This section of the site is
designed to inform visitors about
three main aspects:
- General geology
- How and when the rocks formed
- Description of the two rock types
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.
To understand more of the geology of the Darebin Parklands the following section
provides detail on general geology, and then subsequent information on the two
different rock types within the parklands.
General Geology
The three main rock groups, Igneous, Sedimentary and Metamorphic are discussed
below. You can think of these as families of rocks. Within each family there
are many individuals.
Igneous
Igneous comes from the Latin ignis, or “fire”. Igneous rocks are
new, born from within the earth. They comprise either fresh material from the
mantle or reignited continental material that has been subducted and melted.
Those igneous rocks produced at or above the Earth’s
surface are extrusive
(volcanic), those formed beneath the surface are intrusive (plutonic). They
can also be a combination of the two. Creation of igneous rocks can be spectacular
(e.g. by way of volcanic eruption or rapid continental rifting); or it can
be
rather passive (e.g. slow intrusions or oozing gas-rich eruptions) and is commonly
the result of tectonic activity at or adjacent to plate boundaries. More than
95% of the Earth’s outer 50 kilometres consists of igneous rocks. Igneous
rocks are defined by their mineral composition and their texture. Examples
of igneous rocks are granite, rhyolite, andesite, basalt, diorite, gabbro,
peridotite
and komatiite
Sedimentary
When rocks break down due to the processes of weathering and erosion, sediment
is created. Sedimentary rocks are formed from particles accumulated and consolidated
on the earth’s surface after the rocks breakdown.
Accumulation zones
may be a lake, an ocean, floodplains, and quiet parts of a river or stream,
desert
dunes or even glacial fallout. Sedimentary rocks are named based on the size
and relationship of the particles within them. Features of the rocks give clues
as to the environment of deposition, such as different organisms present at
the time, waves, seasonal aridity, wind and current directions, tidal flows,
major
climate change, Examples of Sedimentary rocks include sandstone, limestone,
mudstone, siltstone, greywacke, conglomerate and breccia.
Metamorphic
Created when pre-existing sedimentary or igneous rocks are subjected to heat
and/or pressure, metamorphic rocks will often retain many of characteristics
of the original rock. They are generally more complicated than the other two
rock types and can be quite spectacular. Metamorphic features can reveal things
about rocks such as whether they have been buried, been in contact with volcanic
products, or even been flushed with hot fluids.
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.
