Tasmania has a complex geological history, with the world's biggest exposure of dolerite which can be seen here in Tasmania.
Rock records of Tasmania contain representatives of each period of the Neoproterozoic, Paleozoic, Mesozoic and Cainozoic eras.
Tasmania is one of the few southern hemisphere areas glaciated during the Pleistocene with glacial landforms in the higher parts. The west coast region hosts significant mineralisation and numerous active and historic mines.
Some History of The Jurassic Period.
A major intrusion of dolerite occurred in the Jurassic. This was a widespread phenomena covering over one third of Tasmania,
This intrusion also affected Antarctica, Argentina and South Africa at 183 million years ago. .
About Three to five million cubic Kilometres of magma were intruded overall, being the planet's fourth largest known magma intrusion.
Tasmania has the largest exposure of dolerite in the world of Some 30,000 km2 and a volume of 15,000 km3, of that being In Tasmania the rock is characteristic of many mountains in Tasmania with its columnar joining and dark blue grey colour.
Most of the intrusions are in the form of sills up to 500 m thick. Mostly the sills are in the Parmeener Super Group rocks.
There are also stepped sills, inclined sheets, cones and some dykes the sils here are of the Stepped type, The dolerite rock at Mersey Bluff formed as a sill (or horizontal) intrusion, faulted down, to Bass Strait.
The rock cooled giving joints and fractures. Some joints are like “PACK OF CARDS”. When the rock eroded it has provided flat surfaces for seating for Tasmanian aboriginals.
They also carved inscriptions and engravings onto the rock, also known as petroglyphs.
The upper parts of sills may be more coarsely grained.
Dolerite is a type of igneous rock, which is formed from in the Earth's crust.
Igneous rocks are classified using grain size, silica content and/or silica saturation.
Grain size reflects the depth at which the molten rocks form in the Earth:
- Volcanic rocks form close to the surface, cool quickly and form fine grains/crystals.
- Plutonic rocks form deep in the Earth, cool slowly and form coarse grained rocks.
- Sub-volcanic rocks form at intermediate depths, generally in dykes (vertical sheet of intrusive igneous rock) or sills (horizontal sheet of intrusive igneous rock) and form medium grains. Intrusives are when igneous rocks have intruded other rocks.
The island (of Tasmania) was adjoined to the mainland of Australia until the end of the last glacial period approximately 10,000 years ago.
Much of the island is composed of Jurassic dolerite intrusions (upwelling of magma) through other rock types, sometimes forming large columnar joints as well as sheets.
Tasmania has the world's largest areas of dolerite (Approximately one third of the Island), with many of the distinctive mountains and cliffs formed from this rock type.
At Mersey Bluff you will find a headland of dolerite rock.
These rocks at Mersey Bluff date back to the Jurassic Age, around 185 million years ago.
This trail covers a truly spectacular part of the state and also has some wonderful geocaches!
The Tiagarra Aboriginal Cultural Centre and Museum is also located at the Mersey Bluff.
Q1. When the molten rock cooled, joints and fractures were formed. What is the jointing likened to ( You will find the Answer in the Description)?
Q2. At the Lighthouse What is the Elevation on the Plaque on the door compared to your GPSr?
Q3. When the rock eroded it has provided "what type of" surfaces for seating for "Which State" aboriginals.
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These is one of many petroglyphs Near the coordinates of S41° 09.532 E146° 21.298
Over 200 petroglyphs are located in the Mersey Bluff area.