Smooth, rounded rocks feel and look nice, but how did they get that
way? Natural rock-forming processes rarely lead to rounded blobs of rock. Physical
weathering is the key to rock rounding.
Erosion and physical weathering
Erosion
is the movement of weathered particles by gravity or a transporting agent
(wind, water, ice). During erosion, rocks and sediments get tumbled, causing
physical weathering. Physical weathering is the breaking of rocks by physical
forces. The greater the forces and distances, the more rounded the sediments (broken rocks) become.
We can tell how strong the force of erosion was by the mass of the
rocks it moves. Boulders are moved by glaciers and landslides on steep slopes.
Fine sediments are moved by gentle breezes or currents and will only settle to
form siltstone or shale in calm waters.
Moving water is a powerful agent of rock rounding. Rocks and sediments
collide frequently and become rounded with time. Softer minerals and rocks are
rounded first. We use this principle when tumbling and polishing rocks. Hard
silicon carbide is used to wear away at softer rocks, leaving them smooth and
shiny.
Erosion and physical weathering can transform angular rocks (like
those on the left) into rounded rocks (like those on the right).
Antarctic sand in Sydney sandstone
The Sydney Basin has thick layers of sandstone that were deposited in
an ancient river delta. Minerals in the sand have been matched with mountains
in Antarctica! When Australia and Antarctica were part of the Gondwana
supercontinent, the Antarctic mountains weathered and the resulting sediments
were transported thousands of kilometres to what is now the Sydney area. This
occurred early in the time of the dinosaurs (Triassic Period).
The river did not always flow at the same rate. Layers of large, round
pebbles indicate more turbulent, stronger river flow. Shale (made of very fine
sediment) layers within the sandstone were deposited when the rivers barely
flowed. You can find out more about the timeline of events that formed the
Sydney Basin here.
This Sydney sandstone has a layer with large, round pebbles,
indicating an episode of more turbulent river flow.
Explore rock rounding and weathering
- Try an experiment to observe rock rounding for yourself.
- This WASP experiment modelling the effect of transport on sediments uses plasticine, but you can also try this with rock (please use a strong plastic, not glass jar for this experiment).
- Explore the erosive forces of wind and water in this PALMS activity.
- You can learn more about weathering in this blog post.
- Explore the role of water and wind in erosion.
- Experiment with physical weathering as shown in this video.
- You can also try out this physical weathering activity about freezing.