Site 9

Mafic Plutonic Rock

Welcome to your last field stop of the day! The GPS coordinates for this site are 0236008 6279092.

Unfortunately you missed out on the cardio assessment and lecture on the importance of cardio fitness after running up the big hill to site 9 like the lucky students in the photos.

The mafic plutonic rocks from site 9 are best exposed in outcrops at the top of the hill. The outcrop patterns are similar to the tors you learnt about at site 1.

Examine the model of a hand sample from site 9 in Pedestal3D below. Click on the < button to hide the side menu and give you more space to view the sample. Choose 'High' instead of 'Medium' from the drop down menu to maximise the quality of the sample image.

Task: Please choose an informative view direction in the Pedestal3D model and make a sketch in your field note book. You may zoom into interesting features and make multiple sketches rather than sketching the whole hand sample if needed. Please use the tools in Pedestal3D to measure grain size etc. for your sketches. It is important to recognise the difference between a fresh and weathered face of a sample. Sometimes weathering can conceal important features and the fresh face gives the best view, however the opposite can also be true. Study both faces in these samples to see which gives you the best view of the rock's microstructures.

Homogeneous Rock with some layering

This rock is a mafic plutonic igneous rock (Figure A). Using your mineral identification skills, identify the two key minerals in this rock.

Mineral Identification

A close up of some of the minerals (Figure B) shows you that the white mineral (red arrow) has an elongate tabular prism shape, while the black mineral (white arrow) has a stubby tabular prism shape. However, these are tricky to confidently identify without making a thin section and examining the sample under a microscope. Two photomicrographs are presented in plane polarised light (PPL) and crossed polarised light (XPL). Both minerals show twinning in the XPL image and the dark minerals in the hand sample are pale green in the PPL image (white arrows).

Question 4: Using the chart below and the images of this rock type (Figures B and C), identify the two key minerals you can observe and note down their distinguishing properties and modal proportions into your field trip workbook.

Rock Identification

At site 8, you observed an intermediate plutonic igneous rock. This field site is made up of a mafic plutonic igneous rock. Given the minerals you have just identified, now identify the rock type.

Remember that plutonic refers to a rock that cooled very slowly, allowing coarse minerals to crystalise.

Question 5: Using this information, try and identify the rock type on the graph below.

Magmatic Differentiation

Are you familiar with Bowen's Reaction Series? If you're not, this reaction series can be used to explain the order of crystallisation for different minerals at different temperatures. Mafic minerals begin to crystallise at higher temperatures whilst felsic minerals rich in silica begin crystallising at much lower temperatures.

The intermediate rock you saw at field site eight began crystalising at a lower temperature, whereas the minerals within the rocks at this field site began crystallising once they reached 900–1100°C.

When the magma chamber is very hot and forming these mafic minerals, the heat allows for the magma chamber to convect. As minerals are crystalising they can start to settle due to gravity and may develop cumulate layers with a layered or banded appearance.

Can you recognise that cumulate texture in the rock shown here?

Question 6: What is the likely origin of the flat paddocks around the Stonehaven homestead outlined with a white dashed line in the Google Earth view?

In the photograph above of all the happy students, you can see some cliff-forming rocks in the far distant background on the right. These are found further to the east in NSW, closer to Sydney.

These rocks are a part of a Permian group of sedimentary rocks and are commonly found around Sydney.

Question 7: What type of rock are exposed in the cliffs throughout the Blue Mountains?

Throughout your field journey you have come across numerous rock types. Try and think of the location of where you found each rock type and whether the field images looked as though they were deposited over rolling hills or if they were more inaccessible in the form of jagged outcrops and cliff faces. Do you think you could make better land usage out of rolling hills or the inaccessible outcrops?

Question 8: Have a go at trying to link the different topography to the rock types you observed in the field and the associated land usage.

Hint for Question 6.

Well done! You have got to the end of the trip. We hope you had fun and learned about rock and mineral identification in the field.

A quick round up back at the bus then you can head on home

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