This Earthcache is the result of me being quite amazed at what I saw here. I had no idea what it was, but with some help from buhfuhguh I gradually understoof what it was and how it formed. So now I’m going to attempt to pass that knowledge on to you….
Let me introduce you to some big words.
“Phreatomagmatic eruption” is the ridiculously complex word that describes a volcanic eruption that occurs when magma and water interact. This is the type of volcanic eruption that occurred around 35,000 years ago here at Tower Hill. At this time, molten magma pushed its way up and met with a layer of water bearing rock. This created an extremely violent explosion resulting in the large, shallow crater that is Tower Hill (http://towerhill.org.au/index.php/about-reserve/history).
“Tephra” is the material produced by a volcanic eruption. It can consist of very fine particles such as ash and cinders, through to larger fragments known as bombs or blocks. The tephra can be distributed over quite some distance, but when it eventually falls to the ground, if it is hot enough, it fuses together into something called “tuff”.
“Tuff” therefore is a type of rock made from volcanic ash after it falls following a volcanic explosion. Interestingly, tuff can be described as both igneous and sedimentary in nature (depending who is describing it) and because of it is a relatively soft rock, it has been used extensively in construction since ancient times.
The volcanic explosion that formed Tower Hill occurred roughly 35,000 years ago. Seems a long time ago, but remember, Indigenous Australians have been in the area longer than that and the 1947 discovery of an axe underneath the tuff layer proves that humans were around when this violent explosion happened. Indeed, local Aboriginal oral traditions reference volcanic activity (https://www.abc.net.au/news/2020-02-26/study-dates-victorian-volcano-that-buried-a-human-made-axe/11991290#).
Here’s a little diagram that maybe might explain it better than words….
So, what has all that got to do with the position that you are standing in for this Earthcache?
Well, if you hadn’t noticed, in the sedimentary rock nearby, there is this distinctive wide layer of black amongst the usual grey and sandy coloured layers. This is the tuff layer resulting from a large volcanic explosion.
Have a go at answering these questions. Send me your answers and feel free to log the cache straight away. If you get the answers hideously wrong, I might ask you to resubmit, but if you’ve been to the posted location, the answers shouldn’t be to tuff (see what I did there?)
- Question 1 – Would you describe the layer of tuff rock as igneous, sedimentary or metamorphic. Why do you believe this to be the case?
- Question 2 – How high (measure) is the tuff layer?
- Question 3 – Examine the tuff layer. Using a sports ball analogy, how big would you say are the largest blocks in the layer (ie marble sized, golf ball sized, tennis ball sized, basketball sized)?
- Question 4 – On a scale of 1 to 10, where 1 is “not at all” and 10 is “extremely”, if you were standing at this spot 35,000 years ago or so when the nearby volcano erupted, how alarmed do you think you would be?
- Question 5 – (Not really a question – more a logging task) - Either take a photo of yourself with the sedimentary rocks in the back ground complete with the expression on your face that matches the level of alarm you indicated in question 4, or take a close up photo of the tuff layer, showing the largest blocks you can find (you don’t need to be in the close up photo)..
Please send me your answers to these questions (including the required photo) either before you log your find, or within a reasonable time after (minutes, hours and days are acceptable – weeks maybe at a push – months and years not acceptable). That way I don’t have to delete your log.
Hope you enjoy your time at Tower Hill as much as I did.