At the moment it may feel that the whole world has stopped,
and everything is on hold. But the weather is still here, still changing, and
slowly carrying out processes that make our landscapes dynamic.
High Temperature?
In Australia, one of the most common types of physical
weathering is caused by thermal expansion. During the day a rock will start to
heat up. Rocks have high specific heat capacity. This means they need a lot of
energy for their temperature to change (one reason why a house made of stone
will take longer to heat up in the summer than a house made of wood – but also
take longer to cool down once they are heated).
Rocks are not particularly good conductors of heat. This means that the
outside of the rock will heat up much more than the inside of a rock. As the
rock starts to heat up it starts to expand. The outside will expand more than
the inside, causing differential expansion.
Video
- A simple experiment you may have seen at school showing expansion of metal on heating
Feeling stressed?
The rock will cool down again at night as temperatures drop,
contracting. This oscillation between
hot and cold, expansion and contraction will create stresses on the rock,
weakening it – particularly on the outer layer where the difference is the
greatest.
Relax – exfoliate!
The outer layer of the rock will start to crack and peel
off. This is known as exfoliation. This type of weathering is often called onion
skin or spheroidal weathering. Onion skin – as the rock starts to exfoliate
layer by layer, making the rock smaller and smaller. Spheroidal – as the rocks
begin to become more spherical and rounded as the outer layers exfoliate.
A simple experiment you can do at home using just some
plasticine and a thermometer to investigate temperature differential is
described here.
Demonstrating thermal expansion
Thermal expansion can easily be demonstrated as shown in
this video.
Webpages
- Investigate thermal expansion and the gas laws with this game
(ABC Education) - Another great interactive
(University of Colorado)