Weathering is the chemical
and physical processes that change the characteristics
of rocks on the Earth’s surface. It is also known as the
preparation for erosion. In order for weathering to occur, the
environment of a rock sample must change and the rock needs to
be exposed to some form of water and the air. Human processes
such as pollution, which can be a large factor in acid rain,
along with the acts of other living organisms, can cause
chemical weathering to occur at faster rates.
process occurs when rocks are exposed to the hydrosphere
(water) and atmosphere (air). These weathering agents can
change the physical and chemical characteristics of rocks. As
rocks are broken down (weathered) they can be classified as
different types of sediments, which are: boulders, cobbles,
pebbles, sand, silt, clay, and colloids. The chart below from
page 6 of the
Earth Science Reference Tables
explains the sizes of each of
weathering occurs when rocks are broken in to smaller pieces
without changing the chemical composition of the rock. Think
of a physical change (e.g., ripping a piece of paper) where
the sample will change in size but all its other
characteristics will remain the same. There are a few types of
physical weathering such as:
Frost action/ice wedging
is the breakup of rock caused by the freezing and thawing
(contracting and expansion) of water. Water can seep into the
cracks of a rock and as the climate cools the water freezes
and expands breaking the rock apart. A very similar process
occurs on roads, which causes potholes.
Abrasion is the physical wearing down of rocks as they rub or bounce against
each other. This process is most common in windy areas, under
glaciers, or in stream channels.
the peeling away of large sheets of loosened materials at the
surface of a rock. Common in shale, slate, and mica.
Chemical weathering occurs
when a rock is broken down by chemical action resulting in a
change in the composition of a rock. Such as the change a
piece of paper would go through after being burned. The main
agents of chemical weathering are oxygen, rainwater, carbon
dioxide, and acids produced by decaying plants and animals
that leads to the formation of soil. There are a few types of
chemical weathering such as:
when oxygen interacts chemically with minerals. For example,
when a nail rusts oxygen combines with the iron in the nail to
form iron oxide.
when water interacts chemically with minerals. For example,
when hornblende and feldspar unite with water they eventually
form into clay.
occurs when carbon dioxide interacts chemically with minerals.
When carbon dioxide is dissolved in water, it forms weak
carbonic acid. Carbonic acid when it comes in contact with the
surface of the earth dissolves large masses of limestone,
creating caves and caverns. Other common terms associated with
carbonation are sink holes, karst topography, stalactites and
overall depend on 3 different factors:
size/surface area exposed to the surface
One of the major products of
weathering is soil. Soil is a combination of particles of
rocks, minerals, and organic matter produced through
weathering processes. Soil contains the necessary nutrients to
support various forms of plant and animal life.
As a result of the
weathering processes and biologic activity, soil horizons
(layers) form. Soil horizons vary in depth depending on an
areas climate and weathering rates.
The below diagram is a
mature soil profile common to New York State:
A soil profile refers to
the layers of soil; horizon O, A, B, and C, and D. Horizon O
refers to the organic material on the upper most part
of the profile (this layer is usually very thin). Horizon A
refers to the upper layer of soil, right below the O layer. It
is commonly known as topsoil. In the woods or other
areas that have not been plowed or tilled, this layer would
probably include organic material (humus), such as
fallen leaves, twigs, decaying plant and animal remains. The
material helps prevent erosion, holds moisture, and decays to
form a very rich soil known as humus. Horizon A provides
plants with nutrients they need for a great life.
The layer below horizon A
is horizon B known as the subsoil. Organic material
from layer A is not present in horizon B and therefore there
is much less humus. Horizon B usually will contain a fair
amount of clay and iron oxides, but also may contain some
elements from horizon A because of the process of leaching.
Leaching resembles what happens in a coffee pot as the water
drips through the coffee grounds. Leaching may also bring some
minerals from horizon B down to horizon C.
If horizon B is below
horizon A, then horizon C must be below horizon B. Horizon C
consists mostly of weatherized big rocks known as Parent
material (the rock that he soil formed from).
Horizon D, which is not
shown in this illustration, usually contains