Retorts

Gondwanaland?

Plate tectonic theory, first proposed in the mid-1960s, explains how the Earth's crust works. In particular, it explains what is observed at its upper surface. The crust is made up of some 15 major plates that are essentially rigid (like plates of armour), and which are all moving with respect to each other at measurable rates. This "movement" is determined by processes within the Earth and, in particular, within the mantle.

Adopting the principle that the present is the key to the past, earth scientists can readily model the changing face of the Earth through time, including the past distribution of the major continental landmasses and New Zealand too.

New Zealand is the emergent part of a large area of continental crust that sits low with respect to the Earth's surface simply because it happens to be relatively thin (about 25 km). "Normal" continental crust is 35-45 km thick, as in Australia. Relative buoyancy of the crust (a product of density and thickness) determines relative height of landmasses.

In terms of plate tectonic theory, New Zealand straddles the boundary between the westward-moving Pacific Plate to the east, and the northward-moving Australian Plate to the west. However, this plate boundary configuration has not always been as it is now. Prior to about 80 million years ago, the New Zealand continent was part of Gondwanaland.

It is the use of this term that I wish to comment on, because many people refer to this continent as "Gondwana" and, indeed, this term was used in my article [A Rocky Riddle Solved, May 2000].

Which is right? Gondwanaland or Gondwana? Well, strictly speaking it is Gondwanaland. This is the name that was first used and formally defined by Medlicott, a geologist with the Geological Survey of India (Calcutta) during the latter half of the 19th century.

Gondwanaland refers to the ancient continental landmass that can be characterised by distinctive fossiliferous sedimentary rocks like those preserved in the Gondwana Sequence of India.

The Gondwana Sequence is a succession of largely terrestrial sediments (coal-bearing) of Carboniferous to Cretaceous age that outcrop in the Gondwana Basin to the southwest of Calcutta.

So, Gondwanaland means "land of Gondwana sequences", and includes Africa, India, South America, Antarctica, Australia and New Zealand, as well as parts of Eurasia. What all these lands have in common are fossiliferous sedimentary sequences like the Gondwana Sequence in India.

Like many earth scientists, I have used the term Gondwana to refer to the ancient continent. However, I am now much more mindful of Indian earth science sensitivities. In India it is crystal clear: Gondwana refers to a sequence of rock and Gondwanaland refers to the ancient supercontinent. Both terms are formally defined scientific names. It seems to me that there is only one appropriate term to use for the continent: Gondwanaland.

Gondwana was the name of a kingdom that existed into the 16th century but that was subsumed with the arrival of the Moghul empire. Gondwana means "land of the Gonds". This is fascinating ancient human history, but utterly different from the reality of Gondwanaland, which is here to stay forever!!

Hamish Campbell, GNS