🌏 Rotten Rocks 🌏 EarthCache

Logging Requirements

THE QUESTIONS:
 

1.) Looking at the rocks, describe the surface you see, colours, crystal size, is the rock fine, medium or coarse grained, what does the surface of the rock feel like? 
 

2.) Look to the ground, describe the type of "soil" you see? What do you believe is the predominant mineral in this?

3.) What role does water play in destabilizing and destroying the feldspar in the rock? What does the feldspar become?

4.) Then post a photo of you at the location in your log. Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence.
 

You may log the cache as soon as you submit your answers to us via messenger. 

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice.
Sending the answers is a requirement not a request.    
Please note, answering the logging tasks is based on your observations at the site and using your understanding of content of the cache page, there’s no need to do any further research.

THE LESSON

Welcome to Nangara Reserve. Adjacent to GZ (just down the path) was a commercial quarry, supplying granitic gravel to the Shire’s of Buln Buln and Baw Baw from the early 1980’s until 1998. It was mostly used for road base. Granite gravel (often used as decomposed granite or crusher run) is considered a premium road base because it blends crushed, angular rocks with fine stone dust. When compacted, this mixture creates a long-lasting, stable, rock-solid foundation. Since this area finished providing commercially to the development of this corner of Gippsland a committed group of locals along with the council have spent hundreds of hours attempting to return the land to its natural state for the benefit of the local flora and fauna and the community.

This part of West Gippsland features a diverse geology defined by ancient Devonian-age granite and granodiorite bedrock overlain by Tertiary-era basalt flows. Underlying much of the Jindivik and nearby Neerim South areas is deeply weathered intrusive granitic rocks that form the rolling to steep topography characteristic of the deep valleys and elevated forested areas around the region. The granitic rocks of the reserve were formed around 350 million years ago when granitic magma cooled slowly at depths of two to five kilometres within the surrounding sedimentary rocks. Cooling slowly gives crystals within the rock time to grow and become larger. The rocks here have been exposed by erosion as well as the quarry. These granite tors are displaying signs of deep weathering which results in the rock decaying and crumbling to sandy gravels and thick kaolin clay in places.

When you look at this granite you will see it has a Phaneritic texture. This means the rock is made up of crystals large enough to be distinguished with the unaided human eye. It has larger crystals (commonly feldspar) embedded in a finer-grained crystalline matrix. This particular granite appears to be composed of plagioclase feldspar, quartz, mica, biotite, hornblende and possibly other minerals.

You will also notice the rock surface has a decaying appearance, it is powdery to touch and the harder crystals stand proud from the surrounding matrix. While looking at one of these boulders the Chief wiggled a lifted piece rock about hand sized and the whole slab, about a centimetre thick just fell off the surface of the rock….you can probably find it on the ground. This piece will eventually disintegrating becoming the coarse gravelly quartz based sand you can see all over the ground.

Does this “sand” look familiar? Decomposed granite, known to geologists as Grus, is a familiar sight underfoot on many walking paths. Grus is the result of granular disintegration (or grusification) of granite due to the hydration of feldspar and biotite minerals which causes those minerals to break down into clays. Some pronounce this word "gruss" as in "Russ", while others pronounce it "grew-ss" as in "truce".

Image: decomposed granite is known as "Grus"

Wait! Granite is usually so hard and resilient! Why is the rock going rotten?

Granite goes "rotten" due to weathering, where moisture and chemicals break down its structural minerals—especially feldspar. Exposure to the elements over millions of years causes some of the minerals in the stone to chemically alter into soft clay, leading to crumbling, staining, and a dull, chalky surface. Its breakdown is controlled by a combination of physical, chemical, and biological weathering processes, and different minerals within the rock decay at different rates.

Climatic variations influence the weathering rate of granites. Dry environments slows decomposition of granite; a great example is “Cleopatra’s Needle” obelisk. The carved relief engravings on the red granite obelisk had survived the arid conditions of Egypt for 2000 years before its transfer to London. Within two hundred years, the granite has drastically deteriorated in the damp and polluted air there and the carvings are becoming increasingly eroded. 
 

Image: Cleopatra's Needle, London.

Feldspars also weather more slowly in cool dry climates, but in climates that are humid (whether cool or warm), the weathering of feldspar within the rock is accelerated.

Bush fires can also contribute to the weathering of granite. The high temperatures reached during a fire—often exceeding 1000 °C—can cause significant physical and chemical processes that alter the rock.

Whatever the reason behind granite becoming more vulnerable, once it becomes micro-fractured, its ability to absorb water increases, and its mechanical strength is significantly reduced. Hence, the decomposition process is underway.

HOW DOES IT HAPPEN:

A typical weathering sequence goes something like this....We start with fresh porphyritic granite, then….

1. Water enters joints and fractures.
2. Feldspar begins hydrolysis.
3. Quartz persists
4. Dark minerals oxidize.
5. Rock weakens internally; exfoliation and cracking accelerate.
6. Corestones develop.
7. Granite converts to saprolite.
8. Saprolite erodes away. Quartz sand and clay remain.

1. Initial Weaknesses: Cracks and Joints: Even fresh granite contains fractures, joints, and microscopic cracks formed during cooling and later tectonic stresses. Water enters these openings and begins the weathering process. As weathering progresses cracks widen, blocks become separated from the parent rock and the surface areas exposed to weathering increase.  This is the first stage in granite decay.

2. Chemical weathering causes Breakdown of Feldspar:  The large feldspar crystals in this granite formed deep underground at high temperatures and pressures, when exposed on the surface it becomes unstable and vulnerable. Feldspar is a silicate mineral composed of a rigid lattice of silica and aluminum, bonded with elements like potassium, sodium & calcium. Rainwater absorbs carbon dioxide from the atmosphere forming a weak carbonic acid. This reacts with feldspar through hydrolysis, causing the chemical composition of the feldspar to change. The hydrogen ions in the water actively "attack" the feldspar, replacing the potassium, sodium, or calcium ions. The displaced elements dissolve into the water and are washed away (via a process called leaching). The remaining aluminum, silica, and water molecules recombine into a new sheet-like crystalline structure—transforming it into clay such as kaolinite. Ultimately the rock loses strength and cohesion. Because feldspar often makes up 50–70% of granite, this process is a major driver of granite decomposition.

3. Quartz Persistence: Quartz is much more resistant than feldspar. As feldspar alters to clay, Quartz grains remain relatively intact. As the rock decomposes it becomes into a loose mixture of gravelly soil composed of sand-sized quartz grains and clay, known as Grus. 

4. Biotite and Hornblende Alteration: Dark minerals such as Biotite and Hornblende weather faster than quartz. These minerals may rust (oxidise), absorb water (hydrate) or break down into iron oxides and clay minerals.This often gives weathered granite yellow, red, or brown staining.

5. Physical Weathering: Chemical weathering weakens the rock, while physical weathering breaks it apart.

*Exfoliation (Sheeting): Granite forms deep underground under high pressure. When overlying rock is eroded away that pressure is released. The granite expands slightly and develops curved fractures parallel to the surface. Large sheets peel off, producing rounded edges.

*Freeze–Thaw: In winter when temperatures fluctuate around freezing: water enters cracks, freezes and expands by about 9%. This results in the cracks widening. Repeated freeze-thaw cycles eventually detach whole blocks.

*Thermal Expansion: In the heat of summer minerals expand during the day and contract at night. This process put pressure on the rock because different minerals expand at different rates. Stresses develop that contribute to fracturing and cracking.

6. Formation of Corestones: Weathering attacks corners and edges of blocks more rapidly because those areas are exposed on multiple sides. A roughly cubic block gradually becomes rounded in this order: Corners weather first; Edges weather next; Faces weather last. The result is rounded boulders called corestones surrounded by weathered material. This process is known as spheroidal weathering.

7. Development of Saprolite: As weathering continues, the granite may retain its original structure but become thoroughly decomposed. The resulting material is called Saprolite.

Characteristics of Saprolite:  the original texture still visible; the Feldspar minerals have chemically altered to clay. The rock surface is soft, it can often be scraped or dug with a shovel. The strength & integrity of the rocks is dramatically reduced.

8. Final Products: After extensive weathering, porphyritic granite is transformed into:

• Quartz-rich sand.
• Clay minerals (kaolinite, illite, etc.).
• Dissolved silica.
• Dissolved potassium, sodium, and calcium ions.
• Iron oxides and hydroxides.

At GZ you can see evidence that the granite rarely "crumbles" all at once.
Instead, the feldspars chemically rot into clay while quartz largely survives. The physical processes then separate the weakened rock into blocks, boulders, sand, and soil over thousands to millions of years depending on climate and exposure.

Resources: 

https://www.machinerypartner.com/blog/understanding-crushed-stone-grades-a-complete-guide

https://en.wikipedia.org/wiki/Granodiorite

https://en.wikipedia.org/wiki/Decomposed_granite

https://research.fs.usda.gov/download/treesearch/7754.pdf