Earth Cache
Mount Gibraltar
Yes, I have to admit it: when I think of a volcano I am influenced by Hollywood creations like The Last Days of Pompeii (1935), Krakatoa: East of Java (1969), When Time Ran Out (1980), Joe Versus the Volcano (1990), Dante's Peak (1997), Magma (2006), and a hundred and fifty-four bazillion others. The list is indeed a long one, but when it comes right down to it, when most people think of a volcano, an image something like this comes to mind...
Shortly after I arrived in the Southern Highlands, when I was told we were living in close proximity to a volcano, I began looking around for volcanic mountains, cones, craters, and other stereotypical signs of Hollywood volcanoes and six o'clock news-worthy disasters. Turns out the truth is: eruptions like that of Pompeii don't happen nearly so often, nor as dramatically, as Hollywood would have us believe.
Origins of Mount Gibraltar
Approximately 350 million years ago volcanoes stretched along much of what was then the east coast of Australia. Also at this time, the Sydney Basin formed on Paleozoic rocks, and it is composed mainly of sedimentary rocks and volumetrically minor volcanic units.
Then, about 250 million years ago, the sea level dropped leaving immense swamps and deltas which accumulated plant material to form peat. Floods and rivers that deposited sand and silt periodically inundated the peat swamps producing major coal seams in the Bowral area.
Next, about 220 million years ago, a massive braided river system deposited the Hawkesbury Sandstone throughout much of the Sydney Basin. A rising sea level eventually drowned the Hawkesbury Sandstone to form the shale and siltstone of the overlaying Wianamatta Group.
This sequence of events set the stage upon which Mount Gibraltar was to make its appearance: ancient rocks topped with sedimentary rocks, followed by coal, sandstone, and shale.
Finally, approximately 180 million years ago, during the Early to Middle Jurassic Periods, large volumes of molten basaltic rock upwelled from deep within the earth and were emplaced near the surface. This episode of magmatic activity was responsible for the emplacement of Mount Gibraltar and several other igneous masses (eg. Mt. Flora, Mt. Jellore) in the Southern Highlands.
Eroded Core Theory
Many people I have spoken with have described Mount Gibraltar as a volcanic plug (eroded core). This notion has found its way into print in "Best Walks of the Southern Highlands" (page 80) and other noteworthy publications.
The basic concept of a volcanic plug is that the lava is so thick it does not flow easily and just fills the volcano's vent - with little or no overflow. Since the magmatic chamber is not emptied there is no back flow and so no caldera or crater forms. The lava cools, hardens, and remains in place. Then, over time, changes to conditions of the earth prevent further eruptions.
In the case of Mount Gibraltar, its hardened intrusive igneous rock - microsyenite (known as trachyte in commercial applications) - resists erosion while the softer sedimentary rock around it has worn away over the past 60 million years.
According to the theory of volcanic plugs and eroded cores, this accounts for Mount Gibraltar's volcanic origins and its present day un-volcanic shape and appearance.
Cryptodome Theory
At least one credible source describes Mount Gibraltar not as an eroded core but as a cryptodome.
According to the science of volcanology, a cryptodome is an uplifted area caused by the intrusion of viscous magma and which results in a bulge in the surface of the earth. A key difference between this and a volcanic plug is that with a cryptodome lava never breaks the surface and therefore there is no volcanic vent.
In terms of Mount Gibraltar, the initial igneous intrusion lifted the Hawkesbury Sandstone and Wianamatta Shale layers.
Then, as the intrusion grew, pressure increased until a fault developed on the west side. The rock on the top and east side rose while the rock on the west side dropped.
Finally, erosion exposed the cooled hard rock.
According to this theory, Mount Gibraltar's microsyenite also contains, in some places, thin pegmatitic veins that consist of much larger mineral grains, typically greater than 10mm in size.
It is the occurrence of pegmatites that indicate a confining pressure and implies that the syenitic mass was not open to the atmosphere; that is, there was no volcanic vent. As such, Mount Gibraltar is not an eroded core but the remains of a cryptodome.
Analysis
Since the outcome of an eroded volcanic core and cryptodomic remains are very similar in appearance - as evidenced by present-day Mount Gibraltar - we might ask ourselves why we'd bother splitting hairs over what 'appears' to be trifling matters.
The differences are not insignificant, however, because they indicate to us what the landscape of the Southern Highlands was like some 180 million years ago - which can affirm or refute current geological theory and understanding. And knowing what happened in the past should allow us to reasonably predict what we can expect to happen in the future.
If the eroded core theory is correct, we might imagine that millions of years ago the Southern Highlands was comprised of rugged, jagged mountains aflame with lava exploding in all directions and extending as far afield as the eye can see. Once the volcanic activity subsided, erosion did its job and wore down the peaks to fairly even altitudes. It's true, there are exposed areas of intrusive rock, and in some cases - Gibraltar included - rock that has cooled very close to the surface. But by and large this theory lacks substantiation because there is virtually no trace of lava flow in the area. That said, extinct volcanic activity might be one explanation for the relative lack of earthquakes in the area - the ground is solid and unmoving.
If, on the other hand, the cryptodome theory is correct then we might imagine that millions of years ago the Southern Highlands rose out of the sea - along with the rest of the Sydney Basin - in a rippling, buckling, bulging, quaking mass of ground that reshaped if not the planet then at least the continent as we now know it. It's true, this might account for why we find sedimentary rocks hundreds of meters above sea level, but at the same time we feel hard-pressed to believe that a few centimeters of pegmatite are the primary link accounting for this enormous process. Also, since the general uplift of eastern Australia is rooted in the theory of plate tectonics we might expect continued activity that would accompany continental drift. Yet there is more erosion than lift, and while there have been a few earthquakes of small scale in the Southern Highlands (usually attributable to distant faults) there are not the tremors one would expect from a sizeable fault like that described at Mount Gibraltar.
Compelling arguments are being made for both sides of this debate.
To claim this cache:
1. View Mount Gibraltar close up.
Visit the posted coordinates where you will find microsyenite aplenty, then...
Take ONE of these walks where you will find yet more microsyenite:
- Quarry Circuit to the Jellore Lookout
Begin at the Heritage Quarries Circuit, then go to the Stone Stair-way (to Bowral Lookout), and continue on the Rim Trail to the Jellore Lookout. See waypoints.
- Via Lookouts Crescent Road
Begin at waypoint one and then stroll around the complete circle, being mindful of traffic
a. Do you see evidence of lava flow?
b. Do you see traces of pegmatite? (Don't be decoyed by lichen and moss)
If yes, provide the GPS location for your find(s).
2. View Mount Gibraltar from afar.
Suggested vantage points are (choose ONE):
- Kirkham Road, Bowral
- Old Bowral Road
- Gibraltar lookout in Gibbergunyah Reserve
- Any elevated position in or between Bowral and Mittagong which affords an unobstructed view of Mount Gibraltar.
Based on what you see as well as the description above, do you think Mount Gibraltar is an eroded volcanic plug or the remains of a cryptodome? Provide a few words of explanation.
Please send your answers to me via the geocaching website - e-mail or message. The answers to these questions should not be included in your log. You do not need to wait for a reply from me to log your find.
References:
Souter, John and Gillian. Best Walks of the Southern Highlands. Warriewood NSW: Woodslane Press, 2012
The Gib - Mount Gibraltar: Southern Highlands. Lemann, Jane, and Jenny Simons, Elizabeth Smith, Carmen Wright, Julie-Rose Moffatt, Michael Elphick, eds. Mount Gibraltar Landcare and Bushcare, Australia. 2007.