Diatremes are relatively rare and most unusual volcanic structures. They are only found where hot magma has risen into horizontal sedimentary layers that contain groundwater. Vertical jointing is a likely prerequisite as well, as the famous diatreme areas in Germany, Australia and South Africa all have this common pattern.
For diatremes to form, magma must enter horizontal sedimentary layers, encountering water. Diatremes develop as hot magma flashes groundwater to steam, and the steam finds a way through the joint system to the surface. Once the steam begins to escape at the surface, the temperature along the escape path progressively increases. Accelerated weathering occurs along the narrow vent path, breaking down the country rock, allowing particles to move upward, increasing the cross-sectional area of the vent. The vent path may have started as an irregular alignment of joints, poorly connected between superimposed sediment layers. However, as the steam flow increases, abrasion increases the size of the vent, and straightens out the kinks.
Rock fragments are stripped from the side of the vent, and rattle upwards to escape at the surface. The movement of such rock through the vent abrades the sides, progressively increasing its diameter, and shaping the mouth of the vent like that of a trombone. Steam now roars out of the vent, and rocks are flung high into the air accompanied by an extremely loud noise.
The shape of the vent mouth determines how much can escape from the pipe. Much of the material that is ripped from the sides and flung upwards will leave the vent moving vertically only to fall back into the vent again. Some, however, will fall just outside the vent, creating a ring of debris called a maar ring. What falls back into the vent is then flung out again and again.
A very turbulent situation develops with a vast amount of material being recycled in and out of the vent. This would be a fearful thing to witness, but even worse to hear. In addition to bedrock debris being ejected from the vent, magma is being introduced at the bottom of the vent, and is mixed in with the rest.
The power and the life of a diatreme depend upon the temperature of the magma and on the water supply. The water supply is probably the limiting factor, being controlled by the permeability of the surrounding bedrock. When the diatreme starts to form, the nearby sandstone is saturated with water, providing a considerable local reservoir. As the process proceeds, this local water is consumed. Additional water arrives only after travelling an increasing distance through the sandstone of restricted permeability.
When the water supply diminishes to the point where the system begins to dry out, the steam pressure drops and the circulating debris in the diatreme vent starts to settle down to fill the bottom of the vent. At the same time molten magma may enter the bottom of the vent without encountering water, but coming into contact with broken up (brecciated) diatreme material. The magma will intrude into the accumulating debris in the bottom of the hole and solidify, plugging the bottom of the vent. This marks the end of the diatreme as an active entity. Water will slowly seep into the half-sediment-filled diatreme and turn it into a maar lake. The sediment that settles into the bottom of the diatreme will progressively compact, more in the middle than at the edges, producing the typical basin shape of diatreme fill.
Fig. 2 in the Geology, topography and soils section in this chapter of the Guide shows a geological interpretation of the internal layering in a hypothetical diatreme. It was made by combining observations made at several diatremes. The Hornsby diatreme shows features in the upper part of the cross-section.
Some 25 diatremes are shown on the Sydney 1:100,000 geological map, which covers the eastern part of the Sydney Basin from about Parramatta in the East, Botany Bay in the South and Northern Broken Bay in the North. The diatremes range in area from less than 1 ha to 44 ha in the case of the Hornsby Diatreme. They are spread from Lugarno in the South to Patonga Creek in the North. Most of the diatremes are located in the bottom of valleys where they are poorly exposed and deeply weathered. Quite a few of them have been covered by urban development or playing fields.
C Herbert 1983 Igneous Rocks in ‘Geology of the Sydney 1:100000 Sheet 9130″ Geological Survey of NSW Mineral Resources of NSW