Download B. Chemical Weathering

B.
C.
Chemical Weathering –
1. Decomposition of rock from exposure to water and atmospheric
gases (primarily CO2 and water vapor). As rock decomposes,
new chemical compounds form.
2. Examples of chemical weathering
a. Role of oxygen
1. Oxygen is abundant in atmosphere, and chemically
active, so it often combines with minerals or
elements in minerals at earth’s surface.
2. Oxygen from atmosphere combines with iron to form
iron oxide. Responsible for red “rusty” rocks. Ex.
hematite (red streak)
b. Role of acid
1. Most effective agent of chemical weathering is acid.
2. Decomposes minerals – often into another mineral
(substitute ions)
3. Sources of acid
a. Most important source is dissolved carbon
dioxide (CO2) in water. This forms carbonic
acid, a weak acid, but it is the single most
effective agent of chemical weathering.
Carbon dioxide is dissolved from air and soils.
In soil, as water percolates down, high
concentrations of CO2 from the decay of
organic matter and respiration of soil
organisms. Water trickles down and attacks
un-weathered rock below.
c. Solution – minerals and rocks are dissolved in water also
known as leeching. Can dissolve the cementing agents in
sedimentary rock and accelerate mechanical weathering.
(1)
Calcite, limestone, and cement are
very susceptible to solution
weathering.
(2)
Forms sinkholes, caves and
destroys material.
Rates of weathering
1. Advanced mechanical weathering aids chemical weathering by
increasing the surface area. The smaller the particles, the more
rapid the chemical weathering. This can be seen with spheroidal
weathering, when angular chunks get rounded because they are
attacked on several surfaces (edges being attacked by two sides ;
the forces of the block are being weathered from only one
direction.
2. Other important factors are
a. Mineral makeup
1.
b.
Marble (calcite) readily dissolves in weakly acidic
solutions.
2. Silicate minerals weather in the same order as their
order of crystallization.
Climate
1. Temperature and moisture are the most crucial
factors.
2. Chemical weathering is most effective in areas of
warm temperatures and abundant moisture.