At GZ you should be facing the large boulder with the odd patterning on it….CBD in the background….
THE QUESTIONS:
1.) Examine the rock at GZ. Describe what you see, include colour, general grain texture. Has the surface texture become powdery? Is it granular? Is it loose?
2.) Go to the other side of the rock (S37° 39.230 E144° 52.431) Compare how this side of the boulder is weathering to the other side. How does the grain texture compare? Is it granular and loose or does it appear less affected by the elements?
3.) Is this side of the rock weathering at the same rate? Give your reasons?
4.) There is a wonderful view here. Post a photo of you at the location with your log, (please do not show the subject of the questions up close in your photo). 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.
Where did these rocks come from…
Gellibrand Hill is one of the few high points from which the whole district can be viewed, so do take a moment to enjoy the view from GZ!
Large tors of Devonian Granodiorite occur at the top of the hill. Granite from this area was used in the first Princes Bridge over the Yarra. The nearby ruins, (near GCB7M26) used blocks of the local stone as construction material as well.
During the Devonian period, Victoria had shallow seas, rivers and lakes teaming with aquatic life. It is known as the “Age of the Fishes” but this period was also witness to some of the largest volcanoes in Earth’s history. The volcanoes responsible for Gellibrand Hill exploded across Victoria late in the Devonian Period, covering several thousand kilometres with volcanic rock.
In Victoria we can find the remains of the Cerberean Caldera, a ancient Supervolcano with a caldera of around 27 kilometres across which was active around 380 million years ago. The caldera is evident in a few places as granite outcrops around the Lake Eildon National Park. It is thought that the Cerberean Caldera underwent a super eruption around 374 Mya, which in turn probably contributed to the Late Devonian extinction event. During this period of active volcanism magma intruded deep into the earth’s crust.
Gellibrand hill is a remnant of this time. The boulders you see here were formed at this time in a magma chamber deep below the surface of the earth, cooling slowly into large solid masses known as plutons. Eventually, over millions of years, the large igneous rock masses emerged at the surface via erosion. The removal of the overlying material dramatically reduced the weight pressing down on the granite leading to a release of pressure, causing them to break & fracture into large blocks.
We have brought you here to look at some of these Devonian granitic rocks….with particularly interesting shapes on their surface. When we saw these rocks we thought it looked a bit like crocodile skin.
THE LESSON:
Normally, granite is relatively resistant to weathering due to its interlocking crystal structure and the durability of the minerals it contains. Granite weathers slowly due to its high quartz (silica) content, which is highly resistant to chemical weathering. However, the feldspar minerals in granite are more vulnerable. The concentric, layered peeling (exfoliation) of rock, which results in a rough, scaled, or crocodile-skin-like surface appearance is mainly due to chemical weathering processes that changes the composition of feldspars, weakening the rock and causing it to break down. These particular shapes are caused by spheroidal weathering where the weathering process preferentially acts on the corners and edges of rocks.
What happens:
If the joints and fractures in rock beneath the surface form a 3-dimensional network, the rock will split & break into cube like pieces separated by the fractures. Water can then penetrate more easily along these fractures, altering the minerals like feldspar into clay and each of the cube-like pieces will begin to weather inward. The rate of weathering will be greatest along the corners of each cube, followed by the edges, and finally the faces of the cubes. As a result the cube will weather into a spherical shape, with unweathered rock in the center and weathered rock toward the outside.
*Weathering attacks more vigorously along the edges & most vigorously at the corners
As the weathering progresses, sections of weakened granite is lost leaving behind more rounded, weathered boulders or "corestones" underneath. The continuous exfoliation of weakened material creates a rough and scaly texture on the surface, which can resemble the "crocodile skin" appearance.
The key processes here is are Chemical weathering processes:
- Hydrolysis: Reaction of minerals with water, often transforming feldspar into clay minerals. Feldspars are aluminosilicate minerals and are highly soluble. They contain sodium and potassium ions. These release bicarbonates during hydrolysis, it is during this process of weathering granite that kaolinite clay is created.
- Oxidation: Reaction of minerals with oxygen, forming oxides such as iron oxides.
- Carbonation: Reaction of minerals with carbonic acid, which is formed from carbon dioxide and water.
This weathering occurs in conjunction with other means of weathering…like
Biomechanical: Living organisms contribute to mechanical weathering, as well as chemical weathering. Lichens and mosses grow on essentially rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock.
Physical weathering: Mechanical forces break the rock apart. Including….
Freeze-thaw: Water freezing and expanding inside cracks, widening them.
Granular disintegration: A related process where individual crystals on the surface weather and fall away, contributing to a rounded surface.
Weathering of granite.
Geologists recognise six general weathering grades for volcanic rock. You will probably be able to identify some these grades around you: grade VI is basically the soil you are walking on.
Weathering Classification System for Granite and volcanic rocks including the grade; description and typical characteristics.
- Grade I: Fresh Rock; unaltered with no visible signs of weathering or discoloration.
- Grade II: Slightly Weathered; hard, but stained near cracks. Still has the strength of grade l rock.
- Grade III: Moderately Weathered; completely discoloured. Weakened but rock material not friable (crumbly) and pieces 55mm in diameter cannot be broken by hand.
- Grade IV: Highly Weathered; decomposing, can be broken by hand into smaller pieces. Does not slake (mix into) readily in water. Surface may be powdery. Individual grains may be plucked from the surface.
- Grade V: Completely Weathered; completely discoloured, rock wholly weathered rock texture preserved. Will mix into water. The surface indents easily and breaks into individual grains.
- Grade VI: Residual Soil; a soil formed by weathering with original rock textures totally destroyed.
(Hencher and Martin, 1982)
https://www.researchgate.net/figure/Weathering-classification-system-for-granite-and-volcanic-rocks-Hencher-and-Martin_tbl1_228495889
https://www2.paradisevalley.edu/~douglass/v_trips/mrcs_lndsld/stop03/3b/3b.html