Alan Collins is a Professor of Geology at The University of Adelaide and is fascinated by the world and by what the rocks of the world can tell us about how the world works and what it looked like in the past. He has worked with a great set of PhD students and colleagues all over the world, with a particular focus on the Gondwana-forming orogens that lace South America, Africa, India and Antarctica. You can read more about his work here.
The continents are incredible records of Earth history. They form a mish-mash of the relics of all types of plate interactions with the eroded remains of plate collision mountain belts, basins that formed as ancient supercontinents were ripped apart by 2,900 km deep upwelling plumes, and they expose a cross-section of these Earth events through the thickness of the crust. You can understand that reading this record, and using it to develop maps of ancient plate tectonics, involves lots of fieldwork in true TravelingGeologist fashion to try and understand the stories held in the rocks from all over the world.
Along with colleagues, I’ve helped publish the first whole-Earth plate tectonic map of half a billion years of Earth history, from 1,000 million years ago to 520 million years ago. The fieldwork for this study was drawn together by Andrew Merdith, an amazing PhD student from The University of Sydney, and his supervisors in the EarthByte group, Prof Dietmar Müller and Dr Simon Williams, but the field work used in the reconstructions came from twenty years work I’ve been involved in with PhD students and colleagues from all over the world and the work of many other teams over many years. This article looks at why anyone would bother trying to reconstruct plate tectonics in deep time. Accompanying articles look at some the specific field studies that went into this work, including PhDs student Diana Plavsa in southern India, Morgan Blades in western Ethiopia, Donnelly Archibald and Sheree Armistead in Madagascar, Brandon Alessio in Zambia and Ben McGee in the bad-lands of western Brazil!
The time range we looked at is crucial. It’s a period when the Earth went through the most extreme climate swings known, from “Snowball Earth” icy extremes to super-hot greenhouse conditions, when the atmosphere got a major injection of oxygen and when multicellular life appeared and exploded in diversity.
Now with this first global map of plate tectonics through this period, we can start to assess the role of plate tectonic processes on other Earth systems and even address how the movement of structures deep in our Earth may have varied over a billion year cycle.
Plate tectonics and the problem with deep time
The modern Earth’s tectonic plate boundaries are mapped in excruciating detail.
In the modern Earth, global positioning satellites are used to map how the Earth changes and moves. We know that up-welling plumes of hot rock from over 2,500 km deep in the planet’s mantle (the layer beneath the Earth’s crust) hit the solid carapace of the planet (the crust and the top part of the mantle). This forces rigid surface tectonic plates to move at the tempo of a fingernail’s growth.
On the other side of the up-welling hot rock plumes are areas known as subduction zones, where vast regions of the ocean floor plunge down into the deep Earth. Eventually, these down-going oceanic plates hit the boundary between the core and mantle layers of Earth, about 2,900 km down. They come together, forming thermal or chemical accumulations that eventually source these up-welling zones.
It’s fascinating stuff, but these processes also create problems for scientists trying to look back in time. The planet can only be directly mapped over its last 200 million years. Before that, back over the preceding four billion years, the majority of the planet’s surface is missing, as all the crust that lay under the oceans has been destroyed through subduction. Oceanic crust just doesn’t last: it’s constantly being pulled, pushed and sucked back deep into the Earth, where it’s pretty hard to sample.
Mapping ancient plate tectonics
To get at where plate margins were in the ancient Earth, and how they changed, we looked for proxies of plate margins in the geological record. We found rocks that formed above subduction zones, in continental collisions, or in the fissures where plates ripped apart. This provides many more details about ancient plate margins than were available to previous generations of geologists.
Using other methods, some latitudinal constraints of continents in the past can be worked out, as some iron-bearing rocks freeze the magnetic field in them as they form. This is like a fossil compass, with the needle pointing into the ground at an angle related to the latitude where it formed — near the equator the magnetic field is roughly parallel to the Earth’s surface, at the poles it plunges directly down. You can see this today if you buy a compass in Australia and take it to Canada: the compass won’t work very well, as the needle will want to point down into the Earth. Compass needles are always balanced to remain broadly horizontal in the region that they are designed to work in.
But, these so-called “palaeomagnetic” measurements are hard to do, and it is not easy finding rocks that preserve these records. Also, they only tell us about the continents and not about plate margins or the oceans. Luckily, we had Dr Sergei Pisarevsky from Curtin University on the team to sort out the reliable palaeomagnetic poles from those that were less trustworthy.
Why map ancient plate tectonics?
The lack of ancient tectonic maps has posed quite a problem for how we understand our Earth.
Tectonic plates influence many processes on Earth, including the climate, the biosphere (the sphere of life on the outer part of the planet), and the hydrosphere (the water cycle and how it circulates around the planet and how its chemistry varies).
By simply redistributing tectonic plates, and thereby moving the positions (the latitudes and longitudes) of continents and oceans, controls are placed on where different plants and animals can live and migrate.
Plate boundary locations also govern how ocean currents redistribute heat and water chemistry. Different water masses in the ocean contain subtly different elements and their various forms, known as isotopes. For example, water in the deep oceans was often not at the surface for many thousands of years, and has different composition from the water presently on the ocean’s surface. This is important because different water masses contain different amounts of nutrients, redistributing them to different parts of the Earth, changing the potential for life in different places.
Tectonic plates also influence how much of the Sun’s radiation gets reflected back out to space, changing the Earth’s temperature. For example, continents are relatively bright and reflective compared to oceans – that’s why oceans look dark blue from space. If more continents are near the Equator then more of the Sun’s radiation is reflected back out to space.
How fast tectonic plates move have also varied over time. At different periods in Earth history there were more mid-ocean volcanoes than there are today. The volcanoes make hot ridges in the middle of oceans which are elevated when compared to the surrounding cold, older oceanic crust away from these ridges. So with more mid-ocean ridges, you get more elevated sea-beds, which pushes up oceanic water over the continents. Because the Earth isn’t expanding, when you have more mid-ocean ridges, you have more ocean crust being subducted, meaning that you also have more volcanic arcs forming above these subduction zones, pumping more gas into the atmosphere – changing its composition.
Mountain ranges form as tectonic plates collide, which affect oceanic and atmospheric currents as well as exposing rocks to be eroded. This locks up greenhouse gases as these rocks get chemically eroded, and also releases nutrients from those rocks into the ocean.
Understand ancient plate tectonics and we go some-way to understanding the ancient Earth system. And the Earth as it is today, and into the future.
Merdith, A.S., Collins, A.S., Williams, S.E., Pisarevsky, S., Foden, J.F., Archibald, D.A., Blades, M.L., Alessio, B.L., Armistead, S., Plavsa, D., Clark, C. & Müller, R.D. 2017. A Full-Plate Global Reconstruction of the Neoproterozoic. Gondwana Research, 50, in press.
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