Four Billion Year Old Continent In Canada's NWT

 

November 16, 2015

A fiery ball of molten lava – the beginning of our planet, four and a half billion years ago. There was no atmosphere, no oceans, and no rock as we know it; the conditions were too hot for the molten material to form solids. But, slowly, things cooled, and five hundred million years later, areas of the earth’s crust had solidified. Amazingly, remnants of these first rocks have remained intact through the ages, and are now being studied by geologists looking for hints of that nascent world. The oldest rocks have been found in Australia, scattered crystals that date back 4.28 billion years. But located in the Canadian Arctic are rocks over 4 billion years old that form a solid mass, the heart of one of the first continents on Earth, perhaps the first.

Figure 1: The heart of the archaic continent is located just north of Yellowknife, NWT.

Just north of Yellowknife, in the Northwest Territories, Canadian geology sleuths Wouter Bleeker and Richard Stern have been examining the composition of these primeval rocks to piece together the first 1.5 billion years of the ancient continent’s history. The rocks stretch continuously for hundreds of kilometers – they are not scattered remnants, but a solid chunk of a continent (Fig. 1)!

The piece of ancient crust is made of gneiss, a metamorphic rock that forms under high temperatures and pressure (Fig. 2, 3). The top of this layer is uneven, an indication that it has been exposed to the elements for some time; it has been weathered.

F

igure 2: Canadian arctic gneiss. Gneiss is a grainy metamorphic rock with a banded appearance that is due to the separation of the different minerals into layers.

Some of the layers on top of the gneiss are much younger–about 2.8 billion years old. This gap in time likely represents a period of uplift, when forces from the mantle below forced the continent upwards, exposing its surface to the elements. After millennia of erosion, the continent then sank below an ocean and sedimentary layers – sandstone, and other, iron-rich rocks – began to accumulate. The 2.8 billion year old layers are volcanic, indicating that the wafer of rock was ripped apart, and lava from below flowed over the crust’s surface (Fig. 4).

Figure 3: Very old gneiss intruded by younger, probably 3.6 billion year old granitic veins or “dykes”.

Along the eastern flank of this archaic landmass, the layers of gneiss, sandstone, and volcanic material are all missing. This is a sure indication that the 4 billion year old continent was once bigger–that the Canadian chunk is only a portion of a larger continent, perhaps even a super-continent. Other pieces of this continent may still exist! In places as far afield as Wyoming and

Zimbabwe, rocks dating to the same age and with the same layering pattern have been found. The history of this archaic landmass is obviously a convoluted one, but it just might be the information gleaned from the relic located in Canada’s arctic that illuminates it.

Figure 4: A cross section of rock, showing the layers that reveal an ancient continent’s history.