Duncan Miller

Imagine a jigsaw puzzle the size of the Earth, with most of the pieces missing. And those that aren’t missing are moving around all the time. This is the task that confronts some ambitious geologists. It is important because it explains why there are oceans and mountain chains, and why we may find rocks of similar ages and composition on far-flung continents. It also satisfies human scientific curiosity, and keeps some people employed and off the streets.

Until the mid-1960s, geology lacked a coherent explanation of how the world worked. Geologists puzzled over the existence of ocean basins (did they sink because of an expanding Earth?) and mountain chains (did they rise in crumples because of a shrinking Earth?). They couldn’t explain the global distribution of volcanoes and earthquakes, the fact that some continental margins seemed to fit together (like Africa and South America), and the distribution of related or unrelated groups of animals and plants.

After the Second World War, seafloor mapping changed all of this quite quickly. A continuous mountain chain was discovered running through the world’s oceans. It was where hot new ocean floor was being generated more or less continuously. Deep trenches were discovered, where cold old ocean floor sank back into the mantle of the Earth. This implied that the surface of the Earth was in continuous movement. The continents, consisting of rocks less dense than the ocean floor, were being dragged around on this conveyor belt, colliding with each other and then being torn apart again. This was driven by slow convection in the mantle, in turn driven by the difference in temperature of the interior and exterior of the Earth.

We are so familiar with this explanation of how the Earth works that is easy to forget that it was only about sixty years ago that the persuasive evidence for the unifying theory of plate tectonics became widely known. Maps of the seafloor, first published in the late 1950s, began to reveal the mid-ocean ridge mountain chain and the deep ocean trenches. These maps were primarily the work of two geologists, Bruce Heezen and Marie Tharp. In 1977 Marie Tharp and Bruce Heezen produced their most famous map, with Austrian painter Heinrich Berann.

This map was highly publicised and widely distributed, including in the National Geographic magazine, so it became well known. The original now resides in the US Library of Congress, along with all of Marie Tharp’s maps and notebooks. This is perhaps the second-most important geological map ever produced. (Arguably the most important is the very first geological map, produced in 1815 by William Smith, a British canal surveyor: In the 1970s much of the topography of the seafloor was unknown, but Marie Tharp drew on her extensive geological knowledge and mapping skills to fill in the missing bits with remarkable accuracy.

A lot has happened since to support the theory of plate tectonics. Geologists have identified fragments of ancient crust, measured their former paths relative to the magnetic poles, and compared their characteristic rocks to match formerly contiguous continental areas now separated by oceans. Another physical constraint on any reconstruction of plate movements is that continental fragments cannot cross over each other. The results are several different reconstructions of the configuration of the continents, former ocean basins, and subduction zones where ocean crust is consumed. Until now, these various reconstructions have focussed on either the fairly recent geological past, or specific earlier time periods, or specific geographic locations. This year a group of geologists has attempted and published the most difficult jigsaw puzzle of all time – to reconstruct the former face of the Earth over the past one billion years. This is a dynamic model, but a direct successor of Marie Tharp’s and Bruce Heezen’s earlier mapping work.

Here is a screen grab of the plate reconstruction around the time the Cape Granites were forming:

Colour scheme:
Green areas = landmasses

Blue areas = shallow seas
Brown lines = convergent boundaries
Purple lines = divergent boundaries

Because it is a dynamic model, you will have to click on this link to see it in action: (If the image is static, right click on it and select Play. You can also slow it down a bit.) The research behind it is Merdith et al. 2021 ‘The first full plate tectonic reconstruction of the world over the last billion years’ Earth-Science Reviews: The research publication is rather heavy going, but the dynamic reconstruction really shows how fragmented the continental crust is and how dynamic the plates are as they scurry around the globe. DM


These are highly recommended reading to discover more of the fascinating story of Marie Tharp’s cartography.