Reading Rocks: Rounded Rocks

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.