed.– Time

• The longer the rock is exposed to the forces of weathering, the more it is broken down.

– Climate • The areas with heavy rainfall and warm temperatures,

weathering takes place more rapidly.

– Type of rock• Some rocks do not weather as rapidly as others do.

– Surface features • On steep slopes, rainwater running off the land erodes the

soil and exposes rock to weathering.