Our aim with this particular Earth Cache is to take you to an interesting spot and teach you some basics, we have presented this earth science lesson on a basic level to accommodate those who avoid EC’s ‘cos they are too hard’ and also to accommodate those travelling with Children (TAWKers) who incorporate Earth caches into their curriculum. Although there is considerable EC notes the questions are basic.
A big thank you to Stephanie Sykora, PhD Consultant Exploration Geologist who allowed us to use her work to put this EC together.
The granite on Freycinet Peninsula is Devonian in age, and is a far contrast to the other main geological units that comprise most of Tasmania (i.e., the young, prominent Jurassic dolerite and the old, deformed Precambrian quartzite).
In geology there are (roughly) two major types of granites: I-type and S-type. They are classified this way in regard to their molten source being either igneous (I) or sedimentary (S).
The I-type granites are enriched in sodium and calcium and have the mineral hornblende.
S-type granites are depleted in sodium but enriched in aluminium, they typically have the minerals muscovite, biotite, corundum, and garnet.
On the Freycinet Peninsula, granites are mostly S-type, and they range from equigranular to porphyritic, with large K-feldspar crystals (Groves, 1967). In fact, the K-feldspar-rich composition of the granites is what gives them their characteristic red appearance, which appears to be amplified when the sunset light hits Mt. Amos and the Hazards mountains.
Granites here are also more radioactive (!!) than other granites in Australia, due to significant amount of Uranium and Thorium.
Weathering is the natural process of breaking down materials. Weathering processes fall in two categories: mechanical weathering and chemical weathering. Weathering processes are essential components of erosion and deposition of sediments.
• Mechanical weathering includes all processes that simply "break big pieces into little pieces."
• Chemical weathering are processes that gradually dissolve or disassemble rocks through electrochemical activity.
In nature, the two types of weathering processes may occur simultaneously, but not always at the same rates.
• Erosion is simply moving material in some fluid (such as air, water, ice).
• Deposition involves processes that stop the surface transport of materials. This is followed by burial or further erosion.
Mechanical weathering is typically associated with processes at or near the surface and may take place with processes associated with erosion. However, rocks are also broken in the subsurface by tectonic forces, such as by fracturing and shattering of rocks from earthquakes, asteroid impacts, volcanic activity, and gravitational forces. Erosion involves all-natural processes that move earth materials, including movement by a fluid in motion (wind, water or ice), downslope movement by the force of gravity, or by biological activity. Many factors influence weathering processes, including bedrock characteristics, climate and precipitation, plant cover, and other landscape characteristics. Examples of mechanical processes include:
• Tectonics - Breaking and Moving Rocks, (jointing, sheeting from release of pressure).
• Freeze and Thaw - Ice formation, daily to seasonally, can fracture and move rocks.
• Heating and Cooling - surficial expansion can cause materials to expand and contract, fracturing them.
• Wetting and Drying - Wetting and drying causes clay minerals to expand and contract, and salts may dissolve and re precipitate.
• Biological Activity
• Mass Wasting (force of gravity pulling earth materials downhill).
Chemical Weathering. Chemical breakdown of earth materials takes place mostly below the lands surface involving interactions of rock, water, and biological agents. Soil that covers the bedrock across the land surface is a residue of weathering processes (both chemical & mechanical). Water is a universal solution, and nearly all earth materials can be chemically altered, dissolved, transported, and deposited by it depending on conditions. The content and volume of materials dissolved or suspended in water can have effects on pH (acid/base), eH (available oxygen), and chemical reactivity of the water and its dissolved or suspend load. Likewise, climatic factor strongly influences the rate of chemical weathering. Chemical weathering rates change with changing weather conditions, seasonally, and over long period climate cycles associated with continental glaciation cycles and other forces driving climate change.
Chemical weathering of bedrock follows fractures and boundaries of materials of composition and hardness. Reactive minerals, such as many salts or carbonate minerals will dissolve in water. iron-bearing silicates common in most rocks and are prone to alteration into clays, iron oxide and hydroxide minerals (like rust on an old car). Only hard and stable minerals, like quartz, and physically isolated materials (such as material in the core of a large boulder) survive long periods. Rock material that loses cohesion by exposure to weathering will break down to form a regolith. At the surface, biological action and ongoing weathering convert regolith to soil and exposure to erosional forces.
To log this Earth Cache, we require you to visit the listed waypoints to make some observations, consider the information given, then message us with the following answers to the best of your ability;
1. At the posted Coordinates there is a boulder who looks like a pop culture icon, explain how you think the boulder has become this shape?
2. At the second waypoint explain if you can see any other granite boulders weathering in the same way?
3. A photo of your team or GPS near the iconic shaped rock, upload as many photos as you like!
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil this Earth Cache requirement will have their logs deleted without notice.
Source: Wikipedia, geologycafe.com, Stephanie Sykora PhD, Consultant Exploration Geologist. Parks Tasmania