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Weathering and Soil
• Weathering – the breaking down of rock
and other materials on the Earth’s
surface. A slow, continuous process. The
effects are not always easily observed and
others are obvious. Example:
paint peeling off a house.
Weathering cont.
• Rocks are broken down by
two types of weathering.
• Mechanical – when the forces of
weathering break rocks into smaller
pieces but do not change the chemical
makeup of the rocks.
• Chemical – the chemical makeup of the
rock is changed.
Mechanical Type 1
• Temperature – heat causes the outside of
rock to expand, cooling causes the rock to
contract. This continuous cycle
causes the rock to go through
exfoliation (curved sheets or
Mechanical Type 2
• Ice Wedging– the
repeated freezing
and melting of
water that has
seeped into the
cracks of the
rocks. Example:
road ways, drive
Mechanical Type 3
• Organic
Activity – a
plant growing
in a crack, roots
of plants loosen
rock material;
or any activity
caused by living
Mechanical Type 4
• Gravity – pulls
loosened rocks off
of cliffs or
mountain sides.
• Example:
Landslide – large
movement of loose
rocks and soil.
• Example:
Avalanche – large
movement of loose
rocks and snow.
Landslide Story
Mechanical Type 5
• Abrasion – windblown sand causes
weathering of rocks.
Carried by wind,
water, or other
forces. Causes
riverbed rocks to be
rounded and
Chemical Type 1
• Water –
solvent) can
dissolve most of
the minerals that
hold rocks
together. Can
also form an acid
when mixed with
certain gases
which speeds up
Chemical Type 2
• Oxidation –
combines with
substance. The
result is an
entirely different
substance. Iron
in rocks turns to
Chemical Type 3
• Carbonation – carbon dioxide
dissolves in rain water, forming a
weak acid called carbonic acid.
Dissolves certain rocks (feldspar
and limestone).
• Acid Precipitation – air polluted
with sulfur oxides (from burning
coal) dissolve in rain. Causing acid
rain to fall which corrodes rocks,
metals, and other materials quickly.
• Plant Acids – produce weak acids
that dissolve certain minerals in
rocks (example: mosses and
Formation of a Cave
Is this weathering your brain?
Differential Weathering
• The composition of rock greatly affects the rate at which
rock weathers
• Softer rock weathers away leaving the harder more
resistant rock behind
• Limestone and other sedimentary rocks that contain
calcite weather rapidly while other sedimentary rock
weather faster depending on what cements them
• Some are more resistant than igneous
Rate of Weathering…
• Amount of exposure can determine the
rate, the amount of rock that is exposed,
and the amount of time
• More surface area means more
• Fractured and jointed rock weathers
• Climate that has extreme seasons allows
the greatest rate of weathering. Hot dry
climates allow the least amount of
• Higher elevations and steeper
topography allow for faster weathering
• Plant, animal, and human activities can
accelerate weathering.
Soil Formation
• Soil- formed when
rocks are
broken down by
weathering and is
important for most
living organisms.\
• Plants - food
• Animals –
plants/animals to
Types of Soils
• Residual soil – remains on top of its parent
rock and has similar chemical composition.
• Transported soil – moved from its origin by
wind, water, glaciers, waves…and can be
completely different than the rock is lays upon
• Humus – decayed material (plants/animals).
Important for the growth of plants because it
helps to speed up the breakdown of rocks into
soil. Moles, earthworms, ants, and beetles help
break down big pieces of soil. Filled with
Soil Composition
• Pieces of weathered rock and organic material
(humus) are the two main ingredients of soil.
• Rock particles are more than 80% and air and
water are present in the pore spaces.
• Clay and quartz are the most abundant
because they are very stable.
• Nitrates (potassium, phosphorous, and
nitrogen) are vital to plant growth and found in
• Why are pore spaces important for plants?
Worm Farm
Soil Texture
• Type of weathering
affects the composition
of the soil and the
texture of the soil (size
of grains).
• Gravel – between 2 and
64 mm in diameter.
• Sand – less than 2mm
• Silt – 1/16th of a mm
• Clay – less than 1/256th
of a mm
Soil Horizons
• Soil Horizons – soil layers.
• Each horizon is different. A cross section is
called a soil profile.
• Soil that has 3 layers is mature (takes many
thousands of years and the right conditions.
• Factors: surface features, time, climate and
• Soil that has 2 layers is immature.
• Horizon O-part of the A
layer where all the organic
material is produced.
• Horizon A – uppermost
layer, dark colored, much
activity by living occurs, topsoil (humus); most fertile.
• Horizon B – leached out
minerals, clay and some
humus, called subsoil,
formed very slowly.
• Horizon C – partly
weathered rocks, extends
down to top of un-weathered
parent rock; composition
similar to parent rock.
Soil and Climate
• Tropical soils- soils develop quickly, top layer
called laterites that contain Fe and AL
• Temperate soils- A, B, and C are thick. Areas that
receive 65 cm of rain have a top layer called
pedalfur that contains clay, quartz, and Fe.
• Areas that receive less rain have a top layer called
pedocal that contains CaCO3
• Desert and Arctic soils- thin with little humus
Try and Get excited about dirt…
Soil Erosion
• Weathered material are transported by
gravity, wind, glaciers, and water
Gullying and Sheet Erosion
• Soil that is washed away with each rainfall
can furrow and eventually become gullies.
• Sheet erosion - when the top layer of soil is
washed away
Soil Conservation
• Contour
A true soil lover!!!!