Story: Limestone country
Page 3 – Dissolving rock
The rate at which limestone dissolves depends on the amount of rainfall and the concentration of carbon dioxide in the water.
Carbon dioxide concentration
In the open atmosphere, carbon dioxide has a concentration of about 0.03% by volume. However, in gaps in the soil, concentrations are often 2%, and can even reach 10%. As rainwater runs through soil, its carbon dioxide content increases. A hundredfold increase in carbon dioxide concentration means that limestone will dissolve about five times as fast.
In terms of rates of weathering, the amount of rainfall is even more significant than carbon dioxide concentration. The wettest places in the world have the fastest rate of karstification (formation of limestone caves and other features). As New Zealand is relatively wet, there is plenty of water to dissolve karst rocks.
- At Waitomo (average annual rainfall of 2,350 millimetres), the limestone dissolves at an annual rate of about 70 cubic metres per square kilometre of outcrop.
- On Tākaka Hill (average annual rainfall about 2,160 millimetres), the marble dissolves at an annual rate approaching 100 cubic metres per square kilometre of outcrop.
These rates are moderately high by world standards.
The top 10 metres
Most dissolving of limestone happens just beneath the soil. This is where carbon dioxide is generated by soil microbes, so percolating water has its highest level of carbon dioxide. Some 90% of dissolving can occur in the top 10 metres or so of the limestone outcrop. The heavily corroded rock layer beneath the soil is known as the epikarst.
Fissures are widest near the surface, and taper with depth, usually reaching about 10 metres. Rainwater drains in much more easily than it drains out, and after heavy rain, water accumulates at the base of the epikarst. This water can remain there for months, eroding the rock.
Karstification makes limestone both more porous and permeable, so it has a great capacity to store groundwater and to let water flow through. Porosity is a measure of the gaps (pores and cracks) in the rock; permeability measures the ease with which water can flow through these gaps.
There are three types of porosity.
- Primary porosity results from the original formation of rocks.
- Secondary porosity refers to fractures and cracks that develop later.
- Tertiary porosity refers to cracks enlarged by water dissolving the rock.
Porosity is highest in the youngest rocks, including the Pliocene shelly limestones of Hawke’s Bay and the Wairarapa (5.3–1.81 million years old). In the older, widespread Miocene–Oligocene limestones (32–22 million years old), the rock’s primary porosity is usually less than 2%. In the marble country of north-west Nelson, it is almost zero.
High secondary and tertiary porosity is found in the Miocene–Oligocene limestones that contain plenty of caves. Tertiary porosity in marble, where water enlarges fractures into caves, helps form aquifers (underground layers of water-soaked rock). The classic example is aquifer-fed Waikoropupū Springs in the Tākaka valley, New Zealand’s largest freshwater spring.