K S 3 G E O G R A P H Y T E A C H E R S ’ T O O L K I T : T H E R O L E O F S T O N E S
© Geographical Association, 2014
Weathering match answers
Thermal stress – happens
when rock repeatedly warms
up and cools down with
changes in temperature. This
is most likely to happen in
deserts that are extremely hot
by day and cold by night. The
surface of the rock expands
and contracts causing stress
within the rock, making it
break up.
Freeze-thaw – happens when
water finds its way into a crack
in a rock. In very cold weather,
especially on mountains, the
water may freeze. When water
freezes it expands, like a
wedge, forcing the crack to
widen. If this happens
repeatedly the rock will
eventually split into
fragments. This is why you
often see lots of broken pieces of rock at the bottom of a
mountain.
Carbonation – happens when
rainfall absorbs carbondioxide
from the atmosphere
to form a weak acid called
carbonic acid. Sometimes, in
polluted air, the rain may
absorb other gases, such as
sulphur dioxide, to form an
even stronger acid. Carbonic
acid reacts with calcium
carbonate in limestone or
chalk to dissolve it. In this way, the rock slowly decays.
Weathering match answers
Oxidation – happens when
oxygen in the atmosphere
reacts with the minerals in a
rock to form a new chemical.
The most common example is
when iron is turned into iron
oxide (rust). As a result the
rock changes colour, becoming
browner, and begins to
crumble. This is most likely to
happen when the rock is wet.
Physical biological weathering
– happens when plant roots
find their way into the cracks
in a rock and begin to grow.
The roots force the crack to
widen and eventually cause
the rock to break apart. Small
animals, such as worms, also
help to break up the rocks.
Chemical biological
weathering – happens when
small plants called lichens – a
combination of fungi and
algae – grow on the surface of
a rock. They produce
chemicals that slowly make
the surface of the rock
disintegrate.
LESSON 7: Information Sheet 7
Photos (clockwise from left) © Joshua Tree National Park; Sharon Mollerus; Courtney; Michael McKechnie; Jason Hollinger;
Shutterstock/Jeremy Richards.