Module 4

Rodinia and the Grenville Province


The northern coast of Georgian Bay is a classic 'Canadian Shield' landscape characterized by rounded mounds of exposed crystalline rock poking up through thin nutrient-poor soils that support a boreal ecosystem. Low-lying depressions between bedrock outcrops are poorly-drained so become filled with bogs and marshes. The distinctive topography results two main factors: the irregular rye-bread like structure of the Precambrian rock, and the smoothing and carving of that rock by flow of glacial ice in the Pleistocene Epoch which ended a mere 14,000 years ago.

The coast is sprinkled with the tiny islands of the worlds largest freshwater archipelago, known as the '30,000 islands'. The name is a misnomer: it's actually closer to 100,000 islands. Wendat legend tells of a warrior god called Kitchikewana who upon learning of engagement of his love, Wanakita, to another warrier, fell into a fit of rage and clawed out the earth of Severn Sound and threw it into Georgian Bay, making the 30,000 islands.

The 30,000 islands and complex coastline are an enduring gift to the people of Georgian Bay. To the Anishinaabe, the extensive waterways provided sustenance in the form of fish and access to inland hunting grounds, as well as protection from both the more difficult waters of the open bay and neighbouring tribes with whom relations were tense. The islands also offered convenient stopping places facilitating trade corridors, primarily trading local fish and game with corn grown in the soil-rich regions occupied by the Wendat and Odawa. Deep, straight, valleys scar the landscape where joints in the Precambrian bedrock have been scoured out by glacial ice. These valleys, like the French River, acted as 'superhighways' for Anishinaabe and the French Voyageurs alike, travelling from the Saint Lawrence valley to the Great Lakes, and out to the fur-rich areas of the west.

Even today, the region is inaccessible except for by boat, which helps to keep the region 'wild'. This wilderness was particularly drawn upon by the Toronto-based Group of Seven, who captured the windswept pines clinging onto barren rock and the powerful forces of nature acting on the shoreline. The inaccessibility of that shoreline is appreciated today because it is a haven for wildlife and large areas have been set aside for conservation in the face of increasing pressures arising from the inevitable northward creep of civilization and people. It is an important area for migratory and island-nesting birds.

One of the pioneers of Plate Tectonic Theory, J. Tuzo Wilson, had a summer cottage on the northern coast of Georgian Bay. Wilson was a geophysicist who embraced the use of new technologies like air photo analysis, gravimetry, airborne magnetometry, seismic and thermal measurements, as well as the radiometric dating of rocks to understand the structure of the Canadian Shield. In 1954, Wilson wrote of the Canadian Shield that "the arrangement of [former mountain ranges] and later ranges outside the Shield suggests that the North American continent has grown by the accretion of successive systems of mountain arranged tangentially along the former margins of the continent" which is an important interpretation of how continental crust grows over time through the process of orogenesis. This bases was expanded into a more unified theory of plate tectonics in the 1960's when Wilson became a strong proponent of the idea that the redistribution of the continents on Earth's surface by plate tectonic processes resulted in cyclical formation and breakup of supercontinents throughout Earth's history. This became known as the Wilson cycle. He was no doubt influenced by the ancient and highly deformed rocks of the Grenville Province which lie on the northern Georgian Bay coast and tell us that the Georgian Bay was once a collision zone between ancestral North and South America when new land was added to ancestral North America.

The last supercontinent was Pangea, which formed about 330 million years ago, but before Pangea, there was Rodinia (named after the russian word for 'motherland') 1.1 billion year ago, and it is formation of Rodinia which is recorded in the rocks of the Grenville Province. The mighty Grenville Mountains may have been the largest mountain chain ever to have existed on earth. Today, the Grenville Mountains have vanished, having been wiped away by the relentless forces of erosion, however, they left their mark in the form of chains of metamorphosed belts found on virtually every continent.

In this lesson, we will explore the Central Gneiss Belt in order to discover what secrets it holds about the process of orogeny and the supercontinent cycle.


By the end of this lesson, you will be able to
  • Describe the plate tectonic situation of the formation of Rodinia.
  • Understand how the Grenville Orogeny 'worked' and identify the parts of the Grenville Province.
  • Link metamorphic rock types to geological processes.

  • Key terms: Rodinia, supercontinent, Grenvillian, Wilson Cycle, allochthonous, autochthonous, terrane, shear zone, island arc, magmatic arc, active margin, orogeny, accretion, obduction.
Part One

Building Rodinia

Part Two

The Grenville Province

Self Guided Learning: Virtual Field Trip
The Grenville Coast of Georgian Bay