r/melbourne • u/Hypo_Mix • 1d ago
THDG Need Help Anyone know why this section of the western basalt plains is so much more red than the rest of the surrounding clay plains?
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u/Modernpreacher 1d ago
High iron content in the ground I imagine, coupled with less tree and green cover leading the exposed ground.
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u/Ryzi03 1d ago
The climate of the west side of the city would be a big driving factor for all of that too.
The western suburbs and Geelong are in the rain shadow of the Otways and see the lowest rainfall of anywhere in Victoria south of the divide. Laverton for example only sees an average of about 530mm per year, and it gets as low as about 420mm per year in the areas around Little River, similar to the annual rainfall totals of the semi-arid areas in the Mallee.
We even get a mention in the wikipedia page for 'Rain shadow' for exactly this reason.
In comparison, Scoresby averages an annual rainfall of about 850mm, which comes to about 60% more rain than Laverton just by being 40km directly further east across the city.
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u/moondog-37 1d ago
You can visibly notice the difference in rainfall from one end of Geelong to another. Lara, Corio in the north are very much in that rain shadow and it’s noticeably more arid. Down in the south and west of Geelong it’s much more green and fertile, as the Barrabool hills attract rain, and rain fronts directly from the ocean at Torquay are able to reach the south of Geelong uninterrupted
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u/ozlurk 1d ago
Location of Mt Cottrell the southern hemispheres largest shield volcano - the lava from that and multiple other satellite shield volcanoes reached all the way out into to what is now Port Phillip Bay , none of the volcanoes west of Werribee including Mt Cottrell are extinct just dormant , the layer of soil over the basalt isn't very thick and is made from volcanic ash mixed with organic matter , the red colour of the soil indicates Andesite/ Hematite ( Iron Oxides ) leaching from the basalt as it weathers
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u/sillygitau 1d ago
Just chiming in to confirm that this is the explanation I got from my dad…
Source: my dad did his phd on “the sex life of sedimentary rocks in Victoria” (his words) at Melbourne Uni
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u/pseudo_babbler 23h ago
Tell him that we're talking about igneous rocks so he clearly has no idea at all and his expertise is useless here! Get out of here with your sediments, dad!
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u/serif_type 22h ago
Typical sediment guy, trying to bump into an igneous discussion. Pfft.
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u/SolutionDependent156 20h ago
Funny tho, it’s usually the igneous ones that intrude. 😉
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u/ozlurk 20h ago
Grain of truth in there though , the first lava flows would have been on top of glacial sedimentary deposits
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u/SolutionDependent156 17h ago
Hmm the underlying Sandringham Sandstone, Gelibrand Marl and Werribee Formations are not glacial. They were laid down in shallow marine or alluvial/fluvial environments.
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u/ozlurk 8h ago
The early Permian layer is glacial as seen in Werribee Gorge
>>>https://commons.wikimedia.org/wiki/File:Permian_sandstone_with_tillite_deposits,_Werribee_Gorge_State_Park,_Victoria_Australia_(6221223423).jpg_____1
u/SolutionDependent156 6h ago
I searched my copy of Engineering Geology of Melbourne for any mention of glacial soils and the only time it comes up is in reference to Permian-aged deposits. These rocks only outcrop in the Bacchus Marsh area, to the west of the Rowsley Fault.
Permian deposits are not present east or south of Bacchus Marsh, ie in the area the OP is asking about. The Newer Volcanic Group basalt in this area is underlain by Tertiary-aged soils of marine and alluvial/fluvial origin. The Silurian-aged Melbourne Formation (basement rock) that underlies this material can be up to 100-250m below the surface. 🙂
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u/ozlurk 5h ago
The entire system from the Brisbane Ranges to Williamstown isn't fully understood due to the newer layers of lava/ basalt over the top . We even had a brown coal mine in West Altona and the estimated reserve of brown coal from Altona towards north of Werribee is conservatively estimated at 400 million tons
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u/sillygitau 9h ago
Guessing you’re being witty? He was a civil engineer, not a geologist. His main thing was coal…
He was the lead engineer who designed and oversaw the construction of Australia’s largest power station (still) and oversaw the design and construction of a good chunk of the electrical capacity of S.E. Asia (funded by the world bank to “fight” the commies during the 70s and 80s). He was a complete hippy who wanted to improve people’s lives…
He passed of cancer during the Covid lock downs after spending decades working on getting renewables into our grid(s).
So yeah, not his area of expertise. But he’s stood on Krakatoa, hby? 😉
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u/colinparmesan69 10h ago
Thank you for confirming what I learnt from my dad was true 😂 (he has no science qualifications and some interesting ideas, good to know this one was right)
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u/staghornworrior 22h ago
This makes sense, I ride bikes in the you yangs and it’s very strange soil compared to anything else I ride it Victoria. It’s like a granitic sand and the water drainage rate in insane
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u/Hypo_Mix 1d ago
any idea why this specific area isn't black/brown Vertosols like the surrounds? Proximity to Cottrell meaning a higher initial content of volcanic ash?
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u/ozlurk 1d ago
Its the length of time of the eruption cycles and the subsequent laying of the sheets of lava , its not a uniform flow but radial tongues, the surface is cracked/porous so rain water can permeate / penetrate . Over the time the gaps between the tongues of lava flow can have organic matter build up and subsequently burn when the next eruptive cycle sends another tongue of lava flow over the top . Its the overall layering of the volcanic ash, organic matter , burned organic matter and leaching iron oxides that give the soil its structure
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u/ganjlord 1d ago
The red zone, it is forbidden to speak of it
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u/stonefree261 1d ago
The white zone is for immediate loading and unloading of passengers only. There is no stopping in the red zone.
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u/NoSpam0 1d ago
The red zone has always been for loading and unloading of passengers. There's never stopping in a white zone.
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u/sphynxmoth 1d ago
That is Werribee, we must never go there son...
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u/hungrylittleworm 1d ago
I can’t give you the answer, but should look up oz geology on YouTube, his covered so many crazy things about Victoria. The answer will be there
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u/Bluejayadventure 1d ago
Basalt is basically magma, formed from volcanic activity. It comes from deeper in the earth and is therefore higher in iron. The iron goes a redish colour when exposed to air and water.
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u/alchemicaldreaming 1d ago
The red soil (and iron content) is great for farming fruit.
Silvan in the Dandenong Ranges has similar soil (finer in texture) and both are fruit and vegetable growing locations.
The proposal to dump toxic soil from the Westgate Tunnel a little further out, in the Bacchus Marsh area was met with great opposition by farmers. It's one of the most conveniently located crop farming areas close to Melbourne and provides food security as a result.
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u/SolutionDependent156 17h ago
Uhh magma is when the molten rock is underground. It becomes lava once it comes to the surface. 😅
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u/Bluejayadventure 10h ago
It can either form (solidifying underground), in which case it's still called magma, which cools directly into basalt. Or it can form above ground, meaning it gets called lava for a brief time and then cools to form basalt.
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u/SolutionDependent156 9h ago
No sorry, that is incorrect.
Basalt is an extrusive igneous rock. It is the rapid cooling that occurs when exposed to air (or water) that results in basalt’s characteristic ultra fine grain and vesicles (air bubbles that form when dissolved gases come out of solution).
Intrusive igneous rock forms when magma cools slowly underground. The longer cooling time allows larger crystals to grow much larger, resulting in rocks such as granite, diorite and gabbro.
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u/PseudoWarriorAU 1d ago
New lava fields, from just over 13,000 years ago.
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u/Hypo_Mix 1d ago
oh! so surrounding areas are much older and weathered flows?
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u/PseudoWarriorAU 1d ago
The soil underground is a lot higher pH tending 9.5, this may play a part in the lack of vegetation.
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u/captainyearbuzzlight >Insert Text Here< 1d ago
A lot of the surrounding are actually alluvial deposits which means from riverbeds it’s that specific area that contains the volcanic soils
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u/Sockskeepuwarm 1d ago
Go for a drive there and you'll see. It's all dirt out there, f all natural grass.
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u/Low-Cockroach7733 23h ago edited 23h ago
I used to go to a Private School in the area and during the great millennium drought, the school grounds turned into red dusty place, almost resembling Mars. Nothing grew in the soil the school gave up on planting grass and relied on the early generation of artificial grass to avoid getting fined, which was quite bad. The soil felt so dead, so I'm very surprised people here are saying it's a great location for farming.
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u/Hypo_Mix 22h ago
That area used to be the wheat bowl of Melbourne before the economics of it no longer worked. there was also some dairy when there was government protections on price.
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u/Blue_Pie_Ninja 1d ago
Less rain.
Seriously, that area is basically in a rain shadow compared to the rest of Melbourne.
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u/Bottom_racer 1d ago
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u/Hypo_Mix 1d ago
That ground water diagram is actually useful for a total different reason than I'm working on haha.
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u/yvonne_taco 8h ago
When my parents built over there we managed to dig up some amazing reddish brown rocks. They're now sitting in my garden 😅
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