BLOG 9: Meet the Rock Stars – treasures from the deep

By Emily Fewster

On this voyage our aim is to use the rocks collected from each dredge to help piece together a story about the age and evolution of each individual seamount and identify which seamount belongs in which “family” chain of seamounts. Pictured below are of some of the types of rocks we were targeting in our dredging activities and why.

Fresh volcanic basalt with its dark-grey colour is prized in each dredge as it can be used to date the age of the seamount using the Argon-Argon dating technique.

 

 

 

 

 

 

Breccia tells us about sedimentary processes that have occurred and contains fragments from past eruptions. This provides context for the type of volcano the seamount was, that is, either effusive with flowing lava or explosive where lava and rock fragments were blown up into the water and possibly air.

 

 

 

 

This rock is vesicular basalt with infills of zeolite from alteration (the process of chemical change over time). This type of alteration can hint at hydrothermal vent activity in the seamount.

 

 

 

 

 

The width and layers of the crumbly, black manganese crust on some of the rocks is useful for a new dating technique where the layers can provide information about what ocean conditions were like at the time of formation.

 

 

 

 

Rocks containing white calcium carbonate point to a time the seamount was above or close to sea level even though it may now be many kilometres below the surface today.

 

 

 

 

 

 

Carbonate, the building material of coral, can be tested for quickly by dropping weak acid on a sample. Fizzing of carbon dioxide bubbles indicate the sample contains carbonate.

Carbonate is ideal for a process called strontium dating which can tell us about subsidence history of the seamount.

 

 

 

 

 

 

 

 

Macro-fossils such as shells and coral along with micro-fossils (called foraminifera and shown here magnified 40 times) can be found within rocks and sediments that we dredge.

 

 

 

 

 

 

Fossil types can also be used to provide relative dating of when the seamount sank beneath the surface.