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.
Source: Diamond Hill Kurrajong
Earthcache requirements:
In order to log this earthcache you must answer the following questions. You do not have to wait for a reply before logging your find. I attempt to reply to all e-mails submitting answers and, if there are any problems I will contact you to obtain more information.
Stage 1 (posted coordinates S33 31.781 E150 40.237) will put you approximately in the middle of the diatreme. Answer the following questions:
- Describe the location where you are standing
- What is the colour and granularity of the soil
Stage 2 (S33 31.771 E150 40.247) is approximately 25 metres away and will put you on the lip of the diatreme. Answer the following questions:
- Describe (colour / approximate size) of the rocks at this location.
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