Lesson for Year 7 class

1. Weathering
How did this happen to the rock?

2. Weathering is the breakdown of rocks by
water, frost and temperature change. Rocks
can also be broken down by the effects of
plants and animals.
There are three kinds of weathering;
physical, chemical and biological. They can all
go on together.
Erosion is the wearing away of rock and its
removal by streams, ice, waves and wind.
Erosion, transportation and deposition help
shape the land.

3. Physical Weathering – freeze-thaw
You may have heard of the
saying, “hard as rocks”.
This is because as water
freezes it expands.
This creates powerful forces
that can enlarge the cracks.
As this freeze–thawfreeze–thaw process
is repeated and cracks
spread through the rock.
Eventually small pieces of
rock (called screescree) break off
altogether.
Most rocks are hard, but
despite this they can be
broken by just a small amount
of water getting into cracks in
the rock.

5. Physical Weathering – exfoliation
Freeze thaw is not the
only cause of weathering.
In places with large daily
changes in temperature
(e.g. deserts) expansion
and contraction of the rock
itself occurs. The surface
gets the hottest and so
expands the most. This
may cause it to “peel off.”
Additionally, some rocks
contain crystals that expand
by very different amounts.
This too can cause cracks.

7. Biological Weathering – plants and lichens
Plant roots can get into tiny cracks and
can physically open them up further.
In addition, decaying plant roots also
produce acid which can chemically eat
away at the rock.
Similarly lichens produce acids
which weather the rocks upon
which the lichens are growing.
Plant roots can
cause cracks in
rocks
pH876543

9. Chemical Weathering – Carbonation
Firstly, there is carbon dioxide gas
which dissolves in rain to form
weak carbonic acid. This very
slowly eats away at certain rocks.
There are also acids in the rain that can
chemically eat away at rocks – especially
rocks consisting of metal carbonates (such
as chalk, limestone and marble).
Secondly, there are nitrogen and
sulphur oxides which produce
much more acidic rain that can
rapidly chemically dissolve the rocks.

12. What type of weathering?
Look at the following photos and
see if you can identify
Which type of
Weathering?