Kenorland: One of Earth's Earliest Supercontinents

Kenorland is a proposed supercontinent that may have formed in the Neoarchean era, about 2.7 billion years ago, when many of Earth’s early blocks of continental crust came together. If it existed, it was one of the first large land masses in the planet’s history. Its shape, size and even its existence are still debated, because the evidence lies in scattered, ancient fragments of crust.

Background: continents in the Archean

During the Archean Eon, between about 4.0 and 2.5 billion years ago, Earth’s first durable continental crust formed. Its surviving pieces are called cratons: old, thick, stable blocks that now form the cores of continents such as North America, Africa and Australia.

The late Archean, especially around 2.7 billion years ago, was a time of unusually rapid crust formation. Many cratons show intense volcanism, the intrusion of granites and mountain building at about this time. Some geologists think these episodes reflect cratons colliding and merging into a larger continent.

The idea of Kenorland

The name Kenorland was proposed in 1991 by H. R. Williams and colleagues. It comes from the Kenoran orogeny, a period of mountain building recorded in the Superior Craton of Canada around 2.7 billion years ago. The orogeny is named after the Kenora district of north-western Ontario.

In the Kenorland model, the Superior Craton was joined with other blocks, possibly including the Slave Craton of northern Canada, the Kaapvaal Craton of southern Africa, the Pilbara Craton of Australia, the Karelian Craton of Scandinavia and others. The continent would have assembled by about 2.7 to 2.6 billion years ago and lasted into the early Proterozoic.

The evidence

Matching rocks

Geologists reconstruct ancient continents by matching features that were once continuous. These include similar sequences of sedimentary rock, belts of mountain building, and above all swarms of dykes: sheets of volcanic rock that filled cracks as the crust stretched.

Where dykes of the same age and pattern appear in cratons that are now far apart, they may once have formed a single swarm. Precise dating of these dykes, using uranium-lead methods, is the main tool for testing Kenorland.

Magnetic clues

Another tool is palaeomagnetism. Some volcanic rocks record the direction of Earth’s magnetic field when they cooled, which reveals their latitude at the time. If two cratons show matching records, they may have moved together. For Archean rocks, however, such data are scarce and often disturbed by later heating.

Rival models

Not all geologists accept a single Kenorland. In 2003 Wouter Bleeker of the Geological Survey of Canada compared the late Archean record to “a puzzle in ca. 35 pieces”. He argued that the cratons fall into several groups with different histories, which he called supercratons. These included Superia, centred on the Superior Craton, and Sclavia, centred on the Slave Craton, along with Vaalbara, made of the Kaapvaal and Pilbara cratons.

In this view, the supercratons broke up at different times, which explains why their dyke swarms and rift histories do not match neatly. Today many geologists regard Kenorland as a possible but unproven supercontinent, and some use the name loosely for the late Archean assembly of cratons in general.

Breakup

The breakup of Kenorland, or of the supercratons, appears to have happened in stages between about 2.48 and 2.1 billion years ago. Large swarms of dykes, such as the Matachewan dykes in Ontario, dated to about 2.45 billion years ago, record the stretching of the crust as rifts opened.

Rifting created new coastlines and shallow seas. Some scientists link this to major environmental changes of the early Proterozoic, including the Great Oxidation Event and the Huronian glaciation. Fresh rock exposed to weathering could have drawn carbon dioxide from the air and cooled the climate, while nutrients washing into the sea may have boosted oxygen-producing microbes. These links remain hypotheses.

Myths and facts

It is sometimes stated as fact that Kenorland was Earth’s first supercontinent. In reality, its existence is debated, and some reconstructions favour several smaller land masses instead. Maps of Kenorland are informed estimates, not established geography. The oldest supercontinent for which most geologists see strong evidence is Columbia, also called Nuna, which assembled about 1.8 billion years ago.

Timeline

• c. 2.72-2.68 billion years ago: Kenoran orogeny and rapid crust formation in the Superior Craton.
• c. 2.7-2.6 billion years ago: Possible assembly of Kenorland.
• c. 2.48-2.45 billion years ago: Early dyke swarms, such as the Matachewan dykes, record rifting.
• c. 2.45-2.2 billion years ago: Huronian glaciation during the breakup period.
• c. 2.1 billion years ago: Rifting largely complete.
• 1991: H. R. Williams and colleagues propose the name Kenorland.
• 2003: Wouter Bleeker proposes separate supercratons instead.

Why it matters

Kenorland is a test case for when Earth began its supercontinent cycle, the repeated gathering and breaking apart of continents that continues today. Whether it existed bears on how early plate tectonics worked and how continental shifts influenced the atmosphere, climate and early life.