Rodinia: The Supercontinent Before Snowball Earth

Rodinia was a supercontinent that joined most of Earth’s land into a single mass from about 1.1 billion or 1 billion years ago until it broke apart, mainly between about 800 and 750 million years ago. Its assembly is recorded in the ancient Grenville mountain belt of eastern North America, and its breakup opened new oceans and may have helped plunge the planet into the Snowball Earth glaciations. Because the evidence is so old, geologists still argue about exactly how its pieces fitted together.

Background: the supercontinent cycle

Earth’s outer shell is divided into tectonic plates that move a few centimetres a year. Over hundreds of millions of years this motion repeatedly gathers continents together and then splits them apart again, a pattern called the supercontinent cycle.

The best-known supercontinent is Pangaea, which assembled around 335 million years ago. Before it came Rodinia, and before that an older supercontinent known as Columbia or Nuna, which existed roughly 1.8 to 1.3 billion years ago. The long interval between them belongs to the Proterozoic Eon, before animals had evolved.

Reconstructing these ancient landmasses is far harder than for Pangaea. Oceans older than about 200 million years have been destroyed by subduction, and most Proterozoic rocks have been deformed, eroded or buried.

Discovery and naming

In 1970 the geologists James Valentine and Eldridge Moores suggested that a supercontinent had existed late in the Precambrian, before the familiar landmass of Gondwana assembled. Evidence gradually built up from matching rock belts, ancient magnetic directions and the ages of mountain-building events.

The name Rodinia was proposed by the American palaeontologists Mark and Dianna McMenamin in their 1990 book The Emergence of Animals. It comes from the Russian rodina, meaning “motherland”, reflecting the idea that later continents grew from its fragments. They also named the ocean that surrounded it Mirovia, from a Russian word meaning “global”.

How Rodinia assembled

Rodinia formed through a series of continental collisions between roughly 1.3 billion and 900 million years ago. The most famous record of these collisions is the Grenville orogeny, an episode of mountain building that peaked about 1.09 to 0.98 billion years ago.

The deeply eroded roots of the Grenville mountains can be traced along eastern North America from Labrador through Quebec, Ontario and the Adirondack Mountains of New York, and south towards Texas. Rocks of similar age in southern Norway and Sweden, known as the Sveconorwegian belt, are thought to record the same series of collisions.

At the centre of most reconstructions lies Laurentia, the ancient core of North America and Greenland. Other blocks that probably joined it include Baltica (northern Europe), Amazonia and other parts of South America, Australia, East Antarctica, India, Siberia and parts of Africa and China.

Life on and around Rodinia

Rodinia would have looked nothing like a modern continent. There were no land plants or animals, so its interior was mostly bare rock, gravel and sand, eroded by rivers and wind. At most, thin films of microbes may have coated damp surfaces, and lake deposits of about this age in north-west Scotland, part of the Torridonian sandstones, preserve microfossils that some researchers interpret as early freshwater eukaryotes.

Life was concentrated in the sea. Shallow waters along Rodinia’s margins hosted microbial mats and stromatolites, and the oceans held single-celled algae and other eukaryotes. One of the oldest fossils that can be linked to a living group, the red alga Bangiomorpha from Arctic Canada, dates from about 1.05 billion years ago, the time when Rodinia was taking shape.

The interval when Rodinia formed also falls within what geologists sometimes call the “boring billion”, a long stretch of apparently stable climate and ocean chemistry. The breakup of the supercontinent coincides with the end of that stability, which is one reason the supercontinent attracts so much attention.

Configuration debates

The exact layout of Rodinia is one of the major open questions in Precambrian geology. Geologists combine three main kinds of evidence: matching rock belts and mountain chains on different continents, ages of minerals such as zircon, and palaeomagnetism, the record of Earth’s magnetic field preserved in rocks.

In 1991 three geologists, Eldridge Moores, Ian Dalziel and Paul Hoffman, independently argued that western North America had once lain against East Antarctica and Australia. This became known as the SWEAT hypothesis, short for “Southwest US-East Antarctica”. Hoffman’s paper in Science suggested that the breakup of Laurentia effectively turned the later supercontinent Gondwana “inside out”.

Rival models placed Australia against the south-western United States (AUSWUS) or against Mexico (AUSMEX). Another proposal, the so-called “missing link” model, places South China between Australia and Laurentia. A widely cited 2008 synthesis led by Zheng-Xiang Li of Curtin University compared these options, but no single model has won universal acceptance.

Breakup and its effects on climate

Rodinia began to split apart from about 825 to 800 million years ago, with major rifting by around 750 million years ago and some fragments separating later still. In the SWEAT model, the departure of Australia and Antarctica from western Laurentia opened the ancestral Pacific Ocean.

Breakup was accompanied by large volcanic outpourings. One of the most important is the Franklin large igneous province, a vast area of basalt in Arctic Canada and Greenland erupted about 720 million years ago, at almost the same time as the start of the Sturtian glaciation.

Many geologists think these changes affected climate. Rifting created long coastlines and shallow seas, which can make climates wetter. Fresh basalt weathers quickly, especially in warm, wet tropical conditions, and that chemical weathering removes carbon dioxide from the atmosphere. Because many fragments lay near the equator, this cooling effect may have been strong enough to help trigger the Cryogenian ice ages. The idea is plausible and widely discussed, but it is not proven.

Legacy

Fragments of Rodinia later reassembled into Gondwana, the great southern landmass that formed around 600 to 500 million years ago. Some researchers also propose a short-lived supercontinent called Pannotia near the end of the Precambrian, though its existence is debated.

The breakup of Rodinia also coincides with major changes in life and ocean chemistry. Oxygen levels appear to have risen in the late Neoproterozoic, and the oceans eventually hosted the first large multicellular organisms of the Ediacaran Period. Whether the supercontinent’s breakup helped drive these changes is an active area of research.

Timeline

• c. 1.3 billion years ago: Continental collisions that will build Rodinia are under way.
• c. 1.09-0.98 billion years ago: The main phases of the Grenville orogeny join Laurentia to neighbouring blocks.
• c. 900 million years ago: Rodinia is largely assembled, surrounded by the Mirovia ocean.
• c. 825-800 million years ago: Rifting begins in several parts of the supercontinent.
• c. 750-600 million years ago: Major fragments drift apart, opening new oceans.
• c. 720 million years ago: The Franklin basalts erupt, shortly before the Sturtian glaciation begins about 717 million years ago.
• 1990: Mark and Dianna McMenamin propose the name Rodinia.
• 1991: Moores, Dalziel and Hoffman publish versions of the SWEAT hypothesis.

Why it matters

Rodinia shows that the supercontinent cycle is a deep, recurring feature of Earth’s history, not something unique to Pangaea. Its assembly built mountain belts whose roots still underlie North America and Scandinavia, and its breakup may have helped push Earth’s climate into its most extreme freeze, just before complex life appeared.