Latitude -33.529831, Longitude 150.671343
Located in the Sydney Surrounds Region of NSW, nearest town Kurrajong
In the Age of the Dinosaurs (around 200 million years ago) over 90 volcanoes erupted in the Sydney Basin. The molten rock and volcanic gases exploded through the numerous layers of coal, sandstone and shale underneath the ground.
One of these volcanoes was at Kurrajong, NSW. Known today as 'Diamond Hill', this low grassy hilltop is only three kilometres from Kurrajong, between Comleroy Road and the mountains on Diamond Hill Drive just pass Asplin Place.
The Diamond Hill volcano is of a type called 'diatreme'. Molten rock forced its way up from deep underground and hit a swampy area. The 1000°C molten rock superheated the water in the swamp, resulting in a massive explosion that blasted out a wide cone of the surrounding sandstone and shale rock. The fragmented rock eventually compacted back down into the crater, forming a rock called 'breccia'. Later more molten rock forced its way up through the breccia, filling part of the crater. This flow of molten rock cooled into a hard black rock called 'basalt'.
Today, all that can be seen of the volcanic diatreme at Diamond Hill is a low grassy hill: the top part of the volcano has been completely eroded away. However, under the ground is a 300 m wide cone-shaped plug of breccia and basalt rock - the remains of Kurrajong's volcano.
As well as producing deposits of breccia and basalt, volcanic diatremes may also bring diamonds and gemstones to the surface from deep underground. Most diamonds were formed about 1 - 3 billion years ago, under extreme pressure about 200 km under the ground. They sometimes get caught up in the volcanic material that is blasted out of a diatreme.
A diatreme is a breccia-filled volcanic pipe that is formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled, relatively shallow crater known as a maar, or other volcanic pipes. Diatremes are sometimes associated with deposition of economically significant mineral deposits.
The best place to view Diamond Hill is from Linksview Place, off Peel Parade, off Comleroy Road at Kurrajong. On the opposite side of the valley you will observe Diamond Hill Drive curves down and around the grassy slope that is Diamond Hill.