Mount Basalt Reserve is home to two peaks composed of basalt. At the posted coordinates, you will find evidence of basalt dykes formed by the intrusion of magma into cracks caused by the rupturing of the Earth's crust including the Walloon Coal Measures. This occurred between 17 and 45 million years ago during the Tertiary period.
At that time, the heat of the magma caused groundwater to boil and form steam. This steam expanded and broke the rocks producing volcanic tuffs. When the magma cooled and solidifed, the basalt would have been over 1000°C. Contraction on cooling produced fractures in the basalt known as columnar jointing. Erosion of the surrounding rocks over the following millenia undercut the fresh basalt in the dykes causing if to fall to form slopes of loose blocks of basalt.
The basalt contains large dark crystals giving the basalt a characteristic mottled appearance. The large crystals are pyroxenes (magnesium and iron silicate) which were rafted up by the rising basalt magma from the Earth's mantle.
To log this earthcache, go to the posted coordinates which are at the top of the second peak. There you will find evidence of the basalt intrusion. From your observations, answer the following questions:
- Describe the basalt that you see with particular reference to size, shape, colour, texture and grain size.
- On what orientation are the basalt columns? Why do you think this is?
- In your message, include a photo of yourself with the basalt columns at the posted coordinates. DO NOT post any spoiler photos in your found log.
Please send a message with your answers. DO NOT put the answers in your found log or post any photos with spoilers.
You are free to log the earthcache without waiting for a response. I will contact you if there are any problems with your answers.