Location
Bondi–Maroubra coast, Sydney NSW
Rock
Hawkesbury Sandstone
Age
Middle Triassic · ~240 Ma
Process
Cavernous (salt) weathering
Status
Public coastal walk
Overview
Along Sydney's eastern cliffs the sandstone is riddled with rounded hollows, some smaller than a thumbnail, some large enough to shelter in. These are tafoni: cavities hollowed from the inside out.
<1 cmto >1 m
Size range of tafoni cavities
1882
First published use of the term
100–300kg/m²/yr
Summer evaporation measured from tafoni walls
Chapter 01
Since the 1970s salt weathering has been the leading explanation. Sea spray wets the rock; as it dries, salt crystals grow in the pores and force the grains apart. Ann Young showed salt acting as an agent of cavernous weathering in Sydney's sandstone in 1987.
Chapter 02
Near Sydney, acidic water infiltrating the rock dissolves the iron-oxide cement that binds the quartz grains, releasing them from the inside. A harder surface crust often survives as a lip while the softened interior is removed.
Chapter 03
Recent monitoring of evolving tafoni found evaporation of 100–300 kg/m² per year in summer but under 15 kg/m² in winter. Cavities deepen where moisture lingers and salt repeatedly crystallises, so each hollow helps to shelter the next.
Chapter 04
Tafoni grow by feedback. Once a hollow exists it shades its own interior, traps moisture and collects salt, so it weathers faster than the open face around it. The deeper it gets, the better it shelters, and the faster it grows.
Chapter 05
Look closely inside a large tafone and its walls are pitted with smaller ones, which are pitted in turn. The same process repeats at every scale, which is why the cliff reads like a honeycomb cut into a honeycomb.
How it forms
Onshore wind throws sea spray at the cliff. Salty water soaks into the pores between quartz grains.
As the face dries, salt crystallises inside the pores. Each crystal pushes on the grains around it.
A shallow pit forms. It stays shaded and damp for longer than the rock around it, so more salt cycles happen there.
The pit deepens inward behind a harder crust. New, smaller pits start on its walls: rooms within rooms.
The cliff is not being worn away from outside. It is hollowing itself from within.
The question
Because weathering is uneven from the start. Exposed faces dry quickly and form a hardened crust; sheltered patches stay wet, keep cycling salt and lose grains. Small differences in moisture become large differences in shape.
Cavernous (salt) weathering
Deep time
~240 Ma
River sands
Vast braided rivers lay down the sands of the Hawkesbury Sandstone during the Middle Triassic.
Millions of years
Buried and raised
The sand is buried and cemented into rock, then uplifted and exposed by erosion along the coast.
~7,000 yrs
The sea arrives
Sea level stabilises near today's line. Salt spray begins its work on the new cliff faces.
Today
Still hollowing
Tafoni keep growing, grain by grain, fastest where they are most sheltered.
Field record
Field data
IGMA simulation · move to heat
Rock Lab
Turn a 3D model of Tafoni with hand gestures, heat its surface with your fingertip and weather it through 250,000 years.
Enter the Rock LabReferences
Photography
Next case · 02
Wedding Cake Rock and the white cliffs