Day 4: Third dredge at Frederick Reef

By Olivia Belshaw

The main scientific research conducted on this voyage involves the dredging of rock material from the ocean floor. The dredging process aims to collect significant samples of black volcanic rock. In many instances, limestone is also often present in the dredged material – these rocks are observed, and any fossils are collected and photographed. The black volcanic rocks collected (commonly basalts) are important as they are extrusive volcanic rocks, which crystallise from lava at the Earth’s surface and are formed in areas of hot spot activity.

Toady the morning-shift research team was up at 4am to prepare and collect the rock samples from the third dredge on Frederick Reef – this dredge occurred at below 2000m under sea level. Once the dredge was hauled back on deck, sledgehammers were needed to break apart the large rock pieces into smaller, more manageable chunks before being transported into the wet lab. The rocks were then carefully scrubbed to remove traces of black manganese oxide, and some were sawed if necessary – this helped show the internal features of the rocks. A significant amount of time was then spent carefully observing, categorising and photographing rock samples, before storing them for further analysis on-shore.

When observing and categorising the black rock samples, researchers look for several indicators. Firstly, researchers describe the rock in terms of the colours present. Observations are then made about the overall colour of the rock which can help determine the nature of the rock (for example, igneous, sedimentary or a combination of these rock types etc.) however often further information is needed. When observing the igneous rocks, lighter rocks are felsic, which means they contain feldspar and quartz minerals. Darker igneous rocks are mafic – these contain minerals such as olivine, pyroxene and amphibole. Researchers then look for alterations – these are small, unusual areas in the rock surface or inside the rock that indicate some sort of chemical process has occurred, such as oxidation.

The hand lenses are used next to investigate the visible mineralogy of the rock. Researchers look for minerals such as feldspar and olivine, glassy masses and sections of volcanic brecciated rock. During this stage, vesicle abundance is also estimated – vesicles are commonly found in volcanic rocks and form when bubbles of gas are trapped as the rock cools. These are often filled with minerals such as calcites and zeolites, and carbonate material. The presence of carbonate material is confirmed by testing the section with acid – the production of bubbles is a positive indicator of carbonate.

Lastly, if the rock is sedimentary or volcaniclastic, the grain size and shape of the clasts are observed. Some clasts are made up of a variety of minerals and components including natural glass, pumice, and scoria. In many instances the clasts are cemented together with calcites or carbonate material.

Once all observations are completed, the rocks are named, photographed, bagged and stored to be transported back on-shore. Researchers hope that further on-shore geochemical and isotopic analysis of these collected black rocks will tell them about the ages of eruptions from volcanic hot spots along the edge of the Australian plate.