Examining an Ancient Sea Floor

Callum Reekie
Friday 5 June 2015

There are few rare instances on the surface of our planet where the geology that underlies Earth’s mighty oceans can be directly observed, and moreover, quantitatively measured. Unst, Britain’s most northerly isle and a constituent of the sub-Arctic archipelago of Shetland, is one of these instances. A vast expanse of eastern Unst exposes rocks of an ancient ocean long vanished. These slabs of oceanic crust that have been accreted on to the surface of the Earth (termed ophiolites) afford an opportunity to examine the processes that operate during the crust’s birth, evolution, and eventual demise.

The remote moors of eastern Unst, Shetland. From left to right the geology changes to reflect shallower levels in the oceanic crust which underlies Earth's oceans.
The remote moors of eastern Unst, Shetland. From left to right the geology changes to reflect shallower levels in the oceanic crust which underlies Earth’s oceans.

One such process that concerns my Laidlaw internship is oxidation of the oceanic crust. Water pertains as the principal oxidising agent and owing to the oceans above is in plentiful supply. However, to observe the cryptic patterns of oxidation in the oceanic crust, we must consult geophysics. Where water is present, particular mineralogies within crystalline rocks preferentially alter to magnetite, and thus, the rocks become more magnetic. Therefore, characterising the magnetic susceptibility of the rocks describes, in effect, the degree of oxidation that the ophiolite has been subjected to.

But why does oxidation matter? Water has an adverse effect on the rheology (strength) of rocks and thus a characterisation of the oxidation patterns within the oceanic lithosphere may help resolve some outstanding geological problems. Foremost, does water serve as a catalyst for the motion of tectonic plates across Earth’s considerably sluggish mantle?

My research so far in Shetland, supplemented with copious amounts of rain, has been enjoyable, and the leadership methods we examined during the leadership weekend have been of considerable use. Whilst I am not directly leading a team, the methods have been of particular use to the management of my time, and they were particularly invaluable during the exam period, although, as I must sheepishly admit, I narrowly avoided missing my ferry.

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