Darebin Parklands
The unique nature of rock outcrops within Darebin Parklands makes it an excellent study site for amateur or professional geologists. The associations within the park are regionally common, but globally rare. Silurian rocks are not internationally prolific and Victoria hosts a significantly large unit beneath Melbourne’s eastern suburbs. Combined with this Silurian unit is an extensive unit of Tertiary basalt. The basalt’s eastern limit is the Darebin Creek and the Yarra River to the south. Briefly to the north, and moreover to the west, the Victorian flood plain basalt creates the gentle undulating topography of the plains of southwestern Victoria.
The Rocks of Darebin Parklands
The two very different rock types within Darebin Parklands are the Silurian silt/sandstones and the Tertiary basalt (known commonly as bluestone). The Darebin Creek valley provides a clear boundary between the two. The words Tertiary and Silurian refer to the periods of time within the geological timescale. Geologists use names to distinguish between enormous time periods so that we can piece together the geological history of our Earth (for more information see section 3). The two rock units are described individually below. There are samples available at the Environment Centre, and in time it is hoped that thin section analysis of each unit can also be provided.
Silurian Sandstone
The Silurian sandstone is a Sedimentary Rock. These rocks consist of particles of variable size bonded together and hardened to form a solid rock. This rock unit was produced after a deep ocean accumulated fine sediments over a very long period. The layers alternate between sand and silt sized particle. Each layer represents a period or event of deposition.
There are no fossils known to exist within the Silurian rocks of Darebin Parklands. Within the same unit further to the north, huge populations of Brachipoda appear in intermittent layers. They have all died together most likely buried by a sudden large influx of sediment. Their size is small and the shells articulated.
Tertiary Basalt
Basalt is a volcanic rock, after the Roman god of fire- Vulcan. This is an aphanitic (fine-grained) extrusive igneous rock produced by rapid cooling. Basaltic rocks are poor in silicon, having typically 40-50%. This does not occur as quartz however, it combines with other elements to form pyroxene minerals. In general, this particular basalt is high in aluminium and silicon, and rich in iron, magnesium and calcium, and has smaller amounts of potassium and sodium. Basalts crystallize from fluid magmas, which often erupt as relatively quiet flows of lava. Basalts reach the surface, either on land or underwater, at between 1000 to 1200 degrees Celsius.
Due to the high gas level within this basalt, we describe the magma as having low viscosity. Highly viscous lavas (low in gas) tend to highly explosive. There are many gases (mainly sulfur, water vapour and carbon dioxide) dissolved within the liquid rock when it surfaces. As the temperature drops the gases return to gaseous state and result is the bubbly nature of the basalt. Very frothy basalt is known as scoria. Scoria results from explosive eruptions where the gas rich lava almost solidifies in the atmosphere prior to returning to the surface.
Quaternary Period
The Quaternary period followed the Tertiary. During this period, a huge amount of volcanic activity took place. This activity brought about the widespread development of volcanic or igneous rocks within the Darebin Creek catchment. Between six to seven million years ago during the Quaternary, over 400 volcanoes erupted from Seymour to Portland the lava spreading over 15,000 square kilometres. This lava also flowed through the Darebin Creek catchment and solidified to form a range of volcanic rocks that included basalts, scoria and ash. In the Darebin Creek catchment, basalt is now a major component of the landscape. Large sheets of this rock that we commonly know as bluestone lay over the surface of the soil. Basalt in the Darebin Creek catchment was mostly a result of eruptions from a volcano located at Hayes Hill north of Woodstock. These eruptions occurred between 4.5 and 0.8 million years ago leading to a number of lava flow events.
When examining basalt flows, horizontal and vertical cracks can often be found. These cracks are called joints and are a result of the cooling and shrinking process of the lava. Vesicles or small bubbles can also often be found. These bubbles are created by the accumulation of pockets of gas from the lava as it cooled.
Old Quarry Wall
This basalt rock outcrop is a remnant of the northern end of the bluestone quarry . This area was mined in the late 1940’s and early 1950’s. The lake is known as the Duck Pond and is filled via an underground spring created through quarry works in the 1950’s.
Quarry workers opened up the southern end to allow seeping water to be pumped out of the Duck Pond area into the Darebin Creek.
This natural amphitheatre area, with an earth stage and power supply, now serves as the Parklands’ main activity and festival centre.
Logging requirement:
To Log this EarthCache, please send an email with answers to the following questions to our Geocaching.com account.
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Your caching name and the EarthCache name
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If you look closely at the rocks a GZ you will see small holes in the rocks what do you think caused this
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Is there horizontal or vertical cracking visible at this location.
Now head over to WP2 at S 37° 46.355 E 145° 2.020
4. If you look up to the top of the wall what can be seen ''hugging'' a rock?
5. If you like include a photo of yourself at GZ, but please don’t include a picture of any of the questions as we will need to delete the photo.
Sources:-
http://www.dcmc.org.au/parklands/geology.php