The Tasmanian Dolerite
Dolerite rock is made up of interlocking crystals of light (feldspar) and dark (pyroxene & iron/titanium oxides) minerals. This sample shows shows relatively fresh rock at centre grading upwards into a rust-coloured weathered rock at top. Scale in cm.
Tasmania has a landscape and an environment that is quite unique. One of the reasons for this is the Tasmanian Dolerite, thick bodies of hard, igneous rock that were intruded 180 million years ago in the Jurassic and which now extend across much of the state. The dolerite resists erosion, forming distinctive mountain plateaus and cliffs for which Tasmania is now famous.
Igneous rocks are rocks which have formed from melted rock or magma. They are named based on both what they are made of (their chemistry) and how they were made (how the magma cooled). Rocks that cool from the same magma can have different names depending upon how they formed. An example is basalt and dolerite, both of which may be made from the same magma. Basalt is a volcanic rock that forms when magma erupts on the surface as lava. It cools relatively quickly and may be a mix of crystals and volcanic glass. Dolerite is an intrusive rock made when magma has pooled within the crust, cooling more slowly to form a rock made entirely of interlocking crystals.
The Tasmanian Dolerite was formed as part of a major earth-event related to the breaking of the supercontinent Gondwanaland. Basalts and dolerites made from the same or similar magma as the Tasmanian Dolerite have been found in Antarctica and South Africa. During the Jurassic, these areas were joined together along what was then the southern edge of Gondwanaland. These rocks are part of what is known as a Large Igneous Province or LIP. LIPs form when huge volumes of basaltic magma erupt or intrude areas covering hundreds of square kilometres of land over a short period of geological time. They have occurred at various times and places throughout the history of the Earth; the eruption of basalt into the Siberian Traps at the end of the Permian (as discussed on the extinction page) is one such event. The Tasmania dolerites, part of what is now called the Karoo-Ferrar LIP, is another.
Igneous rocks are rocks which have formed from melted rock or magma. They are named based on both what they are made of (their chemistry) and how they were made (how the magma cooled). Rocks that cool from the same magma can have different names depending upon how they formed. An example is basalt and dolerite, both of which may be made from the same magma. Basalt is a volcanic rock that forms when magma erupts on the surface as lava. It cools relatively quickly and may be a mix of crystals and volcanic glass. Dolerite is an intrusive rock made when magma has pooled within the crust, cooling more slowly to form a rock made entirely of interlocking crystals.
The Tasmanian Dolerite was formed as part of a major earth-event related to the breaking of the supercontinent Gondwanaland. Basalts and dolerites made from the same or similar magma as the Tasmanian Dolerite have been found in Antarctica and South Africa. During the Jurassic, these areas were joined together along what was then the southern edge of Gondwanaland. These rocks are part of what is known as a Large Igneous Province or LIP. LIPs form when huge volumes of basaltic magma erupt or intrude areas covering hundreds of square kilometres of land over a short period of geological time. They have occurred at various times and places throughout the history of the Earth; the eruption of basalt into the Siberian Traps at the end of the Permian (as discussed on the extinction page) is one such event. The Tasmania dolerites, part of what is now called the Karoo-Ferrar LIP, is another.
kunanyi/Wellingtons famous organ pipes, formed from Jurassic Dolerite.
In Tasmania most of the LIP magma appears to have pushed upwards to a shallow depth in the crust, where it then moved outwards between horizontal sedimentary layers, cooling to form broad sheets of dolerite known as sills. Some magma did make it to the surface to erupt as lava that hardened into basalt – as seen at Lune River in the south of the state. How much basalt there may have been is uncertain as erosion of the land surface has now removed much of the material that was once above the dolerite.
The exposure or ‘unroofing’ of the Tasmanian Dolerite sills by erosion has produced landforms that now characterise the state. Magmas solidifying into rock tend to shrink a bit as they cool, a process which results in the formation of hairline fractures or joints in the new rock. These joints often arrange into regular patterns creating the appearance of columns in the rock. In the relatively cool climate of Tasmania a process known as ‘ice-wedging’ where rain water seeps into joints, freezes and then expands, has also assisted in breaking-up dolerite. Kunanyi/Wellington’s famous Organ Pipes are an example of a rock formation that has been shaped by these processes.
The exposure or ‘unroofing’ of the Tasmanian Dolerite sills by erosion has produced landforms that now characterise the state. Magmas solidifying into rock tend to shrink a bit as they cool, a process which results in the formation of hairline fractures or joints in the new rock. These joints often arrange into regular patterns creating the appearance of columns in the rock. In the relatively cool climate of Tasmania a process known as ‘ice-wedging’ where rain water seeps into joints, freezes and then expands, has also assisted in breaking-up dolerite. Kunanyi/Wellington’s famous Organ Pipes are an example of a rock formation that has been shaped by these processes.
