Site 8

Spot the difference

Welcome to site 8! The GPS coordinates for this site are 0236263 6278698. At site 8 we examine an outcrop of darker coloured rocks at the bottom of a hill.

Think back to the first field site and the felsic plutonic rocks that you examined. How does the outcrop pattern of the rock at site 8 differ? Make some notes in your field note book, and jot down your observations.

Examine the model of a hand sample from site 8 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.

Minerals & Microstructure

Try to identify the two main minerals found in the rock at site 8. These are marked by red and white arrows in the figures but are tricky to confidently identify without making a thin section and examining the sample under a microscope. Note the elongate shape of the mafic minerals (white arrows). Two photomicrographs are presented in plane polarised light (PPL) and crossed polarised light (XPL). The white mineral shows twinning in the XPL image and the dark minerals in the hand sample are (i) green-brown in the PPL image, (ii) display two cleavages at ~60° to each other (white dashed lines), and contain relict pyroxene in their cores (white arrows).

Question 1: List at least one distinguishing property for each mineral and estimate the mode (percentage) of each mineral. Fill in the table in your field trip worksheet.

Question 2: Now have a go at describing the microstructure of this rock. Think about the following: Grain Size, Grain Shape, and Arrangement or relationship of grains to each other. You can measure grain size using the tools in the Pedestal3D model.

Question 3: Now you've made these observations, you're ready to make an interpretation and name the rock. Circle what you think this rock is on the table in your field trip worksheet.

Well done, click here to run up the hill and move on to Stop 9

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