This document discusses weathering and soil formation. It describes two types of weathering - mechanical and chemical - and their effects on breaking down rocks. Mechanical weathering breaks rocks into smaller pieces through processes like frost action and abrasion. Chemical weathering alters the chemical makeup of rocks through reactions with water, oxygen, carbon dioxide and acids. Over time, weathered rocks become soil. Mature soils have distinct layers called horizons that differ in composition and support plant growth. The texture and development of soil depends on factors like climate, rock type and landscape.

1. Chapter 5
Weathering and Soil Formation

2. 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.

3. • 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.

4. 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.

6. • 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.

8. • 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.

10. • 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.

12. • 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.

14. 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.

15. • 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.

17. • 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.

19. • 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.

20. • 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.

21. • 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.

22. 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.

23. 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.

24. • 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.

25. • 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.

26. 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.

27. • 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.

28. 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.

29. Gravel

30. Sand

31. Silt

32. Clay

33. 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.

35. 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.

36. • 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).

37. • 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.

39. • 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.